DFT versus Møller-Plesset conformational profile and vibrational assignments of non-planar phenoxyacetic acid and 2,3,4,5,6-pentafluorophenoxyacetic acid

The structural stability of phenoxyacetic acid and 2,3,4,5,6-pentafluorphenoxyacetic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. For the parent acid the calculations were extended to the MP4(SDQ) level of theory. At the DFT-B3LYP level of calculation the planar Tttp (transoid OCOH) was predicted to be about 0.5 and 1.3 kcal/mol lower in energy than the non-planar Cgcpp and Tgcpp (cisoid OCOH) forms, respectively. At the MP2 and the MP4(SDQ) levels the Cgcpp form was predicted to be about 0.8 and 1.4 kcal/mol lower in energy than the Tgcpp and the Tttcp structures, respectively. On the basis of the Møller-Plesset calculations the Cgcpp and the Tgcpp conformations were adopted as the low and high energy structures of phenoxyacetic acid. The observed spectral intensities of phenoxyacetic acid were consistence with the Cgcpp conformation being the predominant form of the acid at room temperature. At the DFT and MP2 levels of theory 2,3,4,5,6-pentafluorophenoxyacetic acid was predicted to exist predominantly in the Cgcpp structure. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of both molecules.