Rotational spectroscopy of the atmospheric photo-oxidation product XXXX and its monohydrate.

@entity3884 @entity4885 @entity4880 @entity41971 @entity5498
UNASSIGNED: @entity41971 , a photo-oxidation product in the atmosphere, and its monohydrate were characterized in the gas phase by pure rotational spectroscopy. High-resolution spectra were measured in the range of 5-14 Hz using a cavity-based molecular beam Fourier-transform microwave spectrometer. Possible conformers were identified computationally, at the @entity4885 /6-311++G(2df,2pd) level of theory. For both species, one conformer was identified experimentally, and no @entity5498 internal rotation splittings were observed, indicative of relatively high barriers to rotation. In the monomer, rocking of the @entity4880 group is a large amplitude motion, characterized by a symmetrical double-well potential. This and other low-lying out-of-plane vibrations contribute to a significant ( @entity5498 top-corrected) inertial defect (-1.09 amu   (2)). In the monohydrate, wagging of the free @entity3884 atom of water is a second large amplitude motion, so the average structure is planar. As a result, no c-type transitions were observed. Water tunneling splittings were not observed, because the water rotation coordinate is characterized by an asymmetrical double-well potential. Since the minima are not degenerate, tunneling is precluded. Furthermore, a concerted tunneling path involving simultaneous rotation of the water moiety and rocking of the @entity4880 group is precluded, because the hilltop along this coordinate is a virtual, rather than a real, saddle-point. Inter- and intramolecular non-covalent bonding is discussed in terms of the quantum theory of atoms in molecules. The percentage of @entity41971 hydrated in the atmosphere is estimated to be about 0.1% using statistical thermodynamics.
