Magnetic ordering is dominated by the formation of ferromagnetic Fe3+–Fe2+–Fe3+ trimers which are evident above the magnetic ordering transition. Between TN = 289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe3O5 exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe2+ for Ca2+ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixed-valent systems to composition.Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe 3O5 avoids the electronic phase separation reported in a non-stoichiometric sample, and also exhibits an incommensurate magnetic phase.A significant challenge in understanding the behaviour of strongly correlated materials is their sensitivity to composition. Here the authors show that stoichiometric CaFe Modern instrumentation was able to detect anomalous shortening of some Fe–Fe distances at this transition, consistent with localisation of the minority Fe2+ spin into linear three-Fe clusters named trimerons2, thus resolving the complex nature of this transition3. CaFe3O5 is another mixed-valence compound of iron with a mean Fe oxidation state of +2.66, the n = 1 member of a series of general formula CaFe2O4(FeO)n. These compounds consist of CaTi2O4-type CaFe2O4 slabs, separated by rock salt-type FeO slabs of a thickness that increases with increasing n. n is known to take integer values of 1, 2 and 34, and half-integer mean values of 1.5 and 2.5 are found when there are alternating (FeO)n and (FeO)n+1 blocks5. The n = 3 member of the series, CaFe5O7, exhibits a complex structure6, stacking faults that are synthesis dependent8, complex long range magnetic order and Charge Order (CO), an important phenomenon in mixed-valent compounds9. Charge ordering has also been observed in two compounds that are isostructural with CaFe3O5: Fe4O510, a high-pressure-synthesised mixed-valence compound in which charge ordering occurs in Fe dimers and trimers, and MnFe3O5, which undergoes charge ordering with localisation of the charges on distinct Fe2+ and Fe3+ sites11.Mixed-valent compounds of iron and other transition metals show a wide range of physical and chemical properties. Magnetite, Fe3O5 adopts an orthorhombic structure in space group Cmcm with two crystallographically distinct iron sites. Fe1 is an octahedral site (8f) in the CaFe2O4 layer and Fe2 is an octahedral site (4a) in the FeO layer. These layers stack along the c axis as shown in Fig. TN of 282 K12, and suggested a Charge Averaged (CA) state pertaining above TN and a CO state below, with localisation of Fe3+ and Fe2+ on distinct sites. More recent work by Hong et al. confirms the high temperature CA state, but they report a macroscopic phase separation in their sample below the magnetic ordering transition (reported as TM = 302 K)13, They used high resolution diffraction data to clearly observe a single phase above this TM which separates into two phases below. Bond valence sums for the Fe sites suggested that the minority phase remains charge averaged (CA) below the magnetic ordering transition, while the majority phase is charge ordered (CO) with the Fe1 site characterised as Fe3+ and the Fe2 site as Fe2+, reflecting the expectation of the work of Gerardin et al.12. The CA and CO phases both exhibit long range magnetic ordering, clearly presenting separate sets of magnetic Bragg peaks in the powder neutron diffractograms. Hong et al.13. show in their analysis of bond lengths that the CO phase meets the requirements for trimeron formation by means of localising the minority spin in Fe1-Fe2-Fe1 units13.CaFe13, and it is unusual that it was not detected in the Mössbauer work by Gerardin et al.12. From the Rietveld refinements the sample of Hong et al.13. contains 4% substitution of excess Fe for Ca Fe3.040(8)O5), which they suggest may tip the balance between the energies of the CO and CA states, raising the possibility that the occurrence of the phase separation may be sample dependent, and may be sharply dependent on composition. A further recent study of CaFe3O5 as a potential oxide-ion storage material in solid oxide fuel cells also used Mössbauer spectroscopy and was largely consistent with the work of Gerardin et al.12, detecting only one phase at low temperature14. However, it also noted that the room temperature Mössbauer spectrum deviated from the usual 3:2:1 ratio for magnetic features, and concluded that an incommensurate antiferromagnetic hyperfine field distribution was the simplest model to account for the spectrum.The evidence for the phase separation in the work by Hong et al. is unquestionable and well characterised but its origin is unclear3O5 ; much lower than the ~40% reported in the sample of Hong et al.13. In three samples, including the sample measured by neutron diffraction (sample A) there was no detectable CA phase in the PXRD, showing that the synthesis route reported here  reliably produces samples which yield only the CO phase at low temperatures.Powder X-ray diffraction (PXRD) patterns of multiple samples of CaFeCmcm symmetry above and below the magnetic transition.Aside from the phase separation evident in sample B, no peak splitting or superstructure reflections were observed as a function of temperature and Rietveld refinements were consistent with the structure maintaining 4O5 and MnFe3O5 at their respective magnetic ordering temperatures15. In CaFe3O5 the lattice parameters begin to deviate from linearity on cooling at around 410 K, well above the long range magnetic ordering temperature established by neutron diffraction, indicating that short range magnetic correlations influence the crystal structure well above the onset of long range order. From ~410 K down to 280 K the b lattice parameter (10.01770(3) Å at 500 K) undergoes an expansion, whilst the c lattice parameter (12.67159(3) Å at 500 K) undergoes a complementary contraction, conserving the unit cell volume. The a lattice parameter (3.039760(7) Å at 500 K) and the unit cell volume change comparatively smoothly with only slight inflections at the transition. Each of the lattice parameters then contracts smoothly as a function of temperature below 280 K, after long-range magnetic order is established. This behaviour of the lattice parameters quantitatively matches the reported behaviour for the CO phase reported by Hong et al.13.The compound undergoes anisotropic thermal contraction on cooling associated with long range magnetic ordering Fig. , which i3O5 show magnetic transitions between 285 and 310 K in the susceptibility data (see Methods section). The behaviour varies from sample to sample  shows no rise in the DC susceptibility at 295 K on cooling and measurements carried out in a low (3.5 Oe) AC field show a very weak, broad feature in the real component that appears to be frequency independent and has no imaginary signal, confirming the lack of a ferromagnetic component. Sample A’s magnetisation isotherm is complicated by a <0.5% Fe3O4 impurity, but sample E, which contains no Fe3O4 and an estimated 0.2 % of the CA phase shows linear magnetisation versus field curves at both 300 K and 5 K, with no hysteresis when sweeping the field between −5 and +5 T, consistent with the CO state being antiferromagnetic at 5 K.All samples of CaFeA showed strong magnetic Bragg peaks in the neutron powder diffractogram at 5.5 K with no diffuse features, consistent with long-range magnetic order. These peaks index to a single propagation vector of  putting the k vector along the Σ(α00) symmetry line.Sample 16, and TOPAS Academic was used to perform Rietveld refinements against the neutron powder diffraction (NPD) data, testing each of the symmetry adapted modes as a refined parameter18. The magnetic space group Pabca (61.438) (BNS notation)19, corresponding to the order parameter mΣ3 produced excellent agreement with the data, describing ordered moments on both Fe sites with a single value of the moment on each Fe sub-lattice. The other possible space groups Pcbcm (57.388) and Pcca21 (29.106) gave comparable fits but do not give the single value of the moment on each Fe sub-lattice expected for an oxide with localized moments. The Pcbcm (57.388) model allows an ordered moment on only half of the Fe ions, inconsistent with previous Mössbauer results12, while the Pcca21 (29.106) model has four independent Fe sites with strongly correlated values of the moments making the refinement very unstable. We discuss this point further below. Our magnetic structure in Pabca was derived independently of the work of Hong et al. and is consistent with their CO model13. Using the data from WISH with sample A, which shows single phase behaviour in the magnetically ordered regime, we refined stably separate magnetic moments for the two independent Fe sites as discussed below. The Rietveld refinement, shown in Fig. 13 has a k =  propagation vector and would add intensity to nuclear reflections, particularly the (0 2 1) peak, but in our sample this peak is weak and temperature independent showing that it results purely from a nuclear contribution to the Bragg scattering in sample A 2+-(Fe1)3+ linear units  generated through edge-shared octahedra with 89° Fe1-O-Fe2 linkages. The antiferromagnetic coupling of these trimeron units along the a-axis is through the J1 and J2 exchange paths (94° Fe1-O-Fe1 and 86° Fe2-O-Fe2 linkages between edge-sharing octahedra respectively), whereas the coupling along the c direction (J6) is between (Fe1)O6 octahedra through vertex-shared linkages with a 134° Fe1-O-Fe1 angle. Another two pairs of couplings, J4 and J5, between edge-sharing octahedra become inherently frustrated as a result of the ordering. The pair of J4 interactions couples Fe1 to two further Fe1 through shared edges with identical Fe1-O-Fe1 linkages, but one of the pair of interactions is ferromagnetic and the other antiferromagnetic, which is inevitable since the coupled sites are offset from each other by ½ along a. Similarly Fe1 is linked to two Fe2 by the pair of J5 interactions with one being ferromagnetic and the other antiferromagnetic. The two components of each of these frustrated exchange paths  can couple a displacive distortion with propagation vector q = (100) to conserve the lattice translational symmetry. This distortion will transform as the Y3−(σ) irreducible representation of the parent space group and will reduce the nuclear symmetry to Pbcm, effectively breaking the mirror plane perpendicular to a, and making each component of the two pairs of frustrated interactions, J4 and J5, inequivalent. The coupling between the two order parameters in the free energy can be written as −σξ2, indicating that the displacive distortion is a secondary order parameter induced by the magnetic ordering and its amplitude will be very small, explaining why it is not evident in the diffraction data.The magnetic structure is shown in Fig. J3 interaction and the antiferromagnetic J1, J2 and J6. If single values are assumed for all the J4 and J5 interactions, the MC simulation of the magnetic scattering clearly indicates the development of short range correlations leading to diffuse features  distortion splits each pair of these interactions into two inequivalent ones  alternating along the a-axis direction. Good agreement with the