Cooperative behaviour in liquids

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Abstract
Evidence is presented for the existence of strong cooperative behaviour in the liquid state. It is suggested that the far-infrared spectrum of CH3CN is composite in nature and that only a small portion may be attributed to the so-called ‘Poley absorption’ supposedly characteristic of polar liquids in the far-infrared. This is in accord with a recent conclusion of other authors in a quite independent investigation. There is strong evidence from a number of spectroscopic methods that a strong local order exists in acetonitrile that persists in solutions down to 10% and less. The consequence of this association is that absorptions other than those arising from single-particle motions (which peak at ca. 60 cm–1) contribute to the total profile which has a maximum absorption at ca. 100 cm–1. Detail is resolved on the low-frequency side of the far-infrared spectral profile that has not been reported previously. In particular, a strong feature at the frequency of maximum absorption, clearly resolved with an inter-ferometric system, appears to be confirmed with a line from a tunable laser system. At this stage it is only possible to speculate on the origin of this detail but there is significant evidence available that suggests a contribution to the profile from the motions of aggregates of molecules. A whole set of intermolecular vibrational transitions is then possible and they may strongly overlap each other. We present other new observations that also seem to reflect strong cooperative behaviour in the liquid state.

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