Theoretical model of infrared spectra of hydrogen bonds in molecular crystals of 2-thiopheneacetic acid: Fermi resonance and Davdov coupling effects.

Abstract

A general quantum theoretical approach of the υX-H IR lineshape of cyclic dimers of moderately H-bonded species in the crystalline phase is proposed. In this model, the adiabatic approximation is performed for each separate H-bond bridge species of the dimer, a strong non-adiabatic correction is introduced into the model via the resonant exchange between the fast mode excited states of the two moieties, the intrinsic anharmonicity of the low-frequency mode through a Morse potential, direct and indirect damping, and a selection rule breaking mechanism for forbidden transitions in IR. The present model reduces satisfactorily to many models in the literature dealing with more special situation. It has been applied to the cyclic dimers of 2-thiopheneacetic acid (2-TAA) and his deuterated derivative in the crystalline phase. It correctly fit the experimental line shape of the hydrogenated compound and predicts satisfactorily the evolution in the line shapes with isotopic substitution. Numerical calculations show that mixing of all these effects allows one to reproduce satisfactorily the main features of the experimental IR line shapes of hydrogenated and deuterated 2-TAA crystals and is expected to confirm the importance of the Fermi resonances and Davydov coupling in reproducing the experimental spectra