Conformations and vibrations of dicarboxylic acids. An ab initio study

The conformations of the higher dicarboxylic acids HOOC(CH₂)_nCOOH, n = 1-4, the hydroxymalonic acid have been studied at the HF/4-21G level of theory. The conformational characteristics of these acids have been elucidated through potential energy scans at this level. The vibrational frequencies of all the dicarboxylic acids have also been evaluated at this level. Calculations at the HF/6-31G** level of theory were also carried out on malonic, hydroxymalonic and succinic acids. They have been assigned to the solid state Raman and IR spectra of the dicarboxylic acids. The potential energy distributions, Raman activities and IR intensities calculated at the levels indicated have been used as aids in making the assignments. The calculated frequencies have also been compared with the frequencies evaluated at the MP2/4-31G* level for oxalic acid.