experimental data can be achieved in the simulations by fixing the J3 ferromagnetic interaction to unity and with the following set of antiferromagnetic J′s: J1 = J2 = J6 = 0.75, J4 = 0.5, J4′ = 0.3 and J5 = 0.1, J5′ = 0.05. The calculated magnetic scattering now shows sharp peaks  and 2.136(2), respectively, at 300 K, which diverge slightly at low temperature to values of 2.756(2) and 2.095(2) at 5.5 K, mirroring the behaviour of the CO phase of Hong et al. and contrary to their CA phase13, for which the bond valence sums converge at low temperature instead. These values indicate that the system shows a partial charge ordering, however, the difference in the refined ordered moments is too small to describe the average of the system as entirely Fe3+ on the Fe1 site and Fe2+ on the Fe2 site even at 5.5 K, which is consistent with the distribution of the minority spin electron across the Fe1 and Fe2 sites in the localised trimeron units2. The increased separation of the moment sizes at low temperature suggests an increasing degree of localisation of the minority spin on the central Fe2 ion in the trimeron units, which moves towards the description of Gerardin et al. derived from their Mössbauer study: the minority spin first being distributed among trimers12, with the system then becoming more formally Fe3+ and Fe2+ at low temperature.The moments refined for the Fe1 and Fe2 sites shown in Fig. k-vector remains along the Σ(α00) symmetry line. The incommensurate structure was modelled with a spin density wave (SDW) propagating along the a axis, described in the Cmcm1’(α00)00ss magnetic superspace group, corresponding to the mΣ3 irreducible representation with order parameter direction . At 283 K the separately refined amplitudes of the spin density wave are 1.98(4) and 2.08(3) µB for the Fe1 and Fe2 sites, respectively. Given that the difference between the Fe1 and Fe2 SDW amplitudes is within 2σ and the refined moments in the commensurate region converge on warming, both sites were constrained to have the same SDW amplitude in the incommensurate region,  μB). The phase coexistence between the commensurate and incommensurate phase that is observed at 281 K is expected since the lock-in character of the transition (see below) requires it to be first order. At 281 K the model has 55% of the incommensurate phase with propagation vector (0.5047(7) 0 0) and 45% of the commensurate phase with propagation vector (½ 0 0), assuming both phases have the same average moment. The incommensurate-regime is only adopted in a narrow temperature range, with a monotonic increase of the k-vector with increasing temperature to a maximum of α = 0.507(2) at 287 K before the moment falls to zero within error on warming to 289 K. The narrow temperature range for this incommensurate region presumably explains why it was not reported by Hong et al.13.The arrangement of spins persists between 5.5 and 280 K, but on warming to 281 K a commensurate-incommensurate transition occurs. The data at 281 K shows coexistence of the commensurate magnetic peaks with a set of incommensurate magnetic peaks, then on warming further to 283 K only the incommensurate magnetic reflections are present Fig. . A propaJ1, J2 and J6 are still net antiferromagnetic but the moment is described by a SDW along the a axis. The modulation is out of phase between the positions in the unit cell at 0 and ½ along a, which are the positions coupled by the frustrated J4 and J5 interactions. In contrast with the low temperature commensurate regime where a displacive distortion lifts the exchange degeneracy, in this incommensurate magnetic ordering regime, where coupling to a displacive distortion is not possible in principle, the magnetic frustration is resolved instead by modulating the moment along the a-axis at the expense of the entropic energy term. The result is that when one moment is maximised, its J4/5 coupled partner is minimised and vice versa, a situation comparable to that in the Fe2+ oxide selenide Sr2Fe3Se2O3 where the low temperature ordered structure involves inherently frustrated pathways which likewise become out of phase by adopting a modulated magnetic structure in a narrow temperature range close to the magnetic ordering temperature21. In this modulated model for CaFe3O5, the trimeron units are fully maintained in the incommensurate structure as shown by the dashed lines in Fig. J3. The trimerons participate in the SDW as units, each one modulating its moment along the a axis.A schematic representation of the incommensurate structure is shown in Fig. J4 and J5 interactions. The good agreement clearly indicates that the trimeron units start to form and interact well above TN and likely when the anomaly of the cell parameters appears around 400 K.The sharp Bragg peaks of the magnetic structure are lost at and above 289 K, with the majority of the magnetic scattering giving a uniformly increased background. However, the strongest peaks are still visible as diffuse scattering in the diffraction pattern at 300 K as shown for the ½ 0 1) reflection in Fig.  reflecti3O5 into phases that show CO and CA13, both with sharp nuclear and magnetic Bragg peaks indicating that they both present long range order. While the quality of their data shows that this is indeed the behaviour of their sample, we show here that this is not the intrinsic behaviour of stoichiometric CaFe3O5 which remains a single crystallographic phase, and shows only a single long range magnetically ordered phase (the CO phase with Fe2+/Fe3+ charge order) below the magnetic ordering transition.Hong et al. report phase separation in CaFe13, with additional Fe ions occupying the Ca site at the level of about 4% giving a composition of Ca0.952(8)Fe3.040(8)O5. Neutron diffraction shows good contrast between Ca and Fe , so the results suggest that there is indeed a small compositional difference between the samples. We presume that, as Hong et al. suggest13, this small degree of non-stoichiometry enables the CA phase to develop and compete effectively with the CO phase. The diffraction method probes the average structure and we suspect it is not possible to refine separately the compositions of the coexistant CA and CO phases, but we speculate that the CA portion of Hong et al.’s sample13 may be more Fe-rich than the CO phase.Our refinements show that our sample is stoichiometric within the experimental uncertainty, while the refinements of Hong et al. show that their sample is slightly Fe richPabca magnetic space group that does not allow, by symmetry, a ferromagnetic moment in any direction. Instead the CA phase, as reported by Hong et al.13, orders with a k =  propagation vector and adopts the Cm’cm’ magnetic space group (mΓ4+) which allows a ferromagnetic moment along the b direction, resulting in the observation of a spontaneous moment only in samples containing some of the CA phase. Clearly the synthesis of CaFe3O5 may result in a range of compositions. While our synthesis with the sample contained in an alumina crucible within a sealed silica tube enables us to make stoichiometric CaFe3O5, it is possible that higher temperature reactions may result in leaching of Ca from the sample into the silica tube.Our several samples show different behaviour in the magnetic susceptibility with evidence that the phase fraction of the CA phase is proportional to the magnitude of a ferromagnetic signal in the magnetic susceptibility. This is also supported by the magnetic symmetry analysis of the two ordered phases. The CO phase orders adopting the k =  is not a special point of the Brillouin zone due to the C centring of the lattice, and the low temperature magnetic structure can be described as a commensurate spin density wave with the magnetic phase locked to the ϕ = π/4 value. The lock-in of the propagation vector to the commensurate value and the selection of the magnetic phase can be understood considering the symmetry of the transition. As we show in the Results section both the incommensurate and commensurate magnetic phases are described by the two dimensional order parameter  transforming as the mΣ3 irreducible representation of the parent Cmcm space group. Considering this order parameter, the Landau free energy, describing the magnetic transition, contains a fourth degree lock-in invariant FLock = ξ14 + ξ24. This term is allowed only when the propagation vector is locked to the commensurate  value and the energy gain deriving from its activation explains the lock-in transition. The magnetic phase selection of the spin density wave can be understood from the phase dependence of the FLock term in the  plane. This dependence is shown in Fig. ϕ = π/4. This point corresponds to the order parameter direction , and describes the magnetic structure observed experimentally in the Pabca magnetic space group. The maxima at ϕ = 0 and ϕ = π/2 represent the solution with only half of the Fe sites ordered , whereas a general point  with ξ1 ≠ ξ2 ≠ 0 represents the Pcca21 solution (in which the trimerons carry different moments). Since the moduli of the neutron structure factors are insensitive to the phase choice, this explains why the last two solutions gave comparable fits in the Rietveld refinements (see above). Moreover the lock-in of the propagation vector to the commensurate value also allows the coupling of the magnetic order parameter  to that of the Y3− displacive distortion (σ) through the coupling term σξ1ξ2, which breaks the exchange frustration allowing for the long range ordered structure through the spin Jahn-Teller effect.Our investigations on the stoichiometric sample showing only the CO phase show a narrow region of previously unseen incommensurate magnetic order from 289 K down to a commensurate ordering transition at 281 K. As observed experimentally, the propagation vector jumps from (0.5047(7), 0, 0) to the commensurate value , remaining along the Σ symmetry line (α00). It is worth underlining that TN and non-linear behaviour of the lattice parameters as a function of temperature well above TN are evidence of short range exchange interactions being influential in CO-CaFe3O5 well above the Néel temperature. The MC simulations indicate the formation of the ferromagnetic trimeron units in this temperature range above TN. The structure of the diffuse scattering and the MC simulations suggest that these units present short range correlations at RT, and the study of their dynamics merit further investigation. Gerardin et al. originally proposed that the mechanism of electron exchange to average the Fe3+ and Fe2+ charges above TN was confined to an Fe2+O6 octahedron and two neighbouring Fe3+O6 octahedra with which it shares edges12. They referred to this as a trimer, and describe the spin exchange as localised in these trimers up to at least 400 K before eventually becoming fully delocalised. In contrast, in Fe3O42, the trimerons form when the system already shows magnetic long range order, accompanied by much larger changes in Fe–Fe distances than are observed in the CO phase of CaFe3O5 reported here and in ref. 13.Our observations of diffuse magnetic scattering above 3O5 shows CO below the magnetic ordering transition with localisation of the minority spin