Density Functional, Hartree−Fock, and MP2 Studies on the Vibrational Spectrum of Phenol

Vibrational spectra of phenol are calculated with ab initio Hartree−Fock and MP2 methods as well as with density functional theory (DFT) using the 6-31G(d,p) basis set. A clear-cut assignment of the vibrational frequencies is reported on the basis of the potential energy distribution (PED) calculated at the three theory levels. These results are compared with the previously reported ab initio data and with the experiment. Several reassignments are suggested for the phenol modes: OH bend, 9b, 17a, 8a, and 8b. It is demonstrated that the MP2/6-31G(d,p) level fails in predicting the frequencies for two modes, labeled 14 and 4 in phenol. The calculated frequency of the former is about 140 cm -1 too high, and that of the latter is 220 cm -1 too low. Very similar results at the MP2 level have been reported earlier for the corresponding ω 14 and ω 4 in benzene. The HF/6-31G(d,p) method provides incorrect results for the modes related to the OH bend in phenol. It is remarkable that DFT with the BLYP functional gives excellent agreement between the calculated and observed frequencies for phenol. In particular, the modes 4 and 14 are predicted to within 11 and 6 cm -1, respectively, which confirms the reliability of DFT (BLYP) in reproducing vibrational frequencies.