Is cyanide significantly anisotropic? Comparison of cyanide vs chloride: clustering with hydrogen cyanide and condensed-phase thermochemistry

Pulsed high-pressure mass spectrometry shows that the clustering thermochemistry of the CN- anion with HCN behaves in a fashion quite similar to Cl-, Br-, and I-, indicating CN- acts very much like a spherical ion with radius slightly larger than CI-. The falloff of binding energy with increasing number of HCN molecules provides no evidence of any significant shell structure. Ab initio calculations support the low degree of anisotropy of CN-. While the most stable complex with HCN is CN-..HCN, there is little loss of stability when CN- and HCN do not form a linear complex. The central CN- may rotate almost freely in clusters with two or three molecules of HCN, suggesting a nearly spherical character for the anion within these complexes. The calculations also confirm the lack of shell structure since complexes of the type CN-...HCN..HCN are nearly equal in energy to NCH..CN-..HCN. The behavior of CN- as a nearly spherical anion is further supported by a survey of condensed-phase thermochemistry wherein properties such as lattice energies and solvation energies are closely matched by Cl-.