The occurrence and importance of the C4 pathway remains uncertain, however. Here, we demonstrate that carbon fixation in Ulva prolifera, a species responsible for massive green tides, involves a combination of C3 and C4 pathways, and a CA-supported HCO3− mechanism. Analysis of CA and key C3 and C4 enzymes, and subsequent analysis of δ13C photosynthetic products showed that the species assimilates CO2 predominately via the C3 pathway, uses HCO3− via the CA mechanism at low CO2 levels, and takes advantage of high irradiance using the C4 pathway. This active and multi-faceted carbon acquisition strategy is advantageous for the formation of massive blooms, as thick floating mats are subject to intense surface irradiance and CO2 limitation.Most marine algae preferentially assimilate CO Ulva prolifera takes place via a combination of C3 and C4 pathways in combination with the enzyme carbonic anhydrase. The active and multi-faceted carbon acquisition strategy in U. prolifera is advantageous for the formation of massive blooms as the thick floating mats are subject to intense surface irradiance and CO2 limitation.Liu et al. present evidence that carbon fixation in  Rubisco , a key C3 enzyme responsible for carbon fixation, requires inorganic carbon in the form of CO22. However, the demand for CO2 in algal photosynthesis is generally higher than CO2 concentrations in natural seawaters, e.g., the half-saturation constants for CO2 of Rubisco in diatoms are 30–60 μM3 but CO2 concentrations in natural seawater are only 5–25 μM4. Thus, CO2 can be an important factor limiting algal proliferation in the ocean.The Calvin–Benson cycle (C2-concentrating mechanisms (CCMs) that enable accumulation of CO2 via bicarbonate (HCO3−) enzymolysis5. A dominant route in the CCMs is that intra- or extracellular dehydration of HCO3− is catalyzed by carbonic anhydrases (CA) to release CO2 to increase reactions at the Rubisco site  and PEPCase kinase (PEPCKase). CO2 incorporated in this manner eventually enters the C3 cycle to increase reactions at the Rubisco site  gene and enzyme analysis in U. prolifera revealed the existence and activity of the C4-related enzyme pyruvate orthophosphate dikinase (PPDKase)15; and (2) tissue δ13C range of U. prolifera (−21.9 to −14.9‰) at 16 stations in the Yellow Sea indicated a mix of C3 and C4 pathways in carbon fixation16.4 pathway in U. prolifera. That pathway requires the participation of the key C4 enzymes, PEPCase and PEPCKase. PEPCase (in the cytoplasm) supports catalysis of carboxylation of phosphoenolpyruvic acid (PEP), which uses HCO3− as the inorganic carbon substrate leading to C4 compound synthesis. PEPCKase (in the chloroplast) is a decarboxylase, generating CO2 and insuring efficient transfer of CO2 to Rubisco17. PPDKase is responsible for catalyzing the regeneration of PEP in the cytoplasm, but it is not the only way for the C4 pathway to acquire PEP17. Some researchers even believe that PPDKase might not assist with net CO2 fixation but rather has a role in protection against photoinhibition18. Therefore, examining the activities of PEPCase and PEPCKase is a necessary step to confirm the existence of C4 pathway in U. prolifera.These two lines of evidence, however, are not sufficient to characterize a full C13C/12C distributions in plant tissue records the integrated pattern of photosynthetic carbon acquisition20; for example, high CO2 uptake leads to low δ13C values , whereas high HCO3− uptake gives high δ13C values . Therefore, tissue δ13C is an important index using to distinguish C3 and C4 activity. The 13C/12C values in seaweeds, however, often display a wide range that makes definition of the C4 contribution uncertain21. The variation in the ratios depends on ambient temperature, light, salinity, nutrients and CO2 concentrations in seawater22. Growth and respiration in seaweeds23 can also lead to discrimination and fractionation of 13C/12C. As canopies of U. prolifera drift in the Yellow Sea, environmental conditions and physiological activity change, forcing variations that may mask clear evidence for C4 activity.The 4 pathway and/or CA mechanism in U. prolifera. Although several environmental factors  may affect the functional expression and activity of CA mechanism and C4 pathway, light intensity and pCO2 are regarded as the most important two under the condition with sufficient nutrient supply24. Massive floating algal mats in nutrient enriched Yellow Sea are not only exposed to strong surface light, but the thick biomass also can reduce the dissolution of CO2 from the air and lead to CO2 limitation. Consequently, the HCO3− systems operated by CA mechanism and/or C4 pathway would likely become active.To resolve this uncertainty, controlled experiments are needed to examine the correlation between the activities of key enzymes and photosynthetic products to ascertain the activity of C3 (Rubisco), CA (extra- and intra-cellular CA) and C4-metabolic enzymes  in U. prolifera. The relationships between the activities of major enzymes and photosynthetic products were analyzed via the corresponding variations in tissue δ13C, aiming to identify the participation of C3 and C4 photosynthetic pathways and CA mechanism in U. prolifera. The results exhibited the coexistence of C3 and C4 pathways and a CA-supported HCO3− mechanism in U. prolifera. The correlated variations of light intensity and pCO2 with enzyme activity indicated that the C4 pathway was most active under high light but that the CA mechanism became active at low CO2 levels. The joint operation of the C4 pathway and a CA-supported HCO3− mechanism in U. prolifera greatly improved efficiency of inorganic carbon fixation and illustrated why massive biomass accumulation can be formed in a short period, when thick floating mats are subject to intense surface irradiance and CO2 limitation in the Yellow Sea.Here, we describe the results of three outdoor culture experiments designed to examine the daily variations of key C3 and C4 enzymes were much higher on a sunny day  under high light intensity , and stayed low activity between 12:00 and 14:00 h, but increased significantly again from 42 to 143 nmol · min−1 · g · fresh weight−1  between 14:00 and 16:00 under reduced light intensity  = 152.08, p < 0.0001, Fig. p = 0.041, 0.001, and 0.011, respectively, Fig. 4 pathway was only detected on sunny days  was induced under high irradiance. Rubisco  did not display a linear relationship with light , C4 enzymes , and CA enzymes (extra- and intra-cellular CA) and their responses to varying light levels were measured over the course of the day .pCO2  and HCO3− concentration , a result that indicated a CA-dominant photosynthetic pathway late in the day when pCO2 decreased. In contrast, extracellular CA was not active and remained unchanged during the experiment  implied an important contribution of HCO3− in carbon fixation in U. prolifera. We cannot specifically define the contribution of HCO3− fixed separately via the CA mechanism versus the C4 pathway, but we found that the environmental factors inducing the activity of C4 enzymes are different from the CA mechanism. The diurnal modalities of C4 enzymes among the three experiments were consistent, although their concentrations were different depending on the ambient environmental conditions at the time of those experiments  at noon, indicating photosynthesis supported by HCO3−  are light-harvesting operators, and the energy flux they capture will be quenched and redistributed between PSII/PSI to achieve a balance and avoid irreversible damage to the photosynthetic systems31. An induction experiment with U. prolifera showed that the electron transport rate (ETR) between PSII/PSI was still high when light intensity reached 800 µmol photons m−2 s−132. Xu and Gao33 found that the ETR and net photosynthetic rate of U. prolifera remain high and stable even as light intensity increased to 2000 µmol photons m−2 s−1. These results explain the continuous carbon accumulation of U. prolifera exposed to light intensity > 1200 µmol photons m−2 s−1 for more than 2 h in sunny day experiments, which is about two- or threefold higher than that required to saturate the photosynthetic rate (600 µmol photons m−2 s−133). PPDKase might play an important role in dissipating excess energy and ATP in cells, since it is responsible for catalyzing the regeneration of PEP in the cytoplasm, where ATP is consumed during PEP formation from pyruvate  when the U. prolifera bloom is forming, dissolved inorganic nitrogen (DIN) concentrations are 10–80 µM near the coast and 1–15 µM offshore16, and the average light intensity in surface seawaters is ~1072.2 µmol photons m−2 s−134. In this study, the DIN concentrations in the culture medium were about 30 µM and the daily average light intensity on sunny days in experiments 1 and 3 was 1196 and 1011 µmol photons m−2 s−1, respectively. These values fit within the range of environmental condition in the Yellow Sea. The pCO2 supply is limited during the formation of green tide in the Yellow Sea, with a range of 300–450 μatm35, and moreover, it can be affected by phytoplankton consumption. Large floating mats formed by U. prolifera block air–sea exchange, thus, HCO3− uptake and assimilation could become important for rapid biomass accumulation. This was reflected in the tissue δ13C 16.Coastal eutrophication and high light supply in the Yellow Sea are important environmental conditions that favor the initiation of CUlva prolifera assimilates CO2 predominately via the C3 pathway, takes up HCO3− via the CA mechanism at low CO2 levels, and takes advantage of high irradiation by deploying the C4 pathway. This adaptive and multifaceted carbon acquisition strategy in U. prolifera is obviously an advantageous biological approach to support fast biomass accumulation and improve oxygen production in the filaments to sustain respiration for algae while floating in thick mats36. The relative contribution of the C4 pathway and CA mechanism in utilizing the HCO3− system and the environmental thresholds that determine which of these pathways dominate at different stages of bloom development, maturity, and decline need to be further elucidated.Ulva prolifera blooms are initiated from the biomass of thalli dislodged in the intertidal zone where the alga is subject to very high diurnal ranges in temperature and light and can tolerate partial desiccation37. Evolutionarily, the expression of C4 photosynthesis in U. prolifera maybe an adaptive response that enables the species to take advantage of rapidly changing conditions, and which also leads to formation of unusual massive blooms. The ability to harness the benefit of C4 photosynthesis might also be a feature of other types of blooming macroalga and warrants investigation.Ulva prolifera were collected from the Yellow Sea and, after cleaning epiphytes off using sterilized seawater, the sampled thalli were cultured in a laboratory incubator for a week prior to the outdoor experiments. Fifty grams of fresh thalli were placed into each of three replicate transparent plastic containers (53.5 cm × 39 cm × 32.5 cm), with 40 L of filtered seawater. To maintain ambient seawater temperatures in the containers, they were suspended in an outdoor seawater pond (60 × 100 m) at Muping Coastal Station of Chinese Academy of Sciences, China. Each experiment lasted 1 day from 08:00 to 18:00, without stirring. The first two outdoor culture experiments were carried out in July 2018, including a sunny day culture (experiment 1) and a cloudy day culture (experiment 2). The two experiments were to examine the responses of major enzyme