Anharmonic Midinfrared Vibrational Spectra of Benzoic Acid Monomer and Dimer
Jens Antony, Gert von Helden, Gerard Meijer, and Burkhard Schmidt
Anharmonic vibrational calculations for the benzoic acid monomer and dimer in the mid-IR regime (500...1800 cm-1) are reported. Harmonic frequencies and intensities are obtained at the DFT/B3LYP level of theory employing D95(d,p) and cc-pVTZ basis sets. Anharmonic corrections obtained from standard perturbation theory lead to red shifts of 1...3 %. In almost all cases, the resulting frequencies deviate by less than 1 % from previous measurements [Bakker et al., J. Chem. Phys. 119, 11180 (2003)]. Calculated intensities are in qualitative agreement with the absorption experiment, with the cc-pVTZ values being superior to the D95(d,p) ones for a few modes of the dimer. The antisymmetric out-of-plane bending mode of the dimer, which is strongly blue-shifted with respect to the monomer frequency, represents a remarkable exception: The harmonic frequencies obtained for the two basis sets differ notably from each other, and the anharmonically corrected frequencies deviate from the experimental value by 8 % (D95(d,p)) or 3 % (cc-pVTZ). Non-perturbative calculations in reduced dimensionality reveal that the relatively small total anharmonic shift (few tens of cm-1) comprises of partly much larger contributions (few hundreds of cm-1) which are mostly canceling each other. Many of the individual anharmonic couplings are beyond the validity of second order perturbation theory based on cubic and semi-diagonal quartic force constants only. This emphasizes the need for high-dimensional, non-perturbative anharmonic calculations at high quantum chemical level when accurate frequencies of H-atom vibrations in double hydrogen bonds are sought for.