Conformational analysis of carboxylic acid anhydrides: A microwave and computational study

The microwave spectra of four carboxylic acid anhydrides, RCOOCOR′, (R,R′) = (CH₃, CF₃), (C(CH₃)₃, CF₃), (C₆H₅, CF₃) and (CH₃, C(CH₃)₃), have been observed in a supersonic jet. Calculations at the M06-2X/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory predict the lowest energy conformations for all four species to be nonplanar cis structures, i.e., conformations in which the C=O groups point in approximately the same direction, but are twisted out of a coplanar orientation. The observed spectra are consistent with these predictions. In addition, for all but the (R,R′) = (CH₃, C(CH₃)₃) species, higher energy nonplanar trans conformers are also predicted, typically within 1–2 kcal/mole the nonplanar cis form. For (R, R′) = (C(CH₃)₃, CF₃), extensive isotopic substitution has enabled a determination of most of the (non-fluorine) heavy atom structural parameters. Excellent agreement with the DFT and MP2 structures was obtained, thus validating the theoretical methods used. A strong correlation is found between the calculated O=C⋯C=O dihedral angle and the average of the vapor phase C=O stretching frequencies of RCOOH and R′COOH.