electron in so-called trimeron units first proposed for Fe3O42. Long range electronic phase separation is not found in the stoichiometric compound with a 2:1 ratio of Fe3+ and Fe2+ ions, but Hong et al. importantly show that the development of an alternative antiferromagnetic phase, the CA phase without charge ordering and without trimeron formation, occurs when the sample is slightly non-stoichiometric13. Our results, taken together with those of Hong et al.13 and the literature on related compounds underline the subtleties of these mixed-valent phases and suggest that targeted further tuning of the competition between electronic states through adjustment of the Ca:Fe ratio, control of the oxide content and control of the microstructure will be fruitful approaches for subtly controlling the physical properties of these oxides.In conclusion, CaFe3O5 were synthesised from stoichiometric amounts of CaO, (Alfa Aesar 99.95%) Fe2O3 (Alfa Aesar 99.998%), and Fe (Alfa Aesar 99.998%). The reactants were ground together inside an argon-filled dry glovebox using an agate pestle and mortar. The ground powder was pressed into a pellet, placed inside an alumina crucible, and sealed inside an evacuated silica ampoule. The product was found to form in high phase purity with repeated annealings between 800 and 1050 °C. Higher synthesis temperatures led to higher quantities of impurities and discolouration of the alumina crucible. Multiple samples were synthesised, as detailed in Supplementary Table 3O5, with subtle differences as described in the body of the paper.Polycrystalline samples of CaFe22 at the Diamond Light Source Ltd, UK using the high resolution MAC detector at room temperature, 100 and 500 K with temperature controlled by a nitrogen cryostream. The temperature dependence of the lattice parameters was measured using the high count rate PSD detector to continuously collect patterns on cooling the sample from 500 to 100 K, with a pattern collected every 1.5 K on average. Neutron Powder Diffraction measurements were performed from 5 to 300 K on the WISH instrument23 at the ISIS Pulsed Neutron and Muon Facility, UK with the samples contained in indium-sealed thin-walled vanadium cylinders inside a closed-cycle refrigerator (CCR). Structure solution and Rietveld refinements were performed using the TOPAS Academic software version 618, for the nuclear and commensurate magnetic structure and FullProf24 was used to refine the incommensurate magnetic structure. Tables of crystallographic data are given in Supplementary Tables High resolution X-ray Powder Diffraction measurements for structure solution and analysis were performed on beamline I11μ0H/T ≤ 5) were collected by first applying a field of 5 T at 300 K, then cooling to the measurement temperature in the field, then measuring the magnetisation on sweeping the field in steps down to −5 T and back to +5 T. AC magnetometry measurements were carried out at frequencies of 1, 10, 100 and 1000 Hz in a DC field of 3 Oe and an oscillating field of amplitude 3.5 Oe. The 1000 Hz data contained a high level of noise and are only shown in Supplementary Figure Measurements were carried out using a Quantum Design MPMS-XL SQUID magnetometer. The DC susceptibility was measured by cooling the sample to 2 K in zero applied field (ZFC) measuring the magnetisation on warming to 330 K in a 1000 Oe field, then cooling back down in the applied field (FC) and measuring again on warming, Magnetisation isotherms (–5 ≤ 25 based on a Metropolis algorithm26. A 20 × 20 × 20 supercell of the structural unit cell was used and the energy minimisation used Ising spins on each Fe site and the exchange interactions described in the text with cyclic boundary conditions.Classical Monte Carlo simulation was performed using an in-house codeSupplementary Information


Z,4E,6Z,8E)‐tetraenoate motif, and an expedient fragment‐assembly procedure. The result is a highly convergent total synthesis of chivosazole F through the orchestration of three mild Pd/Cu‐mediated Stille cross‐coupling reactions, including the use of a one‐pot, site‐selective, three‐component process, in combination with controlled installation of the requisite alkene geometry.A unified strategy for the chemical synthesis of the chivosazoles is described. This strategy is based on two closely related approaches involving the late‐stage installation of the isomerization‐prone (2 Sorangium cellulosum.The chivosazoles Scheme  are a stE/Z geometry.1) corresponding to the aglycon of chivosazole A (2). Despite, or possibly because of, their complex and delicate nature, there have been limited synthetic efforts reported toward the chivosazoles. Notably, the first total synthesis of chivosazole F was achieved by Kalesse's group,1).From the perspective of molecular architecture, the chivosazoles display an extraordinary array of features. The signature 31‐membered macrolactone ring features ten stereocenters, an oxazole moiety, and nine alkenes arranged in three distinct conjugated polyene arrays with diagnostic alternating Z,4E,6Z,8E)‐tetraenoate motif contained within the southern hemisphere.E,6Z,8E)‐trienoate moiety. Through judicious tuning of the reactivity of the termini of each of the building blocks (3–6), it was reasoned that a carefully designed sequence of site‐selective Stille cross‐coupling reactionsBuilding on the lessons learned from our earlier synthetic efforts,3 was required with a vinyl iodide at C14 and vinyl bromide at C26. A modification to our existing synthetic route  over the installation of the C20 and C22 oxymethine stereocenters. This entailed a tandem boron‐mediated aldol addition2BCl, Et3N] and Sm‐promoted reduction2, EtCHO) sequence, proceeding via the alcohol intermediates 10 and 11 . The amine 129 and the resulting amide cyclized to the oxazoline 3 .2,3 proved incompatible with these conditions, thus dictating that the installation of the oxazole ring should be performed after fragment assembly.Following Approach I, the linchpin fragment 3 necessitated the controlled transformation of the C13 bromide in 13tBuLi, Bu3SnCl) to afford 19. Oxidation (MnO2) and Stork–Zhao olefinationZ)‐vinyl iodide 4 , which corresponds to the second linchpin fragment envisaged in Approach I. A (Z)‐vinyl stannane was also required in the third fragment 2, PdCl2(PPh3)2].15, DCC) to afford 5.The choice of an iodide at C14 in ation MnO and StorE/Z geometry. We elected to initiate this demanding fragment assembly process with the acrylate derivative 6,E)‐vinyl stannane in 6 would be more reactive for steric reasons than the (Z)‐vinyl stannane in 4, and that the C14 iodide in 3 would be more reactive than the C26 bromide, thereby facilitating the site‐selective construction of the C13−C14 bond. In practice, the first Pd/Cu‐mediated Stille coupling reaction of 4 and 6 was found to be completely selective for the (E)‐vinyl stannane in 6 but sensitive to isomerization of the (6Z)‐alkene16. However, this troublesome loss of stereointegrity could be suppressed by the use of tBu3P as a ligand for Pd,16 (72 %). Under these modified conditions , the second coupling reaction between 16 and the linchpin 3 proceeded smoothly at 0 °C to give 17 (71 %), with complete site selectivity for the C14 iodide over the C26 bromide. Using these same mild conditions, the C27–C35 fragment 5 was finally appended to 17 to afford the desired complex polyene 18 (83 %). The successful orchestration of these three separate fragment couplings, with complete control over the desired alkene geometry, led us to next attempt a one‐pot process to assemble the full backbone of the chivosazoles. In the event, careful monitoring of the reaction and addition of each building block in turn  upon completion of the preceding coupling step, afforded 18 in 56 % yield, which is higher than that achieved when running the sequence as three separate operations . Remarkably, this choreographed reaction sequence efficiently assembles the highly elaborate polyene 18 with alternating E/Z geometry from four coupling partners in a single flask, under especially mild conditions, and minimizes the need for the isolation and chromatographic purification of sensitive intermediates.With the three key fragments in hand, the planned Stille cross‐coupling chemistry in Approach I was explored Scheme . Based oE,6Z,8E)‐triene motif in 18 was highly susceptible to alkene isomerization under the conditions examined for elaboration into the chivosazole macrocycle.3, 19, 20, and 21) and effectively rotated the order of fragment coupling in a clockwise fashion. We now envisaged the use of the alcohol 19 instead of 4 as the southern fragment, which would be coupled to the bis‐halide 3 used before. Exploratory studies led us to proceed with a revised north‐eastern fragment 20, which possesses a silylene protecting group and a Still–Gennari‐type phosphonate.Z,4E)‐dienoate and to facilitate the final global deprotection step.At this advanced stage, it was found that the southern (42223 into the triol 24. This was followed by site‐specific silylation (tBu2Si(OTf)2) of the C32 and C34 hydroxy groups, Pd‐catalysed stannylation ,25  to afford the phosphonate 20. Returning to the fragment‐assembly phase of the polyene construction 4, CuTC, [Ph2PO2][NBu4], DMF) to afford 27 (83 %) and 28 (88 %), respectively. Once again, it was demonstrated that these two Stille coupling reactions could be carried out in a highly efficient, one‐pot tandem process to give 28 (80 %).Adopting this revised plan Scheme , 227b wZ,4E)‐dienoate appendage with a δ‐tributylstannyl substituent in readiness for the penultimate Stille macrocyclization step.28 with the (E)‐enal 21Z/E) in favor of the desired ‐dienoate. MnO2‐mediated double oxidation then afforded the oxazole aldehyde 29, and accomplished the challenging aromatization reaction on a delicate substrate. After chromatographic removal of the minor double‐bond isomer to provide the required ‐dienoate, Stork–Zhao olefination gave the seco precursor for the final intramolecular Stille reaction to close the 31‐membered macrocycle. Gratifyingly, this crucial cyclization step gave the macrolactone 30 with retention of all the polyene stereochemistry. Finally, HF⋅pyridine‐mediated removal of the protecting groups, all silicon based, afforded chivosazole F (1). To our satisfaction, all 1H and 13C NMR spectroscopic data for this synthetic material correlated with those reported for natural chivosazole F.We were now poised to explore the planned endgame in Approach II Scheme . This fiIn conclusion, we have completed an expedient total synthesis of the actin‐binding polyene macrolide chivosazole F  through a highly convergent route based on an orchestrated sequence of Stille cross‐coupling reactions for fragment union and macrocycle formation. Notably, such multi‐component Stille couplings constitute a versatile, mild, and powerful method to rapidly build up molecular complexity in polyketide natural products, and can be further enhanced by judicious telescoping into one‐pot processes.The authors declare no conflict of interest.Dedicated to Professor Gilbert Stork on the occasion of his 95thbirthdayAs a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.


