Computational study of the infrared spectrum of acetic acid, its cyclic dimer, and its methyl ester

The IR spectra of monomeric acetic acid, both its trans and cis form, its cyclic dimer, and its methyl ester, were studied using the BLYP, PW91 and B3LYP DFT methods. As expected, comparison with previously reported Hartree-Fock and MP2 level calculations revealed that DFT calculated frequencies are always better for all species studied. Regarding vibrational frequencies, BLYP outperforms B3LYP and PW91. Larger basis sets, e.g. 6-31++G(d,p), do not lead to significantly better results compared to the smaller 6-31G(d) basis. For infrared intensities, however, B3LYP seems to reveal better agreement with experiment.