activities of the C3 and C4 pathways  to diurnal variation of light intensity. The details are given below.Floating specimens of −2 s−1) than cloudy days (10.2–810 μmol photons m−2 s−1).Algal sample was collected every 2 h from each of the three culture containers and stored in a liquid nitrogen tank prior to assays of enzyme activities. Light intensity, salinity, and air temperature were monitored during the culture using a light meter , salinometer , and thermometer , respectively. The initial concentrations of DIN and dissolved inorganic phosphate (DIP) in the culture medium for both the sunny and cloudy day experiments were 34.6 and 0.67 μM, respectively. During experiments 1 and 2, salinity varied from 31.2 to 32.1 and water temperature was not much different on sunny (27.4–32.6 °C) versus cloudy (28.2–31.4 °C) days, but sunlight intensity was much higher on sunny days  was conducted in August 2019 to define the correlation between major enzyme activities and corresponding photosynthetic products. This experiment was carried out on a sunny day, from 08:00 to 18:00. Samples were collected from each of the three culture containers every 2 h and stored in liquid nitrogen for the assays of enzyme activities and tissue δ−1). After the pH in the water sample was stabilized, HCl standard solution was added into the sample to adjust the pH between 3.40 and 3.90. The added HCl volumes and pH values were recorded and total alkalinity calculated as−1); cHCl represents HCl standard solution (mol L−1); VW represents water sample volume (mL); VHCl represents added HCl volume (mL); 38, which was used to calculate the pCO2 and HCO3− concentrations.Twenty-five milliliters of water samples were taken from each of the three culture containers at each 2-h time interval and transferred to a fifty milliliters stoppered polyethylene bottle containing ten milliliters of a hydrochloric acid (HCl) standard solution  were weighed and then soaked in 2 mL of barbiturate buffer . Five milliliters of CO2-saturated water was injected to initiate the reaction. The reaction mixture was cooled in a 4 °C-water bath. The extracellular CA activity was assayed by measuring the time of catalyzed reaction required for the pH to change from 8.3 to 7.3 (Tc). The same procedure was repeated using 2 mL of barbiturate buffer without algal thallus and the uncatalyzed time required for background pH change from 8.3 to 7.3 was recorded (T0). The extracellular CA activity was calculated according to the following formula: Enzyme activity (U/g) = ((T0/Tc − 1) × 10)/algal weight.CA activity was assayed according to the method of Wilbur and AndersonThen, ~0.03–0.04 g of algal thallus was carefully ground in liquid nitrogen and soaked with 2 mL of barbiturate buffer  for the assay of total CA activity. The total CA activity was measured and calculated using the same method and formula as for extracellular CA activity. The intracellular CA activity was calculated by total CA activity minus extracellular CA activity.13C analysis. The samples were analyzed using an isotope-ratio mass spectrometer  at the Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences. Reference gases were calibrated against International Reference Materials . Results are expressed relative to Vienna PeeDee Belemnite. Replicate measurements of a laboratory standard  analyzed with the samples indicated that analytical errors were <0.1‰ for δ13C.After rinsing with 0.1 M HCl and washing with Milli-Q water, algal samples were freeze-dried and ground. Approximately 0.5–1 mg ground sample was placed into a 4 × 6 mm tin capsule for tissue δR2 was used to determine the significance of the results. PEPCKase activity was transformed (χ2) to ensure normality of residuals for the linear regression (Shapiro–Wilk test). Other variables did not require transformation. Two-way analyses of variance  was carried out on data from experiments 1 and 2. These analyses compared the level of enzyme activity under sunny and cloudy conditions and between different time periods (three replicates at each time period). Comparisons for PEPCKase were made across all time periods from 08:00 to 18:00, and for Rubisco and PEPCase for time periods from 08:00 to 16:00. Bartletts test was used to check for homogeneity of variances. A square root transformation was required to correct the variance structure for Rubisco and PEPCase. The 18:00 time period was excluded for Rubisco and PEPCase as variance structure could not be corrected if it was included. Tukey’s HSD was used for the pairwise comparisons. Correlation analyses were conducted on parameters measured in the third experiment for the time period from 08:00 to 14:00  in order to examine the relationships between the activities of key enzymes and photosynthetic products to ascertain the activity of C4 and/or CA pathways in U. prolifera. All analyses were undertaken using Addinsoft’s statistical software XLSTAT.Linear regression analysis was carried out to test the dependence of enzyme activity on irradiance levels using data from both the sunny and cloudy day experiments (experiments 1 and 2). Pearsonʼs Further information on research design is available in the Nature Research Description of Additional Supplementary FilesSupplementary Data 1Supplementary Data 2Reporting Summary


The two-dimensionally incommensurately modulated crystal structure has been investigated by X-ray diffraction experiments. In contrast to the tetragonal average structure with unit-cell dimensions of a = 4.4996 (5) and c = 9.179 (1) Å at 296 (1) K, which was solved and refined in the space group P4/nmm (No. 129), the satellite reflections are not compatible with a tetragonal symmetry but enforce a symmetry reduction. Possible space groups have been derived by group–subgroup relationships and by consideration of previous reports on similar rare earth metal polychalcogenide structures. Two structural models in the orthorhombic superspace group, i.e.Pmmn and β = 0.314 (1)], have been established and evaluated against each other. The modulation describes the distribution of defects in the planar [Te] layer, coupled to a displacive modulation due to the formation of different Te anions. The bonding situation in the planar [Te] layer and the different Te anion species have been investigated by density functional theory (DFT) methods and an electron localizability indicator (ELI-D)-based bonding analysis on three different approximants. The temperature-dependent electrical resistance revealed a semiconducting behaviour with an estimated band gap of 0.17 eV.Crystals of the rare earth metal polytelluride LaTe REX2–δ  with trivalent RE metals has attracted attention because of their structural variety in a quite small compositional range. The sulfides and selenides of this class of compounds have been intensively investigated, illuminating several different (super)structures due to different amounts of defects δ and the formation and arrangements of chalcogenide X2− and polychalcogenide nX2− anions for charge balancing. A comprehensive overview discussing these aspects can be found in Doert & Müller (2016RETe2–δ (RE = La–Nd)   comprise puckered [REX]+ slabs and planar chalcogenide layers, which can formally be stated as [X]−  compounds, vacancies in the planar chalcogenide layers are observed, together with X2− to maintain the overall net charge [X]− for the layer. This structural change is obvious for the CeSe1.9 structure type, but can also be seen for the related, intrinsically disordered Gd8Se15-type structures   by forming larger anionic fragments . In contrast to the respective sulfides and selenides, a motif of statistically disordered Te22− anions and linear Te34− anions are found here. Linear Te34− anions have rarely been reported in REX2–δ compounds, although the presence of an Se34− anion was discussed for DySe1.84, but neglected after computational studies  theory, this situation can be described as a 3c–4e bond  compounds, we were interested in identifying the structural motifs of the early rare earth metal tellurides RETe2–δ with δ > 0. This was especially motivated by the reported differences between the structures of LaTe2, CeTe2 and PrTe2  are a few examples  are presented in the following.Starting from the results of the 2 to the reaction mixture in closed silica ampoules. In a standard synthesis, 500 mg of a stoichiometric mixture of lanthanum  and tellurium  were placed in a quartz tube and flame sealed under dynamic vacuum (p ≤ 1 × 10−3 mbar). The ampoule was heated slowly with a ramp of 2 K min−1 to 1073 K. The reaction takes place in a gradient from 1123 to 1073 K with I2  as transporting agent. After 7 d, the ampoule was cooled to room temperature. As we observed a slow degrading of the compounds under atmospheric conditions, the samples were stored under argon.All preparation steps were carried out in an argon-filled  glove-box . Crystals were grown by the addition of a small amount of IKα1 radiation (λ = 1.54056 Å). The scans covered the angular range from 5 to 90° 2θ. Rietveld refinement using the fundamental parameter approach was performed with TOPAS  K with an Empyrean diffractometer  equipped with a curved Ge(111) monochromator using Cu JANA2006  and two harmonic waves have been used for the fit of the atomic modulation functions.Crystal data, data collection and structure refinement details are summarized in Table 1Ua = 5 kV). The composition of selected single crystals was determined by semiquantitative energy dispersive X-ray analysis (Ua = 20 kV) with a Silicon Drift Detector (SDD) X-MaxN (Oxford).SEM was performed with an SU8020 (Hitachi) with a triple-detector system for secondary and low-energy backscattered electrons . The FHI-aims calculations for subsequent real-space analysis were performed with a (2 × 2 × 2) k-point grid (model in P1) and a (3 × 2 × 2) k-point grid (model in Amm2 and A2) using the zeroth-order scalar relativistic zora correction, collinear spins, the numerical atom-centred basis of light level and the PBE functional  was measured between 7 and 345 K with a mini-CFMS . Four gold contacts were attached to the surface of a single crystal in a linear set-up with a carbon conductive composite 7105 (DuPont) to establish the electrical contact between the crystal and the gold wires.The electrical resistance of LaTe1.82(1) were obtained starting from the elements by alkali halide flux reactions or solid-state reactions with a small amount of I2 for mineralization in fused-silica ampoules. Temperatures above 1173 K in the presence of I2 lead to an attack on the ampoule wall, whereas temperatures of about 1073 to 1123 K are well suited for crystal growth, without noticeable side reactions with the ampoule material. The best results were achieved in a small gradient from 1123 to 1073 K, where crystals of about 0.3 mm in length were grown. The amount of added I2 needs to be compensated by excess La when preparing the experiment due to the formation of LaI3 during and after the experiment, which in turn alters the composition slightly. Solid LaI3 was mainly found at the sink of the ampoule, whereas the desired product was found at the source of the ampoule. A similar procedure at higher temperatures was chosen, e.g. for RETe1.8   and the atom sites of the aristotype, the ZrSSi type, were chosen. The fit shows some additional unindexed reflections, which are not compatible with the space group P4/nmm and its most prominent lower symmetric subgroups  with n = 0, ±1, ±2,… . Slightly stronger reflections can be observed in the layers (hkn) with n = ±0.5, ±1.5,… . These additional reflections cannot be indexed with a commensurate