A theory of a broadening of the infrared absorption spectra of hydrogen-bonded species III. The kinematic and electronic coupling mechanisms

Abstract
The theory of the coupling between anharmonic v (XH) and v (XH. .Y) modes of a hydrogen-bonded complex developed in part II is extended so as to make explicit allowance for the effect of varying hydrogen bond length on the electron distribution of the XH molecule. Sufficient experimental data are available on the temperature dependence of the infrared spectrum of Me 2 O . HC1 to enable the form of the potential energy surface to be deduced. The effective potential curves governing the hydrogen bond vibration in the ground and first excited state of the v (XH) vibration are constructed. It is found that the Me 2 O . HC1 complex is 14 pm shorter in the upper state than in the lower, which has important consequences for the structure of the vibration-rotation bands associated with vibrational Franck-Condon transitions. The shortening is much less pronounced in Me 2 O . DC1. The theory provides strong support for the interpretation of the broad v (XH) bands of hydrogen bonded species in terms of v (XH) +/- nv (XH .. Y) combination bands, and demonstrates the close connection between the infrared spectroscopic anomalies and the Ubbelohde effect.

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