Abstract
This work presents a theoretical simulation of ?O?H and ?O?D band shapes in the polarized infrared spectra of 2-furoic acid dimer crystals measured at liquid-nitrogen temperature. The line shapes are studied theoretically within the framework of the anharmonic couplings between low-frequency hydrogen-bond vibrations and degenerate excited states of high-frequency hydrogen vibrations in hydrogen-bonded dimers and the anharmonic coupling between the first excited state of the fast mode and the harmonics or band combinations of some low-frequency bending modes, which lead to Fermi resonances.This approach takes into account the adiabatic approximation, the intrinsic anharmonicity of the low-frequency mode through a Morse potential, Davydov coupling triggered by resonance exchange between the excited states of the fast modes of the two hydrogen bonds involved in the cyclic dimer, and the direct and indirect damping of the fast-stretching modes of the hydrogen bonds and of the bending modes. The infrared spectral density was calculated within the linear response theory by Fourier transform of the autocorrelation function of the transition dipole moment operator of the fast mode. Numerical results show that mixing of all these effects allows satisfactory reproduction of the main features of the experimental IR line shapes of crystalline H- and D-bonded 2-furoic acid at liquid-nitrogen temperature and for different polarizations. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012