III and the excitation energy migration between the different ligands in [Eu(fod)3dpbt] , exhibiting well‐separated fluorescence excitation and phosphorescence bands of the different ligands, were investigated by using time‐resolved luminescence spectroscopy for the first time. The data clearly revealed that upon the excitation of dpbt, the sensitization luminescence of EuIII in [Eu(fod)3dpbt] was dominated by the singlet EET pathway, whereas the triplet EET pathway involving T1(dpbt) was inefficient. The energy migration from T1(dpbt) to T1(fod) in [Eu(fod)3dpbt] was not observed. Moreover, upon the excitation of fod, a singlet EET pathway for the sensitization of EuIII luminescence, including the energy migration from S1(fod) to S1(dpbt) was revealed, in addition to the triplet EET pathway involving T1(fod). Under the excitation of dpbt at 410 nm, [Eu(fod)3dpbt] exhibited an absolute quantum yield for EuIII luminescence of 0.59 at 298 K. This work provides a solid and elegant example for the concept that singlet EET pathway could dominate the sensitization luminescence of EuIII in some complexes.The excitation energy transfer (EET) pathways in the sensitization luminescence of Eu A further investigation on the energy migration processes among the excitation states of different ligands in a dpbt‐sensitized EuIII luminescence complex is helpful to demonstrate the dominant role of the singlet EET pathway.Our previously reported experimental evidences for the singlet pathway involved the well matched lifetime of the singlet state of dpbt in [Eu(tta)III luminescence in a highly luminescent complex [Eu(fod)3dpbt]  to T1(fod) was inactive and the EET pathway of T1(dpbt)→EuIII luminescence states was inefficient. .In this work, the energy migration processes between the different ligands and the contribution of singlet EET pathways to Eu22.1III luminescence excitation and emission spectra of [Eu(fod)3dpbt] at 298 K. The two excitation bands at 295 and 405 nm can be attributed to the sensitization function of fod and dpbt, respectively. The absorption band at 405 nm of coordinated dpbt is in consistent with the EuIII luminescence excitation spectrum in the visible light region, while the absorption signal below 350 nm is derived from the absorption band tail of fod. The absolute quantum yield for EuIII luminescence in [Eu(fod)3dpbt] upon excitation at 410 nm at 298 K was measured to be 0.59, which was much higher than that of [Eu(tta)3dpbt] (0.45). The lifetime of 5D0(EuIII) state in [Eu(fod)3dpbt], emitting at 614 nm, was measured to be 684 μs at 298 K, which was longer than that in [Eu(tta)3dpbt] (480 μs), implying more efficient depression of the nonradiative deactivation of EuIII luminescent states by the replacement of tta with fod in the complex.Figure 3dpbt] was excited at 298 K, the fluorescence of dpbt at 430 nm exhibited a decay time constant of 1.7 ns that was very close to the luminescence rise time constant (1.8 ns) of EuIII at 585 nm derived from 5D1(EuIII)→7F3(EuIII) transition 3dpbt]). Moreover, the luminescence decay time constant →7F2(EuIII) transition).As shown in Figure 3dpbt] within 10 ns. With the delay time increasing, the background of the fluorescence of dpbt in the complex gradually decreased and the luminescence signal at 585 nm of EuIII luminescence emerged (red dash line). Note that the luminescence rise‐time constant of EuIII luminescence at 614 nm was 582 ns, and the 5D0(EuIII) state had not been adequately populated within 10 ns, so the luminescence signal of EuIII luminescence at 614 nm in the illustration of Figure The inset of Figure III luminescence of [Eu(fod)3dpbt].The aforementioned tight decay‐to‐rise correlation clearly demonstrated the singlet EET pathway in the dpbt‐sensitized Eu2.23dpbt] at 77 K measured 0.5 ms after the photoexcitation of dpbt and fod, respectively, are shown in Figure 3dpbt] was excited at 295 nm →S0(fod) at 500 nm (20000 cm−1) was observed clearly. The phosphorescence decay at 500 nm could be well fitted by a bi‐exponential function with the time constants or lifetimes of 2.1 ms (39.3 %) and 16.5 ms (60.7 %), suggesting that there exist two kind of T1(fod) with different coordination environments in the compound. Upon the excitation at 295 nm at 77 K, the EuIII luminescence decay curve at 614 nm  could be well fitted by a bi‐exponential function with the lifetimes of 826 μs (93.1 %) and 2.1 ms (6.9 %). The long lifetime of 5D0(EuIII) was well matching with the short lifetime of T1(fod) (2.1 ms), suggesting the existence of EET from T1(fod) to the EuIII luminescence states upon the excitation of fod, while the short lifetime of 5D0(EuIII) was the characteristic decay constant for the deactivation process of EuIII in [Eu(fod)3dpbt]. Moreover, the shape of the phosphorescence spectrum at 500 nm changed with delay time increasing, which may be induced by the presence of phosphorescence of the long‐lifetime T1(fod) and T1(dpbt).The phosphorescence spectra of dpbt and fod in  was excited at 416 nm →S0(dpbt) transition at 77 K exhibited a broad band with a maximum wavelength of 521 nm (19194 cm−1), and the decay curve of the phosphorescence intensity could be well fitted with a single exponential function with a time constant or lifetime of 3.3 s . The narrow emission bands at 591, 614, 650, 695 nm could be attributed to the EuIII luminescence signals. The phosphorescence signal of the ligand fod (at 500 nm) could not be observed in the phosphorescence spectrum of [Eu(fod)3dpbt] upon the excitation at 416 nm, which was in consistent with the fact that the energy level of T1(fod) was higher than that of T1(dpbt) by 806 cm−1, indicating that the energy migration process T1(dpbt)→T1(fod) hardly happened in [Eu(fod)3dpbt].When dpbt in , and any energy transfer process started from T1(dpbt) was inefficient within 9 ms.During the period from 0.5 ms to 9 ms after the photoexcitation, the luminescent intensity of 5D0(EuIII)→7F2 (EuIII) was observed in the phosphorescence spectrum of [Eu(fod)3dpbt] measured 9 ms after the pulse excitation of dpbt at 416 nm. Since the EuIII luminescence at 614 nm derived from the singlet‐pathway sensitization has a lifetime of 963 μs as measured at 77 K , its intensity would decay at least by one ten thousandth after 9 ms. The EuIII luminescence at 614 nm derived from the triplet‐pathway sensitization should have an apparent lifetime of 3.3 s (the same as that of T1(dpbt)) at 77 K, and its intensity would not decrease obviously within 9 ms. Therefore, the contribution to EuIII luminescence from the triplet energy transfer pathway could be roughly estimated to be less than 2 % (see supporting information), implying that the population of T1(dpbt) state is inefficient for loosening the forbidden rule of f‐f transition of EuIII in [Eu(fod)3dpbt]. It should be mentioned that, compared with the case at room temperature, the much longer lifetime of T1(dpbt) at 77 K enhanced the contribution of triplet EET pathway to the sensitization luminescence of EuIII.A weak emission band at 614 nm of 3dpbt] was excited at 298 K, the fluorescence of dpbt at 430 nm appeared clearly 3dpbt] also existed upon the excitation of fod and excitation energy migration. Such a unique sensitization manner and the excellent luminescent properties make [Eu(fod)3dpbt] to be a promising luminescence dye in the analysis based on photoinduced luminescence.Furthermore, it was found that when fod in  were investigated. The obtained data including the well matched fluorescence lifetime of dpbt at 430 nm and the luminescence rising time constant of 5D1(EuIII)→7F3(EuIII) transition at 585 nm, the relationships among the lifetimes of T1(dpbt), T1(fod), 5D0(EuIII), as well as the phosphorescence spectra measured with different delay times clearly revealed that the singlet EET pathway dominate the sensitization luminescence of EuIII contribution of more than 98 % at 77 K upon the excitation of dpbt. It was found that the energy migration from T1(dpbt) to T1(fod) hardly happened in [Eu(fod)3dpbt] under excitation of dpbt, while upon the excitation of fod, there was a singlet EET process for the sensitization of EuIII luminescence including the energy migration from S1(fod) to S1(dpbt), in addition to the triplet EET pathway.In conclusion, the energy migration processes between the different ligands and the EET pathways for Eu3dpbt] complex were synthesized according to the previously reported methods.3dpbt], a solution of Eu(fod)3⋅3H2O  in benzene (10 mL) was added to a solution of dpbt  in benzene (10 mL) under argon, and the mixture was stirred for 3 h to give a yellow solution with bright red luminescence in daylight. 1H NMR : δ=30.553 , 13.073 , 7.557 =9.0 Hz, 2H; Ph−H), 6.059 =8.9 Hz, 2H; Ph−H), 4.673 , 2.687 =7.4 Hz, 4H; NCH2CH3), 0.636 =7.0 Hz, 6H; NCH2CH3) 0.153 , −1.567  ppm. EI MS: m/z: 1157.3 ([M‐fod]+). Elemental analysis (EuC53H58N8F21O6): found C 43.55, H 3.97, N 7.70; calc. C 43.78, H 4.02, N 7.71.The dpbt ligand and the  were measured on an Hamamatsu Photonics C9920‐02 absolute quantum yield spectrometer, which is more accurate than the quantum yields measured by relative method. UV‐Vis absorption spectrum was recorded on a UV3600 PLUS spectrophotometer in a 1 cm quartz cell. The luminescence lifetimes at microsecond and second timescale were measured on a Horiba Jobinyvon Deltaflex UltraFast lifetime Spectrofluorometer.Absolute quantum yields of [Eu(fod)III luminescence at 585 nm and the fluorescence of dpbt at 430 nm in [Eu(fod)3dpbt] upon the excitation at 400 nm were recorded with a high‐resolution streak camera system (Hamamatsu C10910). We used an amplified mode‐lock Ti: Sapphire femtosecond laser system  to generate the pump beam with a repetition rate of 1 KHz. The samples were excited by the 400 nm laser, which was generated by the 800 nm pulsed laser passing through a frequency‐doubling BBO crystal with an incident angle of ca. 45°.The kinetics curves of the EuThe luminescence lifetime of μs timescale Figure b was meaThe time evolution profiles at selected wavelengths were fitted to single or multiple exponential decay functions to derive the decay time constants or lifetimes. The kinetics data was fitted by mathematical functions with Origin 8.5 and Matlab 5.2.The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.