superstructure of the basic unit cell and were treated as satellites. Moreover, the distribution of the satellites with respect to the main reflections suggest a two-dimensional modulation. The whole diffraction image can then be indexed with five indices hklm1m2 according to: The diffraction image of a single crystal at ambient temperature reveals, as already indicated by powder diffraction, additional weak reflections in the layers (supporting information). The schematic figure illustrates also that two different q vectors are necessary to index the complete diffraction image. Equivalent satellites are linked by a twofold rotational axis , pointing towards orthorhombic (or lower) symmetry for the modulated structure. Furthermore, the weak additional reflections in the (hkn)  plane correspond to the linear combinations of q1 and q2, which cannot be explained by twinning and are, thus, evidencing a true [3 + 2]-dimensional modulated structure.The modulation wave vector components α and β were determined to be 0.272 (1) and 0.314 (1), respectively. A schematic image of the relative positions of the satellite reflections with respect to the main reflections is displayed in Fig. 2a and b within standard deviations so that a tetragonal unit cell of a = 4.4996 (5) and c = 9.179 (1) Å was chosen. The main reflections are clearly compatible with high tetragonal Laue symmetry and the space group P4/nmm (No. 129) was chosen for structure refinement of the average structure according to the reflection condition h+k = 2n. The refinement resulted in a reasonable structural model (Table S1 in the supporting information), with a reduced occupancy factor of the Te site in the Te layer of about 81.1 (4)%. The composition derived from semiquantitative EDS (energy-dispersive X-ray spectroscopy) measurements points towards a composition of LaTe1.79 (1). Throughout the article, we will refer to this compound as LaTe1.82(1), based on the refined composition of the modulated structure.Single-crystal data collected at ambient temperature indicated the same lattice parameters for 1.82(1) can be described with puckered [LaTe] layers sandwiched by square-planar [Te] layers. The partial occupation of the Te position in the [Te] layer, together with its large oblate anisotropic displacement parameters (ADPs) in the ab plane and prolate ADPs of the La atom along the [001] direction already give hints towards the modulation  Å in β-K2Te2; Böttcher et al., 1993RESe1.85 (RE = La–Nd or Sm)  and 1 × 3.324 (1) Å] and four Te atoms of the [Te] layer [4 × 3.3563 (7) Å], forming a regular tricapped trigonal prism. The planar [Te] layer is a perfect square net of Te atoms, with a Te—Te distance of 3.1821 (4) Å, which is significantly longer than a single Te—Te bond with about 2.80 Å  layer shows a strong displacement in the ab plane, as displayed in the two t plots in Fig. 5Pmmn and Pm21n, as a result of the higher degree of freedom in the noncentrosymmetric space group. In Pm21n, the t plot shows a slightly sinusoidal curve in the c direction with a very small amplitude.The atomic modulation functions  maps decrease considerably if two modulation functions are applied to the ADPs of Te2 as well. However, this leads to nonpositive-definite values for Umin at some values of t and has hence been rejected for the final structure model.The Fourier sections in Fig. 6Pmmn is used in the following paragraph for the discussion of the structural features as the structural differences between both models are negligible, as stated before.The structure model derived in c direction, as expected from the average structure. The motif of the puckered layer is very stable and does not show large deviations between different REXδ2– compounds, whereas the planar [X] layer is the more interesting structural feature and will be discussed in the following.The general feature of an alternating stacking of a puckered [LaTe] layer and [Te] layer is easily seen from Fig. S4 (see supporting information), which additionally shows the displacement of the La atoms along the Pmmn are shown in Fig. 7t-plot for the occupational modulation displays a static behaviour along t, which is shifted for different u values. The changes of the Te—Te distances in the modulated [Te] layer are displayed in the second t plot  Å in GdTe1.8  are in good agreement with the distances found in comparable compounds.The change of the occupation and the variations of the Te—Te distances for the model in ot Fig. 7. Short d8 unit, probably consisting of smaller anions, like Te2−, Te22−, bent Te32− and linear Te34− anions, as well as isolated Te2− anions surrounded by different anionic Te entities. Eight-membered rings of chalcogen atoms and isolated X2− anions are, as already pointed out, common motifs in the crystal structures of the rare earth metal sulfides and selenides with compositions of RES1.9      , a Te4 square is apparent additionally when choosing small cut-off values for the approximant crystal structure for visualization . As all four atoms in these fragments have an occupation value of about 0.52 (5), the presence of all four at the same time seems unrealistic. Instead, an unresolved superposition of a dinuclear Te22− anion with two adjacent vacancies is the most likely explanation. Moreover, there is no evidence for anionic [Te4] squares with Te—Te distances of 2.80 Å in the literature. Cationic Te42+ and the corresponding Se42+ entities, on the other hand, are well known  into account, a substantial number of voids in the [Te] layer is expected, which also supports the idea of a disordered motif of dinuclear Te22− anions and adjacent vacancies instead.In the modulated [Te] layer of LaTe8 rings , these apparent eight-membered rings may also consist of different disordered constituents, like Te32− and Te34− anions, along with the more common Te2− and Te22− motifs, around central vacancies. However, this is hard to resolve solely from the diffraction data. To gain more insight into this structural motif and the chemical bonding situation in the modulated [Te] layers in general, chemical bonding analyses were performed for three different commensurate approximants.A similar discussion of apparent structure motifs and possible (super)positions due to unresolved disorder shall be deduced for the apparent Tegs Fig. 8. Arrangeet al., 20121.82(1) in order to provide additional information on the bonding situation in the Te layers, especially regarding the (presumably disordered) Te4 and Te8 entities. As these calculations require three-dimensional commensurate structure models as bases, a suitable commensurate orthorhombic B-centred 4 × 3 × 2 supercell of the basic ZrSSi-type structure was chosen by approximating the q vector components α and β by JANA2006 software package , b = 17.9812 (4) and c = 18.3446 (8) Å. As the highest possible symmetry, space group Amm2 (No. 38), the space group of the RE8Se15–δ compounds, was chosen for one approximant. The respective Amm2 structure model exhibits bent Te3 units in the Te8 ring, as well as a Te4 square, which cannot be resolved due to the symmetry restrictions in this space group . A second model has been established in the space group A2 (No. 5), i.e. removing the two perpendicular mirror planes. This space group has also been used to describe the disorder in the structures of the compounds RE8S15–δ. Here, only Te22− anions with alternating short (bonding) and longer (nonbonding) distances are considered as building units of the Te8 rings , enabling a direct comparison between the bent X3 fragments in Amm2, and the X22− patterns known from different REX1.9 and RE8X15–δ structures (cf. above). A third model in the space group P1 (No. 1) was developed starting from the previous model in A2 to lift the symmetry restrictions completely. The disorder of the apparent Te4 unit can then be resolved by assuming two vacancies and a single Te22− anion . Bear in mind that energetic comparisons are only possible between models with the same number of atoms. This is the case for the models in Amm2 and A2, but not for P1, due to the additional vacancies when taking the occupational disorder of the Te4 square into account. This means that an identification of the favoured structure is not possible based on the computed net energies only.Quantum mechanical calculations based on density functional theory (DFT) and bonding analyses with the electron localizability indicator (ELI-D) . The corresponding stoichiometric LaTe2 was reported as metallic  structure model in Amm2 shows a lower energy than the corresponding (ordered) model in A2 (ΔE = 0.16 eV). The Amm2 structure would imply the existence of bent Te3 entities with bond angles of about 90.0 (1)° in the planar Te layer, as mentioned before. Angular Te32− anions are well known, e.g. from the binary dialkali metal tritellurides A2Te3 with A = K, Rb or Cs  seem to be used to accommodate the lone pairs of different anionic Te fragments, again supporting the ionic description of the [Te] layer and in accordance with the calculated band gap and the measured semiconducting properties of LaTe1.81 (2) (see below).The ELI-D slices in Fig. 9el Fig. 9b seem t1.95 (Amm2 and A2) and LaTe1.875 (P1), i.e. a higher tellurium content as compared to the actual composition LaTe1.82(1). Thus, additional vacancies would be necessary to get a more realistic image of the Te substructure. The bonding features between different constituents should nevertheless be comparable.Note, that the evaluated approximant structures indicate compositions of about LaTe1.82(1) has been recorded by a four-point measurement between 7 and 345 K. The observed temperature dependence of the resistance of LaTe1.82(1) is characteristic for a semiconductor , where kB is the Boltzmann constant and T is the absolute temperature. The estimated Eg value is 0.17 eV for LaTe1.82(1), which is slightly larger than the calculated value for the model in A2 (cf. above). However, comparable compounds like NdTe1.89 (1), GdTe1.8 and SmTe1.84 show similar band gaps of 0.14  has been solved and refined using the superspace approach. The diffraction pattern evidences that the tetragonal symmetry of the average structure is not preserved in the modulation. Two different models evaluated in superspace groups Pmmn may be best described as a depleted REX1.9 or REX1.875 structure. For the latter two structure types, it is possible to accommodate the respective anionic vacancies structurally isolated, i.e. separated between different (poly)telluride anions and in a commensurate superstructure. In the title compound, 18% of the Te2 positions are unoccupied, which leads to two obvious consequences: a commensurate ordering of vacancies and remaining anions is not possible anymore, and a considerable number of adjacent anion defects occur. In other words, LaTe1.82(1) exhibits a higher propensity for missing Te22− dianions. This description deviates significantly from the structures described for RETe1.8  compounds. Quantum mechanical calculations based on DFT with subsequent ELI-D-based bonding analysis for the ionic Te layer reveal Te22− units as dominant species, however, with significant long-range interactions amongst them. Temperature-dependent resistance measurements suggest a semiconducting behaviour with a band gap of about 0.17 eV, which is in good agreement with comparable rare earth metal compounds.The modulated structure of the rare earth metal polytelluride LaTe10.1107/S2053229620005094/sk3745sup1.cifCrystal structure: contains datablock(s) global, I, II. DOI: 10.1107/S2053229620005094/sk3745Isup2.hklStructure factors: contains datablock(s) I. DOI: 10.1107/S2053229620005094/sk3745sup3.pdfAdditional figures and tables. DOI: 1982148, 1996576CCDC references:


