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Anharmonic midinfrared vibrational spectra of benzoic acid monomer and dimer

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Helden,  Gert von
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Meijer,  Gerard
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Citation

Antony, J., Helden, G. v., Meijer, G., & Schmidt, B. (2005). Anharmonic midinfrared vibrational spectra of benzoic acid monomer and dimer. The Journal of Chemical Physics, 123, 014305-1-014305-11. doi:10.1063/1.1947191.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0011-082F-C
Abstract
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 redshifts 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 blueshifted 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). Nonperturbative 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 semidiagonal quartic force constants only. This emphasizes the need for high-dimensional, nonperturbative anharmonic calculations at high quantum-chemical level when accurate frequencies of H-atom vibrations in double hydrogen bonds are sought for.