MoS2 and MoB2 monolayers. The aim of this study is to assess the influence of MoB2 on electron transport of adjacent MoS2 layer. In present investigation we predict that the electronic properties of MoS2 monolayer is influenced by 4d-states of Mo in MoB2 monolayer. Whereas, the B atoms of MoB2 and S atoms of MoS2 exhibit overlapping of intermediate atomic orbitals thereby collectively construct the interfacial electronic structure observed to be metallic in nature. From charge density calculations, we have also determine that the charge transfer is taking place at the interface via B-2p and S-3p states. The bonds at the interface are found to be metallic which is also confirmed by adsorption analysis. Thermoelectric performance of this HS is found be in good agreement with available literature. Low Seebeck coefficient and high electrical conductivity further confirms the existence of metallic state of the HS.We have performed the density functional theory calculations on heterostructure (HS) of  MoS2) has obtained significant attention over the last years as potential candidates for electronic4 and optoelectronic applications7. These materials consist of stacked quasi two dimensional atomic layers that are physically independent to a large extent and can be separated easily. The combination of two-dimensional (2D) dichalcogenides with other 2D or low dimensional materials, such as graphene8 boron nitride or other dichalcogenides have interesting applications9. In principle it provides a completely new class of artificial materials with designed electronic, optical and mechanical properties11. Tuning of electronic properties of 2D transition metal dichalcogenide (TMD) materials are important for their application in optoelectronics. Monolayer TMDs with formula MX2  describes the class of semiconductors with narrow direct band gap, large exciton binding energies, high photoelectrochemical activity and high optoconductivity. Such materials, due to inversion symmetry breaking are used for the study of valley polarization and spin-vally coupling13. Heterostructures (HSs) are used on a large scale in conventional semiconductors for achieving tunable electronic properties. For the development of future 2D materials, the HSs with Van der Waals interactions have been recognized as promising candidates14 and TMD-based hybrid multilayered structures are a prototype van der Waals HSs15. It has been reported that the protected phosphorene and tunable carrier dynamics and optical properties can be achieved by coupling of phosphorene with stable graphene16 or hBN17 and even phosphorene suboxide18 and organism TiL419. According to a theoretical study, phosphorene/graphene HS has been proposed as an anode material for rechargeable Li batteries20. The phosphorene/MoS2 HS is a suitable type-II semiconductor for ultraviolet photodetector applications21. Moreover, the black-phosphorus/blue-phosphorus HS is predicted to be used in solar energy conversion23. Rapid transfer of photogenerated charge carriers between MoSe2 and graphene has been found in MoSe2/graphene HSs depicting its applications in optoelectronics24; black phosphorene/MoSe2 HS show potential applications in p-n diodes and logical devices26. Insulating perovskite substrate SrTiO3 has also been coupled with MoS2 monolayer that shows direct semiconducting band gap and chemisorption of MoS2 monolayer has been observed27. Cathodoluminescence and photon emission excited by a high-energy electron beam can be applied in the analysis of mineral compositions28, light emitting diodes30 and surface plasmon mapping31. Compared to photoluminescence the cathodoluminescence offers a much higher excitation energy allowing the study of wide band gap materials including diamond32 and hexagonal boron nitride (hBN)33. The thermoelectric (TE) properties in few layer and bulk MoS2 and MoS2 monolayer have been theoretically investigated and found to be reasonably good for TE applications37. TE power factor of 8.5 mWm−1 K−2 for MoS2 monolayer at room temperature had been reported by Kedar et al. which is the highest among all TE materials and twice that of commercially used bismuth telluride (Bi2Te3)38. Whereas, for MoS2 monolayer deposited on substrates, Hasan et al. had reported a poor response in the TE power factor39. For improving the TE performance of MoS2 based systems, the hybridization and doping have been commonly explored. Moreover SiGe alloys, hybrid BN/graphene and MoS2/WS2 nanoribbons show higher thermoelectric properties than single nanostructures41. We reviewed that the investigation for HSs of MoS2 with intermetallic material need to be explored. The transition metal diborides XB2  having hexagonal AlB2 structure received much attention because for their interesting physical and chemical properties such as electronic structure, high melting point, corrosion resistance, wear resistance, high hardness factor and possibility of extensive industrial applications46. Recently, ReB2 and OsB2 has been analyzed for exhibiting high bulk elastic moduli in a particular direction(c-axis) owing to the high valence electron density in the lattice which is comparable to that of diamond48. Recently it is been reported that lattice mismatches in the cell parameters and comparable thermal expansion with GaN, ZrB2 can be optimized as a substrate for hetero epitaxial growth of GaN50. For the reinforcements in various composite materials e.g. steel and TiB2 has often been used52. With reference to the above discussion we have motivated to theoretically examine the electronic and thermoelectric properties of MoS2/MoB2 HS. We have selected this combination of monolayers due to their interesting electronic properties, specifically the Boron terminated MoB2 layer with a wide range of thermodynamically allowed chemical potentials53. In Section I, we have elaborated the computational details applied to probe several significant features of interfacial electronic structure of MoS2/MoB2. The results for electronic structure of HS and isolated sub-systems along with chemisorption, chemical bonding and thermoelectric properties occurring at the interface near to the Fermi level (Ef) are reported and discussed in Section II.The family of layered dichalcogenide semiconductor materials such as molybdenum disulfide 56. We sampled the Brillouin zone (BZ) in the Monkhorst-Pack scheme57, and tested the convergence in energy as a function of number of k-points for the calculations. The k-point sampling of (7 × 7 × 1) was found to be suitable for the BZ corresponding to the primitive unit cell. Atomic positions were optimized using conjugate gradient method, where total energy and atomic forces were minimized. The energy convergence value between two consecutive steps was chosen as 104 eV. The energy interval chosen for density of states (DOS) calculations is 0.1 eV and the broadening used in Gaussian type. The standard value of broadening is considered as 0.001 Ry. An equivalent plane wave cutoff of 750 eV is chosen in all the simulations. Relaxed geometries are obtained with the conjugate gradient method, where all the atoms in the super cell are allowed to relax until the force on each atom is less than 0.02 eV/Å. We modeled the MoS2/MoB2 HS by putting a 3 × 3 × 1 super cell of MoS2 monolayer (lattice constant Å)58 on top of a 3 × 3 × 1 super cell of MoB2 monolayer (Å)53, which reduced the lattice mismatch between the two layers to 4.4% and resulted in a simulation cell containing 32 atoms. This lattice mismatch is small enough that will not effect the electronic properties of the HS, however such contraction in lattice spacings may results in increased DOS16. To minimize interactions between periodic images due to 3D boundary conditions, we introduced a vacuum layer such that the distance between periodic images was at least 25 Å. We have modeled the interaction of the valence electrons with the pseudo atomic cores of all the atomic species present in our studied structures by normconserving pseudopotentials explicitly including the semi-core Mo 4d electrons in the calculations. The equilibrium interfacial distance (deq) between MoS2 and surface of MoB2 monolayers is found to be 1.96 Å which is obtained from a fully relaxed HS. To analyze the thermoelectric properties of HS, semi-classical theory of the Boltzmann package59 has been used.High-throughput density functional theory (DFT)MoS2/MoB2 HS shown in Fig. MoB2 is a non-layered structure with metallic bonding53 unlike MoS2 which possess a layered structure with interlayer Van der Waals interaction. The metallic bonding of MoB2 is possibly the reason for the absence of Van der Waals interaction at the interface of the HS. The electronic band structure of HS is shown in Fig. EF showing the existence of metallic nature. The maximum dispersion of electronic bands is observed at high symmetry point M. In order to further elucidate the band structure we have studied the partial density of states (PDoS) of the HS. Figure MoB2 (left panel) and similarly Mo and (top/bottom) S atoms in MoS2 (right panel) along with total density of states (DoS) at the bottom. In Fig. MoB2 (left) is shown, where 4d states of Mo atoms are found to cross the Ef. The PDoS of B atoms (below Mo) represents the 2p states which are also playing important role in the metallic nature of the HS. Whereas, PDoS of MoS2 monolayer is represented in Fig. MoS2 monolayer, 4d states of Mo are observed to cross over the Ef. However, the relative intensity of 4d states of Mo in MoS2 monolayer (0.35 states/eV) is lower than that of 4d states of Mo in MoB2 monolayer (0.47 states/eV) at Ef. 3p states of S atoms (below Mo) are also contributing in the metallic state of HS. The relative intensity of 3p states of S atoms in MoS2 monolayer (0.11 states/eV) is lower than the 2p states of B atoms in MoB2 monolayer (0.18 states/eV). From the above results obtained from Fig. MoS2/MoB2 HS is attributed to Mo-4d and B-2p states of MoB2 monolayer. MoB2 is not only making the HS a metallic system but it has also modulated the MoS2 monolayer to behave as a metallic system. The valance band (VB) in the range from −0.8 to −2 eV, is mainly composed of B-2p and Mo-4d (of MoB2) states. Whereas, in the conduction band (CB), the energy range from 0.7 to 2 eV is mainly composed of Mo-4d states (of MoS2 and MoB2) and S-3p states. From energy range −0.7 to 0.5 eV, dominance of Mo-4d and B-2p states can be clearly seen in Fig. MoS2 are enlisted in Table MoS2 and MoB2 are also studied to get a clear insight of the mechanism taking place within the HS and at its interface.The lattice arrangement of the MoS2 and MoB2 monolayers) of the HS are also studied separately. The PDoS of MoS2 monolayer in same lattice arrangement of HS, in absence of MoB2 monolayer is shown in Fig. MoB2 monolayer is not present, MoS2 is giving an energy gap of 1.8 eV as expected from it60. 