Microwave spectra of two carboxylic acid anhydrides: Acetic anhydride and acetic difluoroacetic anhydride

Microwave spectra of acetic anhydride, D6-acetic anhydride, and acetic difluoroacetic anhydride have been observed in a supersonic jet. In conjunction with accompanying DFT and MP2 calculations, these systems are shown to adopt a nonplanar configuration in which the C=O groups point in approximately orthogonal directions. Methyl group internal rotation was fully analyzed for both species. The observed conformation of these systems appears to result from an interaction between a CH₃ hydrogen (in acetic anhydride) or the CF₂H hydrogen (in acetic difluoroacetic anhydride) with the carbonyl group to which it is not directly bound, forming a six-membered ring. The fitted rotational constants for both systems are in reasonably good agreement with calculated values, but for acetic anhydride, the agreement is somewhat worse than that previously observed for a series of syn anhydrides. The calculations indicate a pronounced flexing of the heavy atom frame as the CH₃ group in the six-membered ring undergoes internal rotation, and this likely influences the level of agreement between the theoretical and vibrationally averaged experimental constants. The other CH₃ group does not interact with a carbonyl oxygen because of its orientation in the molecule, and its internal rotation does not induce similar changes in the molecular frame. In the acetic difluoroacetic anhydride, it is the CF₂H hydrogen that interacts with its remote carbonyl oxygen, leaving the internally rotating CH₃ group unaffected by participation in a six-membered ring and giving rise to much smaller deviations in the rotational constants as it moves along its internal rotation coordinate. Correspondingly better agreement between experimental and theoretical spectroscopic constants is obtained.