Spectroscopic and molecular dynamics studies of solvation of cyanomethane and cyanide ions

Abstract

Infrared (C—N stretch) and n.m.r (¹³C, ¹⁴N) methods have been used to probe the solvation of MeCN molecules and CN^– ions in water, methanol and a range of mixed protic-aprotic media. For MeCN, the i.r. results are unusual in that protic solvents induce a high-frequency shift in ν_max(CN), whereas dipolar aprotic solvents give a low-frequency shift. The interpretation given of this is that both σ- and π-electron interactions play an important role in hydrogen-bond formation, whereas π interactions are dominant for dipolar solvents such as Me₂SO. The ¹³C(CN) shifts are too small to give useful information, but the ¹⁴N shifts correlate well with the i.r. results. It is argued that in methanolic solution MeCN forms only one hydrogen bond and that ca. 50% of the molecules are not hydrogen bonded. In water, all MeCN molecules are hydrogen bonded. The results do not distinguish between mono- and di-bonding, but other evidence suggests that di-bonding is likely to occur. Similar experiments on CN^– ions show again that protic solvents induce a high-frequency shift. Mixed-solvent studies suggest that at least four methanol or water molecules are normally hydrogen bonded to the ions; this is in accord with other spectroscopic results on the same solvents. The experiments have been complemented by a series of molecular dynamics simulations on dilute solutions of MeCN and CN^– in water and methanol. The predicted solvation numbers from the simulations are in good agreement with those derived from the experimental data, particularly for MeCN. There are some discrepancies for CN^–, but these may in part be attributable to difficulties involved in finding an acceptable definition of solvation number in cases where this quantity is large.