4d states of Mo and 3p states of S atoms are dominantly found in the CB near Ef level. The relative intensity of peaks of 4d states of Mo atoms (3.8 states/eV) are higher than that of 3p states of S atoms (1.6 states/eV). The PDoS of lower S atoms and top S atoms are identical. In the VB, there is a vacant space upto −1.7 eV which shows that the electronic states of MoS2 monolayer are present in the VB in lower energy region ≤−1.7 eV. On observing Fig. MoB2 has strongly influenced the electronic states of MoS2 monolayer when coupled together in the HS MoS2/MoB2 making it a metal. The PDoS of MoB2 monolayer when MoS2 monolayer is removed from the HS, is shown in Fig. Ef level making it a metallic material. Below Mo, PDoS of bottom and top B atoms are shown. We found in Fig. Ef as compared to the top B atoms, which concludes that 2p states bottom B atoms are more actively participating with 4d states of Mo in making the system MoB2 a metal. Our findings from Fig. MoS2 and MoB2 monolayers are metallic bonds.To get a clear insight of electron transport, sub-systems  and adsorbate (MoS2) and hence, new chemical bonds are generated at the interface along with consequential alterations in the electronic structure of adsorbate. On the other hand, in case of physisorption, the electronic properties of adsorbate remains unaltered upon adsorption. The bonds can be seen at the interface between adsorbent (MoB2) and adsorbate (MoS2) in Fig. MoB2 driven modulation in the electronic structure of MoS2 motivated us to investigate the nature of adsorption in MoS2/MoB2 HS system. In general, layered MoS2 exhibits honeycomb like structure at the interface with Van der Waals like weak interactions. In our case we observed the existence of some ionic type bonding between B-2p electronic states of MoB2 and S-3p electronic states of MoS2 monolayers. Therefore it is considered that these interfacial interactions may not be the Van der Waals interactions27. In order to analyze the nature of adsorption at MoS2/MoB2 interface with equilibrium interfacial distance (deq = 1.96 Å), we have randomly shifted MoS2 monolayer upward upto 4.76 Å and then downward upto 0.56 Å normal to the plane of MoS2/MoB2 HS. The result obtained from adsorption curve in terms of change in potential energy with respect to interfacial distance is shown in Fig. MoS2 monolayer over MoB2 is provided in Fig. deq is set to 0 Å which indicates the minima of potential energy curve in Fig. MoS2 and MoB2 is ~4.76 Å which is then reduced to a minimum value ~0.56 Å. In Fig. deq, the chemisorption occurs in the region from A (maxima at 2.1 Å) to C (minima at deq = 1.96 Å), where chemical bonds possibly exist at the interface of MoS2/MoB2 HS. The potential energy curve attains the minimum potential energy value (at C) by change in slope via point B. The rate of change of potential energy with respect to the interfacial distance between A and B i.e. ΔAB is greater than that between B and C (ΔBC). This variation in slope is attributed to initial electronic repulsion. In other words the anomaly at B in the potential energy curve appears due to electronic repulsion between MoS2 and MoB2 layers. Tending from B towards C the actual orbital overlapping between the atomic species of corresponding layers can be realized. Conclusively, at C in the potential energy curve the valley-like feature at deq indicates a clear existence of chemical bonding at the interface. If we further continue to decrease this distance, the energy will tend to infinity under the effect of nuclear repulsion. We can see from Fig. MoB2 surface) and bottom S atoms (of MoS2). This indicates the presence of strong bonding with metallic nature at the interface. Hybridization in Mo-4d states and B-2p states in the MoB2 monolayer is also observed. In MoS2 monolayer, Mo atoms weakly participate in charge transfer. These results are in good agreement with those observed for PDoS  making bonds with S atoms as well as B-B bonds.In the system of present investigation, we have performed the adsorption analysis of S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity and κ (κ = κe + κl) is the thermal conductivity which consist electronic κe as well as lattice κl thermal conductivity and T is absolute temperature respectively. Previous studies signified that the mono-layered MoS2 is semiconductor in nature with band gap 1.8 eV60. Due to high S and low κ, MoS2 system presents a good candidature for the thermoelectric applications. However, small ZT is reported for this system due to low electrical conductivity induced by the large band gap energy. The mono-layered MoB2 have metallic nature as represented in PDoS  from −1.5 eV to 1.5 eV at temperatures 300 K and 800 K for MoS2/MoB2 HS show two peaks in the profile  due to metallic nature of MoS2/MoB2 HS. The MoS2 monolayer is a semiconductor that possess large band gap therefore its Seebeck coefficient must be larger than that of purely metallic MoB2 which is shown in inset of Fig. MoS2 monolayer is modulated by MoB2. It can also be observed from PDoS of HS  as a function of chemical potential  as a function of chemical potential (μ) from −2 eV to 2 eV at the temperatures 300 K and 800 K for MoS2/MoB2 HS is shown in Fig. 19 (S/ms) and 3.423 × 1019 (S/ms), whereas for positive chemical potential it is 5.21 × 1019 (S/ms) and 4.588 × 1019 (S/ms) for 300 K and 800 K respectively. This indicates that the p-type composition possess higher electrical conductivity than n-type. Comparable phenomena had also been observed in MoS2 monolayer41. Further, the electrical conductivity (σ/τ) as a function of temperature for a certain value of chemical potential (μ) is shown in Fig. MoS2 monolayer have been provided in the inset of Fig. MoS2/MoB2 HS. However, the electrical conductivity of MoS2 monolayer found lesser as compared to the investigated HS.The variation of electrical conductivity  is a combination of electronic thermal conductivity κe and phononic thermal conductivity κl. In the present study, we have used the BoltzTrap code59 which calculates electronic contribution only. The consideration of lattice thermal conductivity κl, remains as future task. The calculated electronic thermal conductivity (κe/τ) of MoS2/MoB2 HS as a function of chemical potential (μ) at 300 K and 800 K is shown in Fig. κe/τ occurs with increasing temperature. The highest value of κe/τ induced by 800 K while the lowest κe/τ induced by 300 K. Therefore, 300 K is the optimal temperature that gives the lowest thermal conductivity. Moreover, the variation in thermal conductivity with respect to temperature is represented in Fig. MoS2/MoB2 HS which is in good agreement with the previous study62. However, the thermal conductivity of MoS2 is found less as compared to that of MoS2/MoB2 HS as shown in the inset of Fig. MoS2 monolayer63.Further, we have also focused on the thermal conductivity of μ) at 300 K and 800 K illustrated in Fig. μ = −0.99 eV attributed to significant increment in the electrical conductivity at high electron concentration level. Further, Fig. MoS2 monolayer is shown in the inset of Fig. MoS2/MoB2 HS exhibits comparatively better power factor value than MoS2 monolayer. The large magnitude of power factor is obtained in the case of large electrical conductivity. Consequently, it is clear that the heterostructure of MoS2/MoB2 HS exhibit good thermoelectric response at the higher temperatures.The average power factor is plotted against the chemical potential (MoS2 monolayer over single layer of MoB2 in a bilayer MoS2/MoB2 heterostructure can have noticeable effects on the electronic properties of the MoS2 layer. In presence of MoB2 layer in the HS, monolayer of MoS2 becomes a metallic system. Under the influence of 4d and 2p states of Mo and B atoms respectively of MoB2 layer, 4d and 3p states of Mo and S atoms of MoS2 monolayer appeared to cross the Ef. While in absence of MoB2 layer, MoS2 monolayer shows its ideal electronic structure. Hence the metallic nature of the HS is driven by MoB2 layer. We also observed some bonds at the interface which were analyzed via charge density calculation and adsorption curve and found to be metallic in nature. Based on the calculated Seebeck effect and power factor of MoS2/MoB2 HS as a function of chemical potential and temperature, the maximum power factor is estimated successfully which can offer useful guidelines for tuning and improving the thermoelectric performance of such type of HS.In conclusion, our study suggests that deposition of


Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high‐resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences.The doubly substituted isotopologues  using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues.O35Cl for 17O18O and H35Cl and 36Ar for 18O18O, are sufficiently resolved to enable high‐precision determination of Δ35 and Δ36. The Δ35 and Δ36 values of O2 after photochemical isotope equilibration at −63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ35 and Δ36 values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O2 differ from those expected at equilibrium.Interferences from isobars, mainly 2 at medium mass resolution. The Δ35 and Δ36 signatures in atmospheric O2 are mainly governed by O3 photochemistry, temperature and atmospheric transport. Tropospheric O2 is isotopically well mixed and retains a significant stratospheric signature.The 253 Ultra allows interference‐free clumped isotope measurements of O The atmospheric O2 reservoir exchanges with the biosphere on a timescale of roughly 1200 years.2  have been studied extensively. In nature, these ratios vary primarily in response to biological oxygen cycling, although the influence of stratospheric photochemistry has also been observed.18O18O, 17O18O and potentially also 17O17O, also called clumped isotopes, can be useful to independently constrain the cycling of oxygen in the Earth's atmosphere.Molecular oxygen O is an im2, expressed as Δ35 and Δ36 (see definitions below), are quantified as deviations of the abundances of 17O18O and 18O18O, respectively, from the abundances that are expected stochastically from the bulk isotopic composition. Under thermodynamic equilibrium conditions, the magnitude of the deviations purely depends on the temperature at which isotopic equilibration of O2 takes place. The main challenges to measuring clumped isotope signatures in atmospheric O2 are their low abundances (1.6 ppm for 17O18O and 4 ppm for 18O18O) and the isobaric interferences, for example, the interference of 36Ar on 18O18O. Using careful separation and appropriate corrections for this interference, clumped isotope compositions in O2 were first measured with low‐resolution mass spectrometers.18O18O from 36Ar is possible. Here we demonstrate that the 253 Ultra high‐resolution stable isotope ratio mass spectrometer  can measure the Δ35 and Δ36 values along with the traditional isotope ratios  at a precision close to the counting statistics limit. We show that traces of 36Ar and other interferences can be well separated from 17O18O and 18O18O at medium mass resolution (~10000).Clumped isotope signatures in O35 and Δ36 values are mainly governed by O2 + O(3P) photochemistry in the atmosphere.35 and Δ36 in the Earth's atmosphere than expected from a stochastic distribution of isotopes led Yeung et al3P) + O2 isotope exchange reactions reorder the isotopes in O2 toward an isotopic equilibrium that depends on the ambient temperature. At lower temperatures, the 17O18O and 18O18O clumping should be relatively high, i.e., higher values of Δ36 and Δ35, and vice versa.2 should vary throughout the atmosphere because of variations in temperature. Yeung et al2, above 22 km, are indeed in thermodynamic equilibrium at the low ambient stratospheric temperatures. In contrast, there was a clear deviation from isotopic equilibrium at the higher temperatures in the troposphere. This was explained in terms of the time scales of isotopic re‐equilibration, which are long in the troposphere because of a low abundance of O atoms and short in the stratosphere because of much higher O atom levels there. Thus, in different regions of the atmosphere the isotopic re‐equilibration can be faster or slower than the transport timescales. Therefore, the atmospheric Δ35 and Δ36 values reflect a dynamic balance between isotope exchange (temperature and oxidant dependent) and transport. Interestingly, the biological recycling of O2, which is of primary importance for its bulk isotopic composition, has a negligible