To date both explanations have relied on poorly constrained δ13CCH4 source signatures. We use a dataset of δ13CCH4 from >1600 produced shale gas samples from regions that account for >97% of global shale gas production to constrain the contribution of shale gas emissions to observed atmospheric increases in the global methane burden. We find that US shale gas extracted since 2008 has volume-weighted-average δ13CCH4 of −39.6‰. The average δ13CCH4 weighted by US basin-level measured emissions in 2015 was −41.8‰. Therefore, emission increases from shale gas would contribute to an opposite atmospheric δ13CCH4 signal in the observed decrease since 2008 . This observation strongly suggests that changing emissions of other  CH4 source terms is dominating the increase in global CH4 emissions. Although production of shale gas has increased rapidly since 2008, and CH4 emissions associated with this increased production are expected to have increased overall in that timeframe, the simultaneously-observed increase in global atmospheric CH4 is not dominated by emissions from shale gas and shale oil developments.The accelerated increase in global methane (CH Its globally averaged marine surface annual mean mole fraction in the atmosphere steadily increased from ~1600 parts per billion (ppb) to ~1775 ppb in the 1980–1990s, stabilized around ~1775 ppb during the period 1999–2006, and then returned to the earlier pattern of increases leading to ~1860 ppb in 201810. There are anthropogenic  and natural  sources of CH4 to the atmosphere. The carbon-isotopic composition of CH4 (the ratio of stable isotopes 12C and 13C expressed as δ13C (‰) relative to the Vienna Pee Dee Belemnite standard), together with estimates of flux from each source-type, can be used to infer the relative contributions of various CH4 emitters to the global budget by matching co-constrained global observations of CH4 and its δ13C4. Furthermore, temporal variations in δ13C of atmospheric CH4 are a highly useful indicator of changes in the trends of various emitters and/or CH4 sinks. The global mean atmospheric δ13CCH4 trended upward between 1980 (approximately −47.7‰) and 1997 (approximately −47.1‰), and remained relatively stable until 2008, before decreasing towards the most recent values of approximately −47.3‰6.Methane  relative to globally-averaged atmospheric CH4. Decreasing δ13C of atmospheric CH4 since 2008 implies that emissions from biogenic sources are therefore increasing at a greater rate relative to emissions from fossil fuels. However, recent studies have suggested that emissions from conventional petroleum developments5 and from shale gas/oil developments in particular2 (see Text S1 in Supplementary Information) have been the greatest single cause of the recent global increase of atmospheric CH4. Here, we use a large global dataset of δ13CCH4 from produced shale formations, which leads us to conclude that emissions from shale gas and oil production have not played a dominant role in the increase in atmospheric CH4 since 2008.This recent increase in the atmospheric CHδ13CCH4 data for 1619 samples of produced natural gas from 38 shale formations around the world originally presented in 73 studies , we also include samples collected from unconventional low-permeability (tight) reservoirs dominated by very fine-grained sandstone or siltstone  or mixed clastic/carbonate lithologies  developed through hydraulic fracturing and commonly included in the inventories of produced shale gas17. The produced gas may be free gas associated with relatively little condensate liquids  or oil-dissolved gas . Most samples come from the USA (n = 1238), followed by China (n = 252), Canada (n = 124), United Kingdom (n = 2), Sweden (n = 2) and Australia (n = 1).We analyzed es Table . This shδ13CCH4 for shale gases were derived by first calculating the proportion of gas production from each shale formation in the total production, then multiplying that value by the average δ13CCH4 for the corresponding shale formation, and then summing up the results. Emission volume-weighted δ13C-CH4 values were derived by first calculating the proportion of CH4 emissions from each shale formation in total emissions, multiplying that value by average δ13CCH4 for corresponding shale formation, and then summing up the results.Values of production volume-weighted δ13CCH4 for all shale gas samples is −41.3 ± 0.2‰   and significantly more enriched in 13C than the modern atmospheric δ13CCH4 (−47.3‰6). We note that shale gas is even more enriched in 13C relative to the global average δ13CCH4 (about −54‰4) of all atmospheric sources prior to isotopic fractionation of atmospheric CH4 by all sinks resulting in the modern atmospheric value above.The arithmetic mean 18. In the USA, the cumulative production of shale gas from 2000 to mid-2019 reached approximately 4.5 trillion cubic meters (tcm), including about 4.1 tcm produced since 2008  in 2005 to about 434 bcm in 2015δ13CCH4 dataset to derive δ13CCH4 representative of both produced gas (volume-weighted average) and the δ13CCH4 signature when weighted for measured emissions across plays (emission-weighted average). Table δ13CCH4 for the main producing shale plays in the USA. The 1002 available gas samples with δ13CCH4 data are from plays that account for 94% of cumulative US shale gas production. The average δ13CCH4, when weighted by the amount of cumulative production from each shale play during 2008–2019, is −39.6‰. A large proportion (28%) of cumulative shale gas production comes from the Marcellus Formation where CH4 is significantly enriched in 13C . This latter source significantly influences the average volume-weighted isotope signature of CH4 produced from shales in the USA.In this study, we use the global δ13CCH4 weighted by the amount of emissions measured in 2015 from the main USA shale plays19 is −41.8‰  by about 4–7‰ from 2000 to mid-2019  to both production and emissions increased in that period.The average shale 8‰ Table , Text S3δ13CCH4 values than the USA  indicate somewhat more positive 8‰ Table , Text S4δ13CCH4 weighted by the amount of cumulative production from 2000 to mid-2019 for each shale play is −40.0‰ .The US shale 0‰ Table . The mea.‰ Table . We also4 became more abundant but also depleted in 13C, as δ13C decreased from about −47.1‰ in 2007 to −47.3‰ in 2017. If shale gas (with δ13CCH4 around −40‰ as documented in this study) and conventional oil and gas (with δ13CCH4 around −43‰16) were conceived to collectively dominate recent emissions of CH4 to the atmosphere, then atmospheric CH4 would very simply become more enriched in 13C relative to the current global mean δ13C, which is not consistent with global observations. While we agree that shale developments  represent an important CH4 source, and that emissions from those sources have been likely increasing due to growing production, we conclude that the increases in global atmospheric CH4 concentrations since 2008 are not as strongly attributable to shale gas and conventional oil and gas emissions as some studies claim5, based on our global observations of isotopic fractionation.Recently, atmospheric CHδ13CCH4 signal is the sum-total of all CH4 source and sink terms. For example, a decrease in biomass burning emissions (significantly enriched in 13C (δ13CCH4 −22.3 ± 1.9‰4), and an increase in fossil fuel emissions , could in principle result in the same global average atmospheric δ13CCH4 signal over time as if both sources had no trend5. The biomass burning category includes fires and solid biofuels . Data on global CH4 emissions from fires is not entirely conclusive. Remote sensing data of CH4 and CO (and assuming (i) biomass burning CH4/CO emission ratios and (ii) a partitioning of CO emissions across sectors) suggests decreased fire CH4 emissions of ~3.7 Tg/yr from the 2001–2007 to the 2008–2014 periods5. In contrast, remote sensing of burned fire area suggests no such trend20 . While this data does not indicate an immediately apparent decrease in global biomass burning CH4 emissions, more research is needed. Potential trends in the various CH4 sink processes such as the soil sink22 and the tropospheric OH sink11 can further complicate the diagnosis of source trends. As a result, it is important to account for these processes, as well as other existing evidence such as latitudinal and seasonal CH4 trends, when attributing the global signal9.Additionally, we must emphasize that the measured atmospheric 4 increase to North American shale gas production and another ~1/3 to conventional gas and oil with a simple mass balance approach2 is not supported by observations because of unconstrained uncertainties. Based on long-term airborne CH4 measurements over the US, previous analysis concludes that oil and gas industry CH4 emissions  over the past decade have increased at about the same rate as natural gas production volume7. The existence of unaccounted and poorly characterized emission sources within the oil and gas industry has also been demonstrated through intensive field studies in the USA23, and additional international studies paint a similar picture25, although little independent measurement data exist for many world regions including the Middle East, the Former Soviet Union, and Africa. Further research targeted for these areas, in addition to changing biogenic sources and sinks, will serve to further constrain the conclusions made in this work.From the above, it follows that attributing ~1/3 of the global CH4 source and sink terms and isotopic signatures, additional CH4 emissions associated with increased shale gas development in the USA cannot account for a large fraction of the recent increase in atmospheric CH4. Yet, oil and gas industry expansion remains a significant factor in the complex patterns of global atmospheric CH4 emissions and concentrations25. And, of equal importance, fossil fuel CH4 sources may be mitigated with policy and best (or better) industrial practice that can effectively reduce emissions. We suggest that the rise in global CH4 concentrations is most effectively seen not through a lens of what is the most important or dominant source of emissions, but rather understanding all sources and how they can collectively explain the observed patterns of atmospheric increases. Indeed, a reduction in emissions from any major source (such as fossil fuels or cattle husbandry) would be expected to lead to a reduction in the global CH4 concentration1. Therefore, although our analysis indicates that shale gas and conventional gas and oil production has not played a dominant role in the increase in atmospheric CH4 since 2008, we should not lose sight of the powerful impact of interventions to reduce emissions from sources we have.Based on existing knowledge of CH4 recently increased in the atmosphere and simultaneously became more depleted in 13C. In this study, we compiled a large global dataset of isotopic composition of CH4 produced from shale formations that account for most global shale gas production. Developments of shale gas and oil on average emit CH4 significantly more enriched in 13C than the atmospheric CH4 signal. Given current knowledge of global isotopic data and processes, the increase in US shale oil and gas apparently does not dominate the recent increased emissions of global CH4 to the atmosphere. It is important to understand all sources of CH4 that collectively contribute to recent atmospheric increases, and isotopic data provide key constraints for this.CH


