“Troublesome ” Vibrations of Aromatic Molecules in Second-Order Mo1ller-Plesset and Density Functional Theory Calculations: Infrared Spectra of Phenol and Phenol-OD Revisited

The infrared spectra of phenol and phenol-OD are thoroughly reinvestigated, to resolve the contradictory assignment of some vibrations. The harmonic frequencies, integrated IR intensities, and potential energy distribution (PED) have been calculated by the B3LYP method with the 6-311++G(df,pd) basis set. The Fourier transform infrared (FT-IR) spectra of phenol and phenol-OD have been measured in carbon tetrachloride and cyclohexane solutions, in the frequency range 3700-400 cm-1, and the experimental integrated infrared intensities are reported. On the basis of the results obtained, the detailed assignment of all the fundamental modes of Ph-OH and Ph-OD are presented. The study demonstrates that density functional B3LYP is clearly superior to the ab initio Hartree-Fock (HF) and second-order Möller-Plesset (MP2) methods in reliable prediction of the vibrational spectra of phenol. In particular, it is shown that scaling of the B3LYP-calculated frequencies of the CH and OH(OD) stretching vibrations by the scaling factor, derived by Baker et al. [J. Phys. Chem. A 1998, 102, 1412] gives excellent agreement between theoretical and experimental frequencies of these vibrations. Detailed theoretical investigations are performed for these troublesome normal modes in phenol and benzene, which show the largest deviations between the MP2-predicted frequencies and the experimental ones. It has been demonstrated that these modes have almost identical atomic displacements and potential energy distributions in both the molecules. The electron correlation effects and basis set dependences are examined, and the nature of these problematical vibrations in aromatic molecules is discussed