effect on the clumped isotopic composition because of the slow resetting time scale compared with photochemical isotope exchange.The first clumped isotope studies showed that Δ2 using the 253 Ultra mass spectrometer. After establishing and validating the purification and the mass spectrometric methodology, an empirical transfer function was developed to convert the measured isotope compositions into the absolute reference frame (ARF) by isotopically equilibrating O2 at low and high temperatures. We also report Δ35 and Δ36 values measured in stratospheric and tropospheric air O2 samples. In addition, we discuss the challenges and possibility of measuring the rarest isotopologue, 17O17O, of O2 using the 253 Ultra.Here we report a method for measuring clumped and bulk stable isotope ratios in atmospheric O22.117O = (17Rsam/17Rstd – 1) and δ18O = (18Rsam/18Rstd – 1), where 17R = 17O/16O, 18R = 18O/16O, and subscripts ‘sam’ and ‘std’ stand for sample and standard, respectively. The δ values are expressed in per mill (‰) or parts per thousand. Values of δ17O and δ18O are reported with respect to Vienna‐Standard Mean Ocean Water (V‐SMOW).The conventional singly substituted isotopic composition of a gas is characterized by the delta value, i.e., δ2, also called clumped isotopes, are 18O18O, 17O18O and 17O17O. Their excess abundances compared with a stochastic distribution of the three oxygen isotopes in O2  are expressed using the relationship i refers to mass 36, 35 and 34  and The doubly substituted isotopologues of Oi values are also reported in ‰. For a stochastic distribution of isotopes, Δi = 0. A nonzero Δi value for a sample represents an excess or deficit of multiply substituted isotopes  compared with what is expected based on their bulk isotopic composition. We calculate the clumped isotope ratios following the previously adopted established conventions and data reduction procedures for O2 (Δ35 and Δ36) and CO2 (Δ47)Δ2.235 and Δ36) were carried out with the 253 Ultra isotope ratio mass spectrometer at Utrecht University, which is one of the first instruments produced by Thermo Fisher Scientific on the basis of the prototype described in Eiler et al.8 Ω to 1013 Ω. Operation of the instrument is controlled by the Qtegra software package (Thermo Fisher Scientific).Isotopic measurements including the clumped isotopes  are measured in Faraday cups with resistors 1010 Ω, 1012 Ω and 1011 Ω, respectively, while the minor ion beams  are measured using two CDDs.In the present work, the medium‐resolution slit with a mass resolving power of ~10 000 (M/ΔM) for O−7 mb. Under these conditions, in medium‐resolution mode the ion signal is 1 × 1010 counts per second (cps) at mass 32; the corresponding signals for masses 35 and 36 are ~20 × 103 and ~50 × 103 cps, respectively. A sample is measured for 30 to 60 sample‐reference cycles, divided into 5 to 10 acquisitions, each with 6 cycles. Peak centering, background scanning (60 s) and pressure‐balancing (tolerance 0.3%) are performed before each acquisition. The equilibration time (idle time) and integration time are set as 60 s and 67 s, respectively. Measurement of a sample takes ~7 h which consists of an actual measurement time of ~4.5 h, roughly half of which is required for equilibration after valve switching and the other half for signal acquisitions of the sample and the working gas. The remaining ~2.5 h is used for background scanning, reference and sample signal balancing, peak centering and peak positioning .Isotope ratio measurements are generally made at a source pressure of about 2.0 × 102.32 at different temperatures in order to express our measured clumped isotope ratios against a scrambled reference. At high temperatures (850°C), isotopic equilibration of O2 is carried out in the presence of a platinum catalyst  in a quartz tube of length 42 cm and diameter 1.5 cm. Heating is carried out by inserting ~30 cm of the tube into a furnace set at 850°C. After 2–3 h of heating, the tube is directly connected to one of the bellows of the dual‐inlet system of the 253 Ultra to transfer hot O2 for measurement.We isotopically equilibrate aliquots of O2 isotopically as it does not readily exchange isotopes. Therefore, UV‐induced photochemical isotope exchange2 gases at lower temperatures. UV photolysis experiments are carried out in a spherical ~500‐mL volume Pyrex reactor (diameter 10 cm) fitted with a Suprasil finger  at the center. The Suprasil finger transmits UV radiation (Pyrex is opaque to UV), and it is connected to the reactor using a KF clamp with a Teflon O ring. A Hg lamp  is introduced into the Suprasil finger as the UV source (major wavelengths: 184.9 and 253.6 nm).At low temperatures, it is not straightforward to equilibrate O−5 mbar and then pure O2 working gas  is introduced into the vessel to a pressure of 40–50 mbar. The flask is immersed in an ethanol or water bath and cooled to various temperatures between +26°C and −78°C. While the outside of the reactor is at the temperature of the cooling fluid, determination of the temperature of the gas inside the reactor is challenging, because the temperature of the Suprasil finger with the glowing UV lamp is much higher than the outside temperature of the reactor. For example, when the outer surface is at dry‐ice temperature (−77.8°C), the temperature inside the Suprasil finger is found to be 10 ± 5°C at steady state. The effective temperature of the gas inside the flask is calculated from the change in pressure using Gay‐Lussac's pressure law P1/T1 = P2/T2, where P1 is the pressure inside the chamber at room temperature (T1), i.e. before putting the chamber inside the bath and without UV lamp, and P2 and T2 are the pressure and temperature with the chamber inside the dry‐ice bath and the glowing UV lamp inside the finger after stabilization. For the lowest bath temperature of −77.8°C the effective gas temperature is −63 ± 5°C. This effective temperature is then used to assign the expected isotopic ordering of O2 inside the flask based on the theoretical calculations of Wang et al.2 samples are separated from the O3 formed during the UV treatment by passing the gas through a spiral tube immersed in liquid nitrogen. It is observed that 3–4% of the O2 inside the chamber is converted into O3 at steady state. Therefore, immediate separation of O3 is necessary to prevent further O2 formation from decomposition of O3, which would probably change the isotopic signatures. The isotopically equilibrated O2 samples are measured without any further purification.The reactor is connected to a gas mixing system2) with nominal δ17O and δ18O values of 9.254‰ and 18.542‰, respectively, versus V‐SMOW, assigned by E. Barkan at the Institute of Earth Sciences, Hebrew University of Jerusalem .All samples are measured against a working gas (IMAU O2.42 is separated from air using a gas chromatography (GC) column . Air is filled into a sample loop of 10 mL volume  filled with silica gel. O2 is released from the U traps by removing the liquid nitrogen and then transferred to a glass ampule of ~2 mL volume filled with a few pellets of preconditioned silica gel. O2 from the ampule is directly introduced into the bellow of the mass spectrometer to measure its isotopic composition. After each sample extraction, the GC column is baked at 200°C for 1 h at a He flow rate of 25 mL/min and longer baking (overnight) is performed at the end of the day. The two silica gel U traps for sample collection are also baked at 200°C for 1 h under high vacuum (1*10−5 mb) after each sample collection.O2.52 during sample storage, purification (e.g. GC column) and analysis in the mass spectrometer.2 (18O2 ~700‰) in a 2‐L volume canister by diluting 97.2 atom % 18O2  with our IMAU working gas O2. The enriched O2 was then measured before and after passing through the GC column to quantify the isotopic reordering in the GC column and the source of the mass spectrometer.There is a possibility of isotopic reordering in O2.62, and to check the reproducibility of measurements, we have used ambient air compressed to ~140 bar in a ~40‐L cylinder. This air was collected outside the Center for Isotope Research on the campus of Groningen University , in August 2017. Air is compressed using a Rix oil‐free air and gas compressor . O2 is extracted from the compressed air cylinder as described in section To determine the clumped isotope composition in lower tropospheric air O2 was extracted as described in section 35 and Δ36 values were measured on purified O2 in the 253 Ultra as discussed in section Stratospheric and upper tropospheric air samples were obtained with a whole air sampler operated on the high‐altitude aircraft M55 GEOPHYSICA on two flights carried out within the project StratoClim (stratospheric and upper tropospheric processes for better climate predictions). StratoClim is a collaborative research project under the European Commission's 7th Framework program (H2020) to improve the understanding of the key processes in the Upper Troposphere and Stratosphere with the goal of producing more reliable climate projections. We analyzed 13 samples collected on 1 and 6 September, 2016 during the StratoClim campaign over the Mediterranean  at heights between 10 000 and 20 000 m and covering latitudes between 33 and 41°N and longitudes between 22 and 29°E. The temperature was −58°C near the highest Geophysica sampling altitudes (~ 20 km) and −42°C at the lowest sampling altitudes (~10 km). Stainless steel canisters of ~2 L volume were filled and compressed to ~4 bar with ambient air at different heights. O2O mole fractions are measured together with the N2O isotopic composition in a Delta Plus XP continuous flow stable isotope ratio mass spectrometer (Thermo Fisher Scientific) following the method developed by Röckmann et al.2, and the N2O (together with remaining traces of CO2 and other condensable species) is preconcentrated cryogenically. The cryo‐focused sample is purified on a capillary GC column  and N2O is introduced into the mass spectrometer via an open split interface. From the area of the peaks, the concentration of N2O is calculated.The N33.13.1.133 values, −0.004 to 0.047‰ for δ34 values, −0.176 to 0.112‰ for δ35 values and 0.008 to 0.132‰ for δ36 values  is the limiting factor here. A slightly positive value is observed for δ36, but it is within the error associated with the individual measurements. Therefore, a zero‐enrichment correction is not applied for the sample measurements.Table 35 and δ36 values are compared with the errors expected from counting statistics and found to be similar  on 18O18O (mass 35.9983 u) is demonstrated using an Ar‐O2 mixture gas that is prepared by mixing 0.16% pure Ar with our pure O2 working gas. The mass difference between these two isobars is 0.0308 u and the resolving power required to separate their peaks is ~1160, which is easily achieved with the 253 Ultra at medium mass resolution. Figure 2 and the Ar‐O2 mixture. The O2 isotopologue peaks are about 0.03 u wide and they are in general very stable in terms of signal intensity, width of the plateau and collector positions. In the Ar‐O2 mixture, the 36Ar peak appears about 0.03 u before the 18O18O. There is a region of 0.009 u where the peaks overlap. The tilted peak for 36Ar is probably due to inaccurate adjustment of the CDD operating voltage. With further tuning of the operating voltage we later also achieved a flat peak for 36Ar (not shown here).It is difficult to achieve full separation of Ar from O36Ar, other interferences can be identified by the mass differences relative to the O2 isotopologue peaks. Chlorinated species are particularly important, especially in new mass spectrometers, as chlorinated solvents are used to degrease the parts, and we have regularly observed increases in these interferences after heating the ion source. 