M−L bonds in 3d transition metal complexes, analysis by energy decomposition analysis and natural orbital for chemical valence model (EDA‐NOCV) is desirable as it provides a full, quantitative and chemically intuitive ab initio description of the M−L interactions. In this study, a generally applicable fragmentation and computational protocol was established and validated by using octahedral spin crossover (SCO) complexes, as the transition temperature (T1/2) is sensitive to subtle changes in M−L bonding. Specifically, EDA‐NOCV analysis of Fe−N bonds in five [FeII(Lazine)2(NCBH3)2], in both low‐spin (LS) and paramagnetic high‐spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L6 fragmentation, tested against a family of five LS [FeII(Lazine)3](BF4)2 complexes; this confirmed that three Lazine are stronger ligands  than 2 Lazine3+2 NCBH , as observed. 2) Analysis of Fe−L bonding on LS→HS, reveals more ionic (ΔEelstat) and less covalent (ΔEorb) character (ΔEelstat:ΔEorb 55:45 LS→64:36 HS), mostly due to a big drop in σ , and a drop in π contributions . 3) Strong correlation of observed T1/2 and ΔEorb,σ+π, for both LS and HS families , but no correlation of T1/2 and ΔΔEorb,σ+π(LS‐HS) (R2=0.11). Overall, this study has established and validated an EDA‐NOCV protocol for M−L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral transition metal complex. This new and widely applicable EDA‐NOCV protocol holds great promise as a predictive tool.To improve understanding of  A generally applicable fragmentation and computational protocol for EDA‐NOCV M−L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral complex is established and validated in the first EDA‐NOCV analysis of spin crossover‐active complexes. As expected the combination of the σ and π bonding  in the Fe−L bonds correlates very well with the observed switching temperature (T1/2). M−L interactionsM−L interactions in any octahedral complex,The function of metalloenzymes,M−L complex. Italics are used for low spin (LS) and high spin (HS) state abbreviations. Bold and italic formatting is used for the organic ligand family, Lazine, used in the complexes investigated in this study.Bold formatting is used to highlight the fragmentation scheme used for the M−L interactions during bond formation: the various contributions to the total interaction energy (ΔEint) are assessed by the use of EDA, and then a breakdown of the orbital contribution (ΔEorb) to quantitatively assess the M−L bond character is achieved by the use of the NOCV scheme.Specifically, energy decomposition analysis (EDA) and natural orbital for chemical valence theory (NOCV)n+M+6L vs. 5MLn++L for a general 6ML complex) suitable for EDA‐based bonding analyses and direct comparison of any metal complex—so we rigorously, and successfully, address this issue herein.Whilst EDA‐NOCV methodology has been extensively used to study diamagnetic systems,Our first, and key, step was therefore to establish a suitable, generally applicable fragmentation and computational protocol for the EDA‐NOCV analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral complex. To do this, a test system that enables validation of the outcomes must be chosen.M−L bonds as L is modified, as the transition temperature (T1/2) at which the complex switches between the low‐spin (LS) and high‐spin (HS) states in solution is sensitive to these changes.[II(Lazine)2(NCBH3)2] complexes that vary in the choice of the azine ring ‐active complexeserature T1/2 at wherature T1/2 at wherature T1/2 at wherature T1/2 at wherature T1/2 at wherature T1/2 at whT1/2 values of the complexes. Doing this enabled us to determine whether or not the theoretical findings are consistent with experiment, and hence provide quantitative and chemically intuitive insights into the nature of the M−L bonds under consideration.Application of the resulting new protocol to this family of SCO‐active complexes then enabled us to evaluate the changes in the bonding properties across the family, obtained by EDA‐NOCV calculations,II(Lazine)2(NCBH3)2] complexes was then used for the closely related [FeII(Lazine)3](BF4)2 family of LS complexes,Lazine ligands produce a stronger octahedral ligand field than a combination of 2 Lazine+2 NCBH3, which is in line with experimental findings.Finally, the optimized EDA‐NOCV protocol developed for the SCO‐active [FeIILazine22(NCBH3)2] complexes in both the LS and HS states are required, so density functional theory structure optimizations of the complexes were performed with the ORCA 4.1 software package.3).II(Lpyridine)2(NCBH3)2] complexII(Lazine)3](BF4)2 complexes had been previously optimized by using the same protocol.Introduction to EDA‐NOCV: The EDA‐NOCVM and the surrounding ligands L in a chemically intuitive manner. To do so, EDA‐NOCVEint of the M−L bonds formed between the two (or more) fragments in the frozen (unrelaxed) geometry of the molecule is then assessed.Eint, is comprised of four main contributions  is negative and accounts for the electron density distortion associated with the electron flow between 1) two different fragments to give the individual orbital contributions to the σ, π and δ bonds formed  and 2) two regions of the same fragment to give the polarization term . The dispersion term (ΔEdisp) is an extra contribution obtained from the explicit calculation of dispersive interactions, complexes comprise one metal ion (Fe2+), two constant axial anionic co‐ligands (NCBH3−) and two varying equatorial neutral bidentate Lazine ligands. In the first step, a full test of five possible fragmentations that the LS [FeII(Lazine)2(NCBH3)2] complexes could be broken into  are present, removal of a single ligand.[L=[NCBH3]− (fragmentation 1) or [Lazine] (fragmentation 2) is removed, so these provide detailed information on a single type of Fe−L interaction. However, the presence of another ligand of the same type in the other, iron‐containing, fragment makes these two fragmentation choices less than ideal here. Hence fragmentations 3 and 4  or both [2×Lazine] (fragmentation 4) ligands, should provide a cleaner analysis of the details of the different types of Fe−L bonds. These fragmentation schemes are described in detail in Sections S3.1–S3.4. However, all four of these fragmentations, 1–4, would only really be useful for examining trends within a family of very closely analogous complexes—confidently comparing very different coordination environments around M will be rather difficult, as the fragmentation is not general enough for that: The remaining metal‐bound ligands will surely affect the electronic environment of the metal ion so will subsequently influence the M−L bonding character.Fragmentations e ligand.28 Here 5 , this is not the case in fragmentation 5. Due to the absence of partial ligand fields, which are induced by lone‐pair containing ligands surrounding the metal ion containing fragment in the other fragmentation schemes (1–4), the attractive potential of the Fe2+ centre is not “buffered” by electron density in the vicinity anymore and is therefore fully experienced by the d electrons.In light of this, fragmentation 5 Figure , in whic2+ instead of Fe0 as a fragment appears intuitive and convenient at first, the resulting Fe2+ d atomic energies for fragmentation 5 a , are very low in energy . A slightly different approach was taken with scheme 5 e: Here the electron density of the isolated Fe2+ AOs was mapped onto the neutral Fe0 AOs. These approaches, 5 a–5 e, are described in detail in Section S2.9, but are summarized as follows:This strongly challenges the physical justification for this description of 5 a: Fe2+Fragmentation 5 b: Fe2++6×−0.425eFragmentation 5 c: Fe2++6×−1.0eFragmentation 5 d: Fe2++6×−2.0eFragmentation 5 e: Fe2+ density mapped onto Fe0(AOs)Fragmentation 5 b–5 e) effectively rescaled the Fe2+ d orbitals towards more positive energy levels  so both were applied for in depth analysis of the complexes depending on the quantity in question. Specifically, 5 b allowed for identification of chemically intuitive bonding interactions by NOCV analysis but underestimated the Pauli repulsion (ΔEPauli) in the EDA, whereas for 5 e it is the other way around. Hence, we have employed fragmentation 5 e to obtain information about the contributions to the intrinsic bond energy  and—in separate calculations—fragmentation 5 b to gain deeper insight into the orbital interactions and the relative contributions within by NOCV decomposition analysis.Fragmentations 5 b/5 e , in the EDA‐NOCV analysis of three families of complexes follows.As the purpose of this work is to provide a robust computational protocol to enable the application of EDA‐NOCV analysis to any monometallic complex, regardless of spin state or the exact nature of the coordination pocket provided by the coordinating ligands, a detailed description of the results of applying these two general fragmentations, Eelstat and ΔEorb [Eq. (1)] are the EDA quantities that give indications of the ionic and covalent character of the chemical bonds formed between the two fragments .As noted above, Δ5 b; n+M+6L) that is expected to be most sensitive to the differences in the azineM−L bonds (due to the 5 different azines), and hence reflects the changes in the SCO properties, is ΔEorb [Eq. (1)], in particular the σ and π contributions that involve the metal ion . Visual representations of all the σ and π contributions to the M−L bonding are provided by the NOCV deformation densities Δρ(i) for each of the fragmentations employed. It should be noted that the general appearance is the same for the other four complexes in the respective family (treated with the same fragmentation), regardless of the different Lazine ligands.The term (fragmentation Lpyridine)2(NCBH3)2] and for the LS family of [Fe(Lpyridine)3]2+ . Hence, as expected, the variation in ΔEorb,σ as the Lazine changes from L4pyrimidine to Lpyrazine is due to changes in the σ interactions formed by the Fe2+ dz2and dx2-y2orbitals . The Lpyridazine complex shows significantly smaller dz2 and dx2-y2 orbital interactions than are seen in the other complexes .Focusing first on the M→L π‐back‐donation contributions, ΔEorb,π, reveals: Δρ3 is mainly associated with the interaction of M with the diazine ring in the yz plane ; while Δρ4 is mainly associated with the interaction of M with the triazole ring in the xz plane . Δρ5 lies in the Lazine plane (xy) so both the diazine ring and the triazole ring of each Lazine ligand participates in this bond . As for the ΔEorb,σ values, the ΔEorb,π values do not parallel the order of T1/2 values: again the Lpyridazine complex is the outlier, with a significantly bigger ΔEorb,3  than the rest .Focusing next on the analysis of the three Lazine)2(NCBH3)2] complexes , unsurprisingly, the change in FeII spin state dramatically affects the M−L interactions. The EDA (fragmentation 5 e) shows that on going from LS to HS the ΔEint stabilization for the [Fe(Lazine)2(NCBH3)2] family (Table S22) decreases by ca. 25 %, from about −500 to −370 kcal mol−1. The exact values depend on the Lazine present; those for Lpyridine are shown in Figure Eint  lies between −330 and −350 kcal mol−1 and almost two‐thirds of this orbital interaction is provided by ΔEorb,σ, in particular by the formation of M−L σ bonds involving the M dz2and dx2-y2(unoccupied) orbitals . In contrast, in HS [Fe(Lazine)2(NCBH3)2] these two orbitals are now half‐occupied so M−L antibonding interactions are also present, dropping the ΔEorb,σ stabilization energy values to less than −35 kcal mol−1 each; consequently, the total ΔEorb,σ+π stabilization energy drops to between −145 and −160 