Solvent-solute hydrogen bonding in dilute solutions of CN^- and CH₃CN in water and methanol

Abstract

Molecular-dynamics calculations have been carried out for dilute solutions of CN^- and CH₃CN in water and methanol at a temperature of 300 K. Results are reported for the numbers, energy distributions and lifetimes of hydrogen-bonded solute-solvent pairs. The numbers of bonds formed to methanol (c. 5·2 for CN^-, 0·5 for CH₃CN) are found to be fewer in number than those formed to water (c. 6·3 for CN^-, 1·8 for CH₃CN), but the lifetimes of the bonds in methanol are longer by factors of approximately three (for CN^-) or eight (for CH₃CN) than those in water. In the case of CN^-, bonding to the nitrogen atom is favoured both in water and in methanol, and there is also evidence for the formation of short-lived, bifurcated structures. The results for CN^- in solution present a more consistent pattern than those for CH₃CN, and it is concluded that the description in terms of hydrogen bonding is less appropriate for the molecule than for the ion. However, the results for both solute species are in satisfactory overall agreement with those obtained experimentally by spectroscopic methods.