35Cl (mass 34.9688 u) causes the apparent increase in the background  on 18O18O is visible on the left side of the 18O18O peak in the case of pure O2  is much higher than that required to resolve most of these isobars . We note that the background for mass 33 is ~1.7% of the maximum peak height , the background signal should return to zero after the width of an interfering peak of 0.03 u, as the width of any species falling on any cup other than the fixed Centre cup is ~0.03 u , no significant 36Ar interference is observed (Table 18O18O can be easily resolved from 36Ar and other interferences even at medium resolution (resolving power: ~10 000 in this case). Therefore, interference‐free measurements of the clumped isotopic composition of O2 are possible even if the samples are not fully free from Ar. All the measurements presented here are made near the right edge of the peak plateauxs  is used  is 1.6% when He‐O2 is passed through the column cooled to −75°C and 1.7% at −80°C. For our measurements of atmospheric O2 with a typical Δ36 value of 2.4‰, 1.7% isotopic reordering towards a scrambled isotopic distribution would induce a systematic error of ~0.04‰ (assuming the same relative reordering of 1.7%), which is below our reported uncertainty. The observed values for isotopic reordering are similar to the 1.1% reported by Yeung et alTo investigate reordering in the purification system (in particular the GC column), an aliquot of the isotopically spiked O34 value of the spiked O2 compared with the unspiked O2. This may occur because in the fragmentation‐recombination process, some 18O18O will be converted into 18O16O. The natural abundance of 18O18O is 4.2*10−6; therefore, the spiked gas with Δ36 ~684‰ contains 2*684‰*4.2*10−6 ≈ 5.7*10−6 extra 18O atoms. If 1% of the added 18O18O is converted into 18O16O, this would produce 5.7*10−8 18O16O molecules, which would cause a change in the δ34 value of 0.014‰. The actually observed δ34 difference between the spiked and unspiked O2 is 0.02‰  are plotted against the corresponding values calculated for O2 in thermodynamic isotope equilibrium35 and Δ36 values for O2 into the Absolute Reference Frame (ARF).35 and Δ36 in the present case are Δ35(ARF) = (0.970 ± 0.035), Δ35(meas) + (1.281 ± 0.018) and Δ36(ARF) = (0.974 ± 0.026), Δ36(meas) + (2.429 ± 0.035), respectively  of our working gas (IMAU‐O2).The Δ35 and Δ36 values before and after application of the transfer function are provided in Table 35 and Δ36 values before converting to the absolute scale are the measured values of the samples versus those of the heated gas assuming that the heated gas is isotopically scrambled. No significant difference is observed in the Δ35 and Δ36 values before and after conversion to the ARF  external reproducibility (1σ standard deviation) of the measured values is 0.115‰ for δ17O, 0.215‰ for δ18O, 0.021‰ for Δ17O, 0.101‰ for Δ35 and 0.098‰ for Δ36 . The isotopic ratios of the two gases, viz. IMAU O2 and GEO O2 before and after heating, are presented in Table 2 is our working gas and GEO O2 is a commercial O2 gas supplied by Air Liquide . GEO O2 has very low bulk isotope ratios and high Δ35 and Δ36 values compared with IMAU‐O2, but after isotopic equilibration at 850°C (for 2–3 h) in the presence of platinum, the Δ35 and Δ36 values of the two O2 gases are identical, indicating full isotopic scrambling at this temperature. This also indicates the absence of any dependence of the Δ35 and Δ36 values on the bulk isotopic composition within our measurement precision. We note that heating does affect the δ17O and δ18O values, which is possibly due to some exchange with oxygen in the quartz material of the reactor at high temperature. However, this exchange does not affect the Δ35 and Δ36 values as these signatures are purely driven by exchange temperatures.Clumped isotope measurements with isotope ratio mass spectrometers may exhibit a dependence of the clumped isotope ratio Δ on the bulk isotopic signature δ. A corresponding correction is found to be necessary for most of the CO3.1.717O17O . The biggest challenge is the interference from 16O18O . The required mass resolution to separate these two masses is 8132, so they can in principle be resolved, in the medium‐ or high‐resolution mode. However, the detection is complicated by the very low abundance of 17O17O compared with16O18O. Under the present measurement conditions (at medium resolution) the expected signal strength for 17O17O is 1450 cps when the signal for its isobar 16O18O is 4.11*107 cps, so 17O17O needs to be resolved on the tail of the very large 16O18O peak. When going to high‐resolution mode the signals decrease correspondingly, and the small signal of 17O17O (a few hundred cps) is only observable with a CDD. High‐precision determination of 17O17O will therefore take even longer than that of the more abundant clumped isotopes. The 17O17O isotopologue is expected to carry similar information to the other two clumped isotopologues and its measurement is therefore not yet being targeted rigorously.We also investigated the possibility of measuring the rarest isotopologue 3.235 and Δ36 values along with the conventional isotope ratios measured in the stratospheric and upper tropospheric air O2 are presented in Table 17O and δ18O values show only a small spread, between 11.89 and 11.98‰ for δ17O values and between 23.60 and 23.77‰ for δ18O values  − 0.516*ln(δ18O + 1) is found to be −0.23 ± 0.03‰. Note that the three‐isotope slope (λ) value used here is 0.516. When using the same slope, the data from Luz and Barkan17O value of −0.17, close to our result. Laskar et al17O value of atmospheric O2 to be −0.22 ± 0.02‰ based on indirect measurement of car exhaust CO2, as the source of the O2 in the exhaust's CO2 is atmospheric O2, consumed during the combustion of fuel. The agreement with previous values for the bulk isotopes including Δ17O provides confidence that the extraction of O2 from air is made without strong fractionation.The Δes Table . The δ1735 and Δ36 values measured in stratospheric and upper tropospheric O2 obtained from the GEOPHYSICA air samples. The values lie in the range of 1.1 to 1.6‰ for Δ35 and 2.2 to 3.1‰ for Δ36  + O2 reaction in the stratosphere caused full thermodynamic equilibration above ~22 km, whereas equilibration was not complete below 22 km. For our samples in the lower stratosphere, due to larger errors, it is not possible to decide from the clumped isotope values alone whether the isotopic equilibration is complete. However, the correlation with N2O (see below) indicates that the air in the lower stratosphere still has a partially tropospheric character. This demonstrates that full isotope equilibrium has not yet been achieved in the lower stratosphere.Figure 35 and Δ36 values are lower than the equilibrium values, whereas the lower tropospheric values are higher . The discrepancy in estimating the effective temperature of equilibration of O2 inside the chamber is probably a major reason for the differences in the Δ35 and Δ36 values between the present measurements and those reported by Yeung et al.35 and Δ36 values, however, would be smaller. Another potential point of concern is that the effective temperature, however determined, is an average over very different temperature conditions in the reactor, changing from hot near the illuminated Suprasil finger to cold at the outer surface. Thus, reactor geometry and illumination conditions may also play a role. Although the differences between stratospheric and tropospheric results reported by Yeung et alThe two sets of samples have different geographical origins, but a difference of ~0.4‰ is not expected as the mixing times in the troposphere are fast (weeks within one hemisphere and 1 year for inter‐hemispheric exchange) compared with the isotope reordering time scale of the order of a decade. In addition, the samples from Yeung et al36 in O2 and the N2O mole fractions measured on the same samples. N2O is almost exclusively produced at the Earth's surface from natural and anthropogenic sources and destroyed almost completely in the stratosphere by UV photolysis and reaction with O(1D).2O is a useful tracer for stratospheric photochemistry and is often used as “pseudo vertical coordinate” for stratospheric studies, where lower N2O values indicate higher altitudes. A negative correlation is observed between Δ36 and N2O values for stratospheric samples . The photochemical N2O removal is a slow process with timescales of months to years. The correlation between N2O and Δ36 therefore probably reflects mixing between fresh tropospheric air with higher N2O and lower Δ36 values and older stratospheric air with lower N2O and higher Δ36 values. This also implies that the lower stratospheric air is not in full isotopic equilibrium and retains some signatures of the troposphere, since isotope equilibration would eradicate the tropospheric signal and the correlation with N2O. The correlation is not observed in the troposphere because the troposphere is well mixed without any significant vertical gradient in N2O and Δ36.Figure 42 in atmospheric air. Isobaric interferences of 36Ar and H35Cl for 18O18O and 35Cl for 17O18O are easily resolved at medium mass resolution. Isotopic equilibration experiments using O2 heated to 850°C and photochemical isotope exchange via O(3P) + O2 at lower temperatures are used to convert the measured values into the Absolute Reference Frame for clumped isotope analysis through an empirical transfer function. A critical parameter for this calibration is the effective isotope exchange temperature in the photolysis reactor, and uncertainties in assigning the correct temperature could be the cause of an absolute difference of ~0.4‰ between the data presented here and a previously published dataset.2 is close to thermodynamic equilibrium at the ambient temperature, because of rapid equilibration due to the O(3P) + O2 isotopic exchange reaction. However, the correlation with N2O suggests that isotopic equilibration is not complete in the lower stratosphere. The clumped isotope composition of tropospheric O2 deviates significantly from the thermodynamic equilibrium at the respective local temperatures and there is no significant vertical clumped isotope gradient in the troposphere. This shows that the isotopic resetting time 2 retains part of the isotopic signature acquired in the stratosphere or that tropospheric O2 is isotopically reordered in the upper troposphere at an effective temperature of ~ −40°C. This is being investigated with measurements and model data.We demonstrated that the 253 Ultra high‐resolution stable isotope ratio mass spectrometer allows clumped isotope analysis of O35 and Δ36 values of the equilibrated and heated O2 samples at different temperatures expressed in ‰ against IMAU O2, the working gas. These values are plotted against the corresponding thermodynamically predicted values to construct the empirical transfer functions .Table S1. Δ35 and Δ36 values before (measured) and after conversion to the absolute scale using the empirical transfer function . The measured clumped isotope data are the difference between the Δ35 and Δ36 values of the samples and the heated O2. Tropospheric air from the surface level was collected from Groningen and Utrecht, The Netherlands and stratospheric and upper tropospheric O2 samples were collected using the GEOPHYSICA M55 aircraft (see main text for details and discussion).Table S2. ΔClick here for additional data file.Data S1. Supporting informationClick here for additional data file.