kcal mol−1 in the HS state  .The ΔEorb,π) to ΔEorb,σ+π are small in both spin states , those involving the t2g orbitals donating electron density back to the ligands show a large reduction in magnitude of stabilization on going from LS to HS  due to the lower number of electrons present in them.Whilst the π contributions  provide greater stabilization in the HS state, by about −30 kcal mol−1  between M and 6L, by ca 50 %, while the electrostatic interactions (ΔEelstat) only drop by =10 %, reflecting the reduction in the hardness of the metal ion as the radius increases (from 0.75 Å LS to 0.95 Å HS).M−L bond becomes more ionic and less covalent, with longer and weaker bonds due to decreases in both the σ and π interactions.In a nutshell, as expected by the occupation of antibonding orbitals, spin state switching from LS to HS Figures , 5 and 6Eorb,σ+π values obtained from the EDA‐NOCV analysis were expected to correlate with the ligand field strength of the bonds formed between the fragments n+M and 6L. This is a useful test of whether or not this approach can provide a useful, general, quantitative and predictive tool for predicting T1/2 for an SCO system.Given the above, the Δ2=0.95) between the EDA‐NOCV calculated ΔEorb,σ+π and the experimentally observed T1/2 is observed, regardless of whether the family of LS and HS state complexes is examined  is observed . Rather, the single spin state trend  should be used, as it appears to be a good predictive tool. In summary, it is evident from these results that the change of Lazine induces different alterations in the σ and π interactions, which only correlate (extremely well) with the T1/2 values when the synergy of the two contributions  is considered  to the closely related family of LS [Fe(Lazine)3]2+ complexes 3]2+ family the EDA revealed a ΔEelstat:ΔEorb ratio of about 45:55 , revealing greater covalent than ionic bonding, in contrast to the LS [Fe(Lazine)2(NCBH3)2] complexes in which this ratio is reversed (ΔEelstat:ΔEorb=55:45) (Table S23).Application EDA‐NOCV 2(NCBH3)2] complexes two anions are involved. This results, when going from [Fe(Lazine)2(NCBH3)2] to [Fe(Lazine)3]2+ (Table S23), in a large decrease in ΔEelstat stabilization  and a slight increase in ΔEorb stabilization . The same magnitude of increase in stability observed for the ΔEorb term is observed as an increase in ΔEPauli stabilization . This is consistent with the general trend that these two terms, ΔEorb and ΔEPauli, are intimately connected in describing the covalent bonding between fragments (Table S23). The NOCV analysis reveals that on stepping across the five Lazine ligand from L4pyrimidine  Lto pyridazine  that: 1) the σ bonds  involving the dz2and dx2-y2orbitals strengthen by about −5 to −10 kcal mol−1 per bond per step and 2) the π backbonds  involving the dxy, dzx, dzy orbitals strengthen by about −5 to −15 kcal mol−1 per bond per step .On the other hand, as before, the bonds involving s, pLazine. The σ strength  of the Lazine ligands follows the order:Analysis of the σ and π contributions shows that the σ interaction is almost eight times larger than the π interaction regardless of pyridazine>L4pyrimidine>L2pyrimidine>Lpyrazine>Lpyridine.LEorb,π) of the Lazine ligands differs :Interestingly the order of the π strength 2(Lazine)2] family, with an average magnitude decrease in ΔEorb,σ+π stability of about 30 kcal mol−1 between LS [Fe(Lazine)3]2+ and LS [Fe(Lazine)2(NCBH3)2] (Table S21 and Table S23):Adding those two contributions together gives Δ4pyrimidine>L2pyrimidine>Lpyridine>Lpyrazine>Lpyridazine.L6M+L) can provide a useful, general, quantitative and predictive tool—in this case to compare the field strength of a pair of ligands in different types of complexes 2(NCBH3)2] family,II(Lazine)3](BF4)2 complexes are all LS,3− anions by one bidentate Lazine ligand increases the ligand field experienced by the iron(II) centre. The ΔEorb,σ+π values 2(NCBH3)2]  and [Fe(Lazine)3]2+  show that replacement of 2x3NCBH− by one Lazine leads to an increase in the stabilization  of −33 kcal mol−1 2(NCBH3)2] family are SCO‐active whereas the [Fe(Lazine)3]2+ family are solely LS.In contrast to the SCO‐active , was chosen as the test system for this study, as the experimentally observed solution switching temperatures (T1/2) provided the order of Lazine ligand field strengths. Also, the chance to work on both spin states, diamagnetic LS and paramagnetic HS, enabled us to significantly increase the small handful of reports of EDA‐NOCV analysis of paramagnetic transition metal complexesM−L bonding in both of the thermodynamically accessible spin states, diamagnetic LS and paramagnetic HS.A family of SCO‐active Fexes, [FeLazine2(NCLazine)2(NCBH3)2] complexes was examined by EDA‐NOCV, the analysis identified a good correlation  between decreasing T1/2 and increasing ligand field strength as quantified by the ΔEorb,σ+π term. In addition, comparison of the results for [Fe(Lazine)2(NCBH3)2] with those subsequently obtained on the LS [Fe(Lazine)3](BF4)2 complexes revealed that only the corrected n+ + L6M fragmentation provides a general protocol suitable for comparing different types of complexes. It should be noted that the above analysis neglects any entropic contributions, which are known to be key in SCO, so the next big step in the development of this approach for applications in the SCO field will be understanding how the inclusion of computed entropic contributions can be included so that the unbiased determination of the T1/2 values on the basis of the EDA values will be possible.Regardless of whether the LS or HS family of [Fe(n+ + L6M) that provides a clear, quantitative and chemically intuitive description of the M−L bonds in these paramagnetic and diamagnetic transition metal complexes. This new protocol should be widely applicable, a point we are currently testing further (with more families of SCO and/or redox‐active 3d coordination complexes) in order to prove that it is general, thereby unlocking the great promise it holds as a predictive tool.In conclusion, the EDA‐NOCV protocol developed and validated herein employs a new and general fragmentation type  should be addressed to the authors.SupplementaryClick here for additional data file.


C‐terminal intrinsically disordered tail of PP1 has been demonstrated. The associated phenotype of cell‐cycle arrest was repeatedly proposed to be due to autoinhibition of PP1 through either conformational changes or substrate competition. Here, we use PP1 variants created by mutations and protein semisynthesis to differentiate between these hypotheses. Our data support the hypothesis that pThr exerts its inhibitory function by mediating protein complex formation rather than by a direct mechanism of structural changes or substrate competition.Phosphoprotein phosphatase‐1 (PP1) is a key player in the regulation of phospho‐serine (pSer) and phospho‐threonine (pThr) dephosphorylation and is involved in a large fraction of cellular signaling pathways. Aberrant activity of PP1 has been linked to many diseases, including cancer and heart failure. Besides a well‐established activity control by regulatory proteins, an inhibitory function for phosphorylation (p) of a Thr residue in the  Interrogating a mechanistic black box: Phosphoprotein phosphatase 1 (PP1) is among the major serine/threonine phosphatases and is involved in numerous pathways. Autoinhibition by phosphorylation of Thr320 at the C‐terminus leads to cell‐cycle arrest and neuronal defects. However, the mechanism underlying this regulatory phosphorylation site is unknown. Herein, we investigate potential mechanisms. In the PP1 structures deposited in the Protein Data Bank , whereas the second detected inorganic phosphate from dephosphorylation of a peptide carrying the sequence of the well‐established substrate site Thr3 of Histone 3 (H3pT3).[in vitro. This finding presents a contrast to the suggested mechanism for direct autoinhibition of PP1 by phosphorylation.[Since no structural rearrangements could be observed for the PP1α T320E variant, we next investigated the effect of the additional negative charge on PP1α on its enzymatic activity. PP1α T320E was tested in parallel with wild‐type (wt) PP1α  (H3pT3).21, 22in vitro auto‐dephosphorylation occurring in the required conditions for CDK2/cyclin A phosphorylation of recombinant PP1,in vitro phosphorylation of the recombinant protein is not suitable to study the effect of Thr320 phosphorylation on PP1 activity. Therefore, we designed a semisynthetic proteinPfa) , purified, and characterized (see the supporting information) before coupling to PP1α‐Δ316 by native chemical ligation, which yielded the semisynthetic PP1α variants LigT320 and LigPfa320  in parallel with the unphosphorylated semisynthetic enzyme (LigT320) in vitro, using DiFMUP and H3pT3 as substrates as described above. Introducing Pfa320 did not result in reduced activity compared to the unphosphorylated semisynthetic PP1α, consistent with our observations using PP1α‐T320E , we also tested the enzymatic activity after a limited trypsinolysis. As demonstrated previously, trypsin hydrolyzes the C terminus of PP1 but cannot readily digest its catalytic core.[a by prior trypsinolysis, consistent with a previously observed inhibitory effect of the C‐tail on the phosphatase activity. However, the effect of trypsinolysis was similar for the wt, T320E, LigT320 and LigPfa320 variants, indicating that the phosphomimetic proteins were inhibited to a similar extent by their C‐tail as the wt and LigT320 enzymes. . Substituting Asp64 in the catalytic center with Asn has previously been shown to yield a PP1 variant with strongly decreased activity,a. Titration of increasing amounts of PP1  into this reaction led to efficient dephosphorylation of glycogen phosphorylase a, without any evidence for intermolecular dephosphorylation of pThr320 on PP1αD64N‐pT320, providing further evidence that PP1α‐pThr320 is a rather poor substrate of the PP1 catalytic subunit alone for auto‐dephosphorylation in trans, irrespective of the PP1 isoform. To investigate whether dephosphorylation‐independent autoinhibition of PP1α via pThr320 could take place in trans, we tested the dephosphorylation of the PP1 substrate glycogen phosphorylase a in the presence of T320/Pfa320‐containing C‐tail peptides  compared to unphosphorylated PP1α or LigT320 mixed with T320E or LigPfa320 mutants, respectively, to dephosphorylate glycogen phosphorylase a . However, these experiments did neither reveal interactors above levels of background binding proteins, nor proteins binding the PP1C‐tail in a phosphorylation‐dependent manner with high fold changes.In an y Figure a. TwentyC‐terminal tail and its binding to the PP1 catalytic core. Consequently, our findings suggest a mechanism involving pThr320‐specific recruitment of (a) protein(s), which then inhibit PP1 directly or through an additional  modification, such as promoting oxidation of the Fe2+ in the catalytic center,PP1 plays a role in numerous signaling pathways, regulated by interacting proteins and by phosphorylation of its own C‐tail at residue T320.13, 15C‐terminal PxTPP sequence is due to an indirect mechanism, involving complex protein‐protein interactions. Therefore, future efforts should be focused on dissecting the possible cellular mechanisms of PP1 inhibition through C‐tail phosphorylation.In conclusion, the structural, semisynthetic and biochemical approaches taken here have clearly shown that, contrary to previous assumptions, the inhibitory effect of the phosphorylation of the 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.


