Davydov coupling as a factor influencing the H-bond IR signature: Computational study of the IR spectra of 3-thiopheneacrylic acid crystal

Abstract

In this work, we have studied the role played by Davydov coupling in the evolution of the fine structure of the IR spectra of 3-thiopheneacrylic acid dimer crystal. For this purpose, we took into account the linear dependence of the Davydov coupling on the position coordinate Qi(i=1, 2) of the slow frequency O←H…O→ mode of the two H-bond bridges of the cyclic dimer. The Davydov coupling VD term was expanded up to first order with respect to the slow mode coordinate Qi. We have considered: VD(Q1,Q2)=VD0+Θ(Q1+Q2). That leads to the anharmonic expression of the Davydov coupling parameter VD: VDQg=VD0+2ΘQg within the symmetrized coordinates. The effect of the second order parameter (Θ) of Davydov coupling on to the spectral density was undertaken. The numerical calculations clearly show that the crystal spectral properties of the infrared υOH stretching band remain in a close relation with the new anharmonic coupling parameter and provide a direct evidence of the increase of the level density and the spectral broadening.