Theoretical modeling of the O–H stretching IR bands of hydrogen-bonded dimers of benzoic acid in S and S1 electronic states

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Abstract
Theoretical model for vibrational interactions in the hydrogen-bonded dimer of benzoic acid is presented. The model takes into account anharmonic-type couplings between the high-frequency O–H and the low-frequency O⋯O stretching vibrations in two hydrogen bonds,resonance interactions (Davydov coupling) between two hydrogen bonds in the dimer, and Fermi resonance between the O–H stretching fundamental and the first overtone of the O–H in-plane bending vibrations. The vibrational Hamiltonians and selection rules for the C 2 h geometry in the S 0 state and for the C s in-plane bent geometry in the S 1 state of the dimer are derived. The model is used for theoretical simulation of the O–H stretching IR absorption bands of benzoic acid dimers in the gas phase in the electronic ground and first excited singlet states. Ab initioCIS and CIS ( D ) ∕ 6 - 311 + + G ( d , p ) calculations have been performed to determine geometry, frequencies, and excited state energies of benzoic acid dimer in the S 1 state.