I2(HCO2)3− and CuIIn(HCO2)n+12−, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with CuI and CuII copper ions is observed. For the CuI species, transitions between copper d and s/p orbitals are recorded. For stoichiometric CuII formate clusters, the spectra are dominated by copper d–d transitions and charge‐transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low‐lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In CuII(HCO2)3−, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge‐transfer excitations, specific excited‐state reaction channels are observed in addition, such as formyloxyl radical loss. In CuI2(HCO2)3−, the system relaxes to a local minimum on an excited‐state potential‐energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a CuII(HCO2)3Cu0− biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.The electronic structure and photochemistry of copper formate clusters, Cu Oxidation state is key: Copper formate clusters in oxidation states +I and +II in the gas phase are investigated by using a combination of UV/Vis spectroscopy and quantum chemical calculations. The electronic transitions and photochemistry are governed by the oxidation state. Whereas ligand‐to‐metal charge‐transfer states in CuII can lead to photochemical formyloxyl radical loss, ejection of a neutral copper atom is observed in the CuI species. With global warming becoming an ever‐increasing problem, efficient activation and transformation of carbon dioxide becomes a desirable option for carbon capture and usage (CCU). Transformation into formic acid and methanol are very attractive candidates.+ center.2CuO5+ complexes have been found to selectively convert methane into CH2O.2+(H2O)n, underwent charge separation until a critical size of n=8.For a mechanistic understanding, gas‐phase investigations of well‐defined, size‐selected model systems, in combination with quantum chemical calculations, are useful.II formate anions have been found to fragment in a cascade towards clusters with one or two copper centers, upon which redox reactions can be observed, followed by decarboxylation towards CuI hydrides as the predominant process.+ core thought to be responsible for many reactions in nature within copper‐containing enzymes has been characterized,+ ion.3)3− anion, ligand‐to‐metal charge‐transfer (LMCT) transitions and transitions within the nitrate ligand were observed in the UV region.3 radical, similar to collision‐induced dissociation (CID) experiments, which indicates efficient internal conversion to the electronic ground state.Recently, we showed that the formation of formic acid within copper formate clusters took place in the gas phase in a redox reaction through proton‐coupled electron transfer for clusters with two copper centers, whereas it proceeded through hydrogen atom transfer with a single copper center.Herein, we present an investigation into the electronic structure of copper formate clusters in the gas phase by using UV/Vis spectroscopy along with quantum chemical calculations. We also explore selected photochemical reaction pathways.λ=225 to 2600 nm. The obtained photodissociation spectra were corrected for fragmentation caused by blackbody infrared radiative dissociation (BIRD)2 prism and window. The beam profile was estimated as a homogenous Gaussian beam with a divergence of 0.002 mrad and a beam diameter of 0.005 m at the laser output, along with a constant output angle, leading to uncertainties in the absolute cross‐section calculations. To account for changes in the laser beam profile and alignment upon switching between the different laser stages, a correction factor was applied at the signal to idler beam transition (>710 nm), signal to UV stage (<410 nm), and SF/SH beam transition (<296 nm). Here, the cross section of the signal region was taken as a reference because this was the most reliable value.Mass selected, isotopically enriched copper‐63 formate cluster anions were produced through electrospray ionization . To compute electronic transitions in larger clusters, time‐dependent (TD) DFT with the BMK functional was applied, which was chosen based on its performance compared with EOM‐CCSD for copper formate in oxidation state +II, see Table S5 in the Supporting Information. Orbitals participating in electronic transitions were analyzed by using the natural transition orbital (NTO) scheme at the TD‐BMK/aug‐cc‐pVDZ//B3LYP/def2TZVP level of theory. All calculations were carried out with the Gaussian 16 program.For the computational description of the excited states, equation of motion‐coupled cluster singles and doubles (EOM‐CCSD) calculations were performed for small clusters (<150 electrons). The aug‐cc‐pVDZ basis set was used here because it provided results close to the triple‐zeta basis set, see Table S5 in the Supporting Information. All excited states within 6 eV, or at least 12 states (in the case of convergence issues), were considered. For structural optimization in the electronically excited states, the 6‐31+g* basis set was employed. Near conical intersections, the optimization started to oscillate. Therefore, reduced convergence criteria were used (predicted change in energy <2×10I2(HCO)3−, CuII(HCO)3−, CuII2(HCO)5−, CuII3(HCO)7−, and CuII8(HCO)17− are shown in the 0.9–5.5 eV region, along with the calculated electronic transitions for selected isomers. The CuI system, CuI2(HCO)3−, exhibits absorptions only in the UV region, starting at about 4.0 eV. The CuII systems exhibit two absorption bands in the 1.0–2.4 and 3.5–5.5 eV regions. The intensity of the absorption bands is comparable for all CuII clusters, with the low‐energy absorption being about five times less intense than that of the high‐energy one.In Figure II8(HCO)17−.There is no significant shift in energy observed between systems with one or two copper centers. We can therefore conclude that the bonding interaction between two neighboring copper centers in stoichiometric copper(II) formate clusters is marginal. Instead, the oxidation state of the copper center has the largest influence on the spectra. In the UV spectrum of larger clusters, the electronic transitions appear with a less pronounced maximum and occur deeper within the UV region for the largest case, CuI compounds and low‐lying states in CuII species, a very good agreement with the experiments is reached. The higher‐lying states in CuII compounds, on the other hand, are shifted systematically to higher energies, see, for example, the CuII(HCO)3− spectrum in Figure The calculated EOM‐CCSD electronic transition energies are shown in Figure II2(HCO)5− and the large Cu−Cu distance in copper formate clusters.In contrast to the strong effect of the formate ligands on the transitions, the change in the number of copper centers from one to two does not significantly change the position of the transitions, in agreement with experiment. This is also consistent with the previously described extremely weak antiferromagnetic coupling of the unpaired electrons in CuThe observed absorption band structures can be rationalized by the electronic configuration of the copper ion Scheme . In oxidII, however, makes low‐lying d–d transitions and LMCT transitions from the negatively charged formate ligands possible, which correspond to the absorption bands at 1–2 and 3.5–5.5 eV, respectively  only affords excitations from the filled d orbital into the empty s/p shells, which corresponds to the band starting at about 4 eV 3−, CuII(HCO2)3−, and CuII2(HCO2)5− confirms these qualitative arguments 3−, a transition from copper d‐type orbitals into the unoccupied copper s/p orbitals is observed. In the ground state of CuI2(HCO2)3−, the 4 s/p orbitals of the two copper ions are unoccupied. The fully filled d orbitals have a nonbonding character for isomer 1, whereas isomer 2 exhibits a Cu−Cu σ bond. This means a very limited bonding interaction between the two copper centers in S0 for isomer 1, whereas isomer 2 has a bonding interaction, which is reflected in the bond length of d(Cu−Cu)=3.3 and 2.5 Å in isomers 1 and 2, respectively. Comparing the isomers, one can see why excitation energies within CuI2(HCO2)3− are so sensitive to the orientation of the formate ligands. In isomer 1, two s orbitals form a bonding σ(Cu−Cu) orbital that is compressed by the presence of formate ligands, which extends into the space along the Cu−Cu axis. The single electron is thus confined to a relatively narrow space, which results in a less favorable electronic state. In isomer 2, the s/p orbitals extend freely along the Cu−Cu axis, shifting the first excitation energy by about −1.0 eV. The target orbitals are almost degenerate, irrespective of their phase, showing the independence of both copper ions; see also below for a discussion on the resulting photodynamics.Our analysis of orbitals participating in electronic transitions for CuII(HCO2)3− 3−, with only monodentate formate ligands 3− is in good agreement with the previously studied CuII(NO3)3− anion, for which TDDFT calculations predict d–d and LMCT transitions, along with energetically higher transitions on the nitrate ligand.For CuIIHCO3− 5− are independent. This is reflected in the first two participating orbitals of each absorption band illustrated in Figure d(Cu−Cu)=3.0 Å.The singly occupied d orbitals of the copper centers in CuI2(HCO2)3−, CuII(HCO2)3−, and CuII2(HCO2)5−, as representative examples. The predicted reaction channels for the most intense fragments of the three species are listed in Table I2(HCO2)3−, we observe CuI2H3−, CuII(HCO2)2H−, CuI(HCO2)2−, and CuI(HCO2)H−, reactions (1 a)–(1 d), with roughly constant branching ratio across the absorption band. Reaction (1 a) includes three decarboxylation steps, requiring 2.29 eV, which is the same decomposition pathway as that observed in infrared multiple photon dissociation (IRMPD) experiments.II(HCO2)2H−, reaction (1 b). Alternatively, they could lose a neutral copper formate unit and partially decarboxylate to form CuI(HCO2)2− or CuI(HCO2)H−, reactions (1 c) or (1 d), respectively. Reactions (1 b)–(1 d) have not been observed upon IR heating,In Figure II(HCO2)3− and CuII2(HCO2)5−, fragment branching ratios depend considerably on wavelength 3−, all observed fragments, namely, CuII(HCO2)2H−, CuI(HCO2)2−, CuI(HCO2)H−, and Cu0(HCO2)−, can be explained by previously observed reactions (2 a)–(2 