Shaping the infrared spectrum of the acetic acid dimer in the OH-stretching range: Multiple conformers and anharmonic coupling

The potential energy surface of the acetic acid dimer relevant to the intermolecular hydrogen bond dynamics is studied. Apart fr the minimum energy structure exhibiting C2h symmetry the H-transfer dynamics is strongly influenced by two rotamer structures obtained by a 60° internal rotation of one or both methyl groups. It was found that the characteristic multi-band structure of the OH-stretching range is caused by the coexistence of these three conformers. It is also shown that the large frequency shift of the OH stretch bands with respect to the harmonic values, arises from the anharmonicity of the two local OH-stretch modes.