On the similarity of the IR lineshapes of weak H-bonds in the gas and liquid phases: Quantum combined effects of strong anharmonic coupling, multiple Fermi resonances, weak dampings and rotational structure

The broadening of the ν S ( X - H → ⋯ Y ) infrared lineshapes of weak H-bonds in gas phase was studied within the linear response theory. Working within the strong anharmonic coupling theory, according to which the ν S ( X - H → ⋯ Y ) high-frequency mode is anharmonically coupled to the H-bond bridge, and after performing the adiabatic approximation, we incorporated multiple Fermi resonances between the high-frequency mode and harmonics of some bending modes, as well as the rotational structure and Doppler effect. At last, we introduced the direct relaxation of the fast mode, the indirect one of the H-bond bridge, and the damping of the involved bending modes. We showed that when passing from the condensed to the gas phase, that is when reducing the damping parameters magnitude and introducing the rotational structure, the substructure of the ν S ( X - H → ⋯ Y ) bandshape is mostly invariant.