d′) upon IR heating.For CuII2(HCO2)5−, mainly the formation of CuI2(HCO2)3−, CuI2(HCO2)2H−, and CuII(HCO2)3− is observed, with appearance energies in the range of 1.0–1.2 eV, see reactions (3 a)–(3 c) in Table II(HCO2)2H− and CuI(HCO2)2− are formed through reactions (3 d) and (3 e), albeit with low intensity. For energetic reasons, two photons are required for reactions (3 d) and (3 e), indicating that these ions are secondary products of CuII(HCO2)3−. Reactions (3 a)–(3 e) have all been observed previously in IRMPD experiments,2 (reaction (3 a)). Subsequent CO2 elimination from the product is energetically feasible, and becomes more important with increasing photon energy (reaction (3 b)). The entropically favored product, CuII(HCO2)3−, reaction (3 c), requires slightly higher photon energies, in line with experiments, and becomes dominant at the high‐energy end of the absorption band.For the low‐energy band of CuII(HCO2)2H− is the dominant fragment, followed by CuI(HCO2)2− (reactions (3 d) and (3 e)). Both reactions require more than 2.4 eV.II2(HCO2)5− are only observed in trace amounts, such as CuII(HCO2)3− and CuI2(HCO2)3−. Only one fragment is observed that is not part of the IRMPD cascade, namely, CuI/II2(HCO2)4−, reaction (3 f), which is comparable in intensity to CuI(HCO2)2−. Formation of the formyloxyl radical requires an energy of 2.08 eV. Notably, HCO2 might further decompose into H and CO2, with a reaction energy of −0.30 eV and a barrier of 0.16 eV. Alternatively, a HOCO unit might be formed with a reaction energy of −0.36 eV. Given that the UV absorption band is due to a LMCT transition, elimination of the neutral formyloxyl radical seems entirely plausible because the alternatives require more extensive rearrangements. The HCO2 loss channel after LMCT would also be feasible for CuII(HCO2)3− through reaction (2 e).In the UV absorption band in the 3.3–5.5 eV region, CuI2(HCO2)3−, we calculated excited‐state potential‐energy curves along the vector connecting the minima of ground and first excited states by using internal coordinates. Figure 1. During geometry optimization, the two monodentate formate ligands rotate out‐of‐plane and the Cu−Cu distance decreases from 3.3 to 2.3 Å, which indicates a strong Cu−Cu interaction due to the occupation of a bonding orbital in S1. The resulting Cu−Cu distance is close to the bond length of 2.35 Å in the Cu2− ion.1 minimum lies 1.8 eV below the S1 energy in the FC point. Relaxation through fluorescence into the ground state would be substantially redshifted with respect to the excitation, since the S1 minimum lies only 1.3 eV above the respective ground‐state structure. Consequently, a total of (hv−1.3 eV) would be available after fluorescence as vibrational energy to support the fragmentation cascade leading to Cu2H3−, which requires 2.29 eV.1 local minimum at which the extrapolated ground‐ and excited‐state potential‐energy curves nearly coincide, and the entire photon energy would be available for fragmentation on the ground‐state potential‐energy surface. The photodynamics thus rationalize the formation of CuI2H3−, as previously observed in the IRMPD cascade of CuI2(HCO2)3−.To gain some insight into the photodynamics of Cu1 state, the system relaxes to a local minimum with a slightly elongated Cu−Cu bond , which does not seem to be relevant for the observed photochemistry. Within reach, however, there is a conical intersection seam that is, at most, 0.1 eV above the excitation energy of S1 in the FC point. This is easily surpassed with the thermal energy content of the cluster ion before excitation, in addition to surplus energy from excitation into higher states in the experiment. Passing through this conical intersection, the system reaches a minimum with pronounced biradical character 3Cu0−, with the excited state manifold separated by only about 0.6 eV from the ground‐state surface (EOM‐CCSD/6‐31+g*). To reach the biradical minimum, one bridging formate ligand is transferred towards the CuII center, along with an elongation of the Cu−Cu bond length to 2.7 Å. Furthermore, the Cu−O bonds on the bridging formate ligands are elongated on the Cu0 center, providing space for its singly occupied s orbital. In the ground electronic state, the CuI oxidation state for both copper atoms is maintained. It is conceivable that a neutral Cu0 atom can be released from this structure at a cost of 3.18 eV, with respect to the FC point. With the remaining energy, the CuII(HCO2)3− structure can then sequentially decarboxylate for reaction (1 b), or even afterwards lose a hydrogen radical and another CO2 (see reactions (2 b) and (2 c)) to give rise to the CuII(HCO2)2H−, CuI(HCO2)2−, and CuI(HCO2)H− fragments. An internal conversion to the ground state becomes accessible through a conical intersection close to this local minimum 3−, all products already appear in the established IRMPD cascade, sharing a sequential reaction pathway (see reactions (2 a)–(2 c)) or branching from it in smaller amounts (see reactions (2 d) and (2 d′)).0/D1 and D4/D5 conical intersections, again using the internal coordinates. Excitation into the first excited state, D1 3− into the lowest lying state of the second absorption band, D5, the bidentate formate ligand starts dissociating on the way towards a D4/D5 conical intersection, which is reached again without any barrier 3−. This behavior can be explained by the charge‐transfer character of the LMCT excitation 2− unit resembles the most stable isomer of the product ion, CuI(HCO2)2−. After passing the D4/D5 conical intersection, the ion might either follow the dissociation pathway and evaporate a neutral HCO2 radical or eventually reach the ground state, as discussed above. HCO2 dissociation from CuII(HCO2)3− in reaction (2 e) is in good agreement with the experimental observation of CuI(HCO2)2−, which exhibits the highest branching ratio in the LMCT band in Figure 2 loss for CuII(HCO2)3− cannot be distinguished from sequential dissociation of CO2 and H in reaction (2 b), as observed in the IR experiment.For the excitation of CuIIHCO3− into 2(HCO2)4− from CuII2(HCO2)5− in Figure II2(HCO2)5−.II formate clusters can also reach the electronic ground state through conical intersections and redistribute the full photon energy in the system, similar to CuII(HCO2)3−.On the contrary, the formation of CuII(HCO2)3− and CuI2(HCO2)3− from CuII2(HCO2)5−, within the d–d band in Figure II2(HCO2)5−, followed by the formation of formic acid, features a tight transition state. The competitive evaporation of neutral copper diformate CuII(HCO2)2 can be expected to be entropically favored. However, formic acid formation through reactions (3 a) and (3 b) requires 0.89 eV, compared with 1.36 eV for reaction (3 c), as calculated at the B3LYP/def2TZVP level.I2(HCO2)3−, along with its sequential decarboxylation product, CuI2(HCO2)2H−, is observed with high selectivity at the start of the absorption band at 1.0 eV, at which the entropically favored evaporation is not yet accessible. Once enough energy is introduced into the ion, the evaporation of neutral copper formate becomes the predominant channel at about 1.5 eV. The production of formic acid can be expected to be highly temperature dependent in catalytic processes, due to the high energy dependence between the two competing reaction channels.Due to this rapid internal conversion, the competitive formation of CuII formate clusters  and depend mainly on the absorption band. For the d–d transitions in CuIIn(HCO2)n+12−, n>2, predominantly the evaporation of neutral copper formate is observed, whereas, for n=1 or 2, decarboxylation happens as the first step, as also observed in IRMPD experiments.II3(HCO2)7−. However, for a large copper formate cluster, CuII8(HCO2)17−, no formyloxyl radical dissociation is observed. Instead, only evaporation of neutral copper formate clusters, similarly to IRMPD,The decomposition trends stay roughly the same for larger CuThe electronic structure of copper formate clusters has been investigated in the gas phase by using UV/Vis spectroscopy between 0.9 and 5.5 eV. For copper centers in oxidation state +I, d–s/p transitions on the copper centers have been observed within the UV region. Copper formate with copper centers in oxidation state +II exhibit copper d–d transitions in the visible/near‐infrared range, along with intense LMCT transitions, which involve the formate ligands and the singly occupied d orbital on the copper center in the UV region.IIn(HCO2)n+12 − does not change the observed electronic transitions, and thus, provides evidence for the absence of copper–copper bonding, with a singly occupied d orbital at each copper center. The orientation of the formate ligands, however, plays a key role in the spectrum, heavily shifting the intensity and energetic position of the transition.The number of copper atoms in CuI2(HCO2)3− depends heavily on the ground‐state structure. Isomer 1, with one bridging formate ligand, most likely dissociates statistically in the electronic ground state, either after fluorescence from a local minimum in S1 or by reaching a conical intersection in the vicinity of the S1 local minimum. For the paddlewheel structure of isomer 2, Cu−Cu charge transfer initiates the photochemical loss of a neutral copper atom, which competes with statistical dissociation in the electronic ground state. Copper formate clusters in oxidation state +II are able to relax ultrafast back to the ground state through conical intersections in both excitation bands, and the redistributed energy is available for statistical dissociation. However, in the UV region, LMCT makes photochemical formation of formyloxyl radicals possible. For large copper formate clusters, no formyloxyl radical loss is observed, which indicates that the photon energy is redistributed into vibrational degrees of freedom. Through the efficient energy redistribution by conical intersections, the selectivity in the formation of formic acid can be tuned via the photon energy in the d–d transition of CuII2(HCO2)5−. Higher energies favor entropically preferred dissociation pathways with loose transition states, such as the evaporation of CuII(HCO2)2. A large temperature dependence in the formic acid yield is therefore expected under catalytic conditions.The photochemistry of CuThe 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.