RAMAN-SPECTROSCOPIC STUDIES ON THE DYNAMIC AND EQUILIBRIUM PROCESSES IN BINARY-MIXTURES CONTAINING METHANOL AND ACETONITRILE

Raman isotropic band profiles of the nu-2 mode of acetonitrile in binary mixtures with methanol have been studied over the whole concentration range and between 196 and 330 K. Attempts have been made to understand the spectral behaviour in terms of variations in vibrational dephasing as a function of environment and in terms of rapid chemical exchange between complexed and non-complexed acetonitrile molecules. If exchange dynamics are assumed to be important it is found that the dissociation rate constant (k21) for this reaction is of the order of 10(11) s-1. This rate seems unrealistically high although similar rates have been obtained for other hydrogen-bonded systems. Nevertheless, the band shape changes dramatically across the temperature range and this demonstrates clearly that a "merging" band profile does not necessarily prove that exchange dynamic processes are important. Bandwidth and frequency shifts across the concentration range could be attributed to increases in exchange rate as the amount of methanol increases or the temperature increases. However, the most probable explanation is that there is a change in vibrational dephasing rate due to environmental fluctuations. We clearly demonstrate that even at 0.001 molar fraction of CH3CN in CH3OH a finite number of CH3CN molecules are "free" (on the vibrational timescale) from the hydrogen-bonded interaction. An explanation for this rather surprising behaviour has been sought (and found) in terms of multiple hydrogen-bonding equilibria in this system. The effect has been shown to be associated with extensive methanol aggregation. An equilibrium model has been devised which predicts accurately the relative intensities of the two nu(CN) bands and the unusual behaviour in binary mixtures of this type.