Analysis of the vibrational spectra of 3,4-dihydroxy- and 4-hydroxy-3-methoxyphenylacetic acids and the ring breathing mode in their raman spectra

The structural stability of 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid were investigated using the DFT-B3LYP and the ab initio MP2 methods together with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (np) forms as the lowest energy conformer of the acids. Another very low lying conformer of the former acid in the gas phase seems to play no role in the condensed phase. The observed spectral intensities of the acids were consistent with the np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and vibrational assignments were provided on the basis of combined theroretical and experimental infrared and Raman data of the molecules. As reported earlier in the case of chlorine substituted anilines, the ring breathing mode, which is expected to have a very high intensity in the Raman spectra of substituted benzenes, diminishes in intensity with increasing electron density in the ring (activation for electrophilic attacks).