Profiles of hydrogen stretching ir bands of molecules with hydrogen bonds: A stochastic theory. I. Weak and medium strength hydrogen bonds

Abstract

A theory is proposed to describe the shape, in inert solutions, of hydrogen stretching ir bands of complexes with hydrogen bonds. The theory uses stochastic arguments and is related to general theories of ir band profiles of liquids. The cases of weak and medium strength hydrogen bond are treated separately. In the former case the band profile is, essentially, a broad asymmetrically distorted Gaussian produced by an anharmonic coupling between the high frequency AH stretching mode and the perturbed low frequency AH⋅⋅⋅B stretching mode. In the latter case the profile is, essentially, a very broad Gaussian intersected by a number of flat or deep, broad or narrow Evans-type transmission windows. Here, the band shaping mechanisms are the anharmonic coupling between the AH stretching and different low frequency external motions and the Fermi resonance between states involving the AH stretching and some other internal modes. Spectral manifestations of hydrogen bonded liquids represent the most extreme case of vibrational broadening and the presence of the Evans holes is its characteristic feature.