The shapes and energetics of dimeric lithium and sodium cyanides. Ab initio molecular-orbital calculations

Abstract

The geometries and energies of many different isomers of (LiCN)₂ and (NaCN)₂ have been investigated by ab initio molecular-orbital calculations, employing large gaussian basis sets with both polarization and diffuse functions. The geometries were fully optimized by gradient techniques. For both dimers, there are three different isomers of very similar stability. The energy difference between the lowest-energy structure, a four-membered planar ring of alternating M (M = Li or Na) and N atoms of D_2h symmetry, and two different six-membered planar rings of C_2h and C_2v symmetries, is ca. 20 kJ mol^–1 for (LiCN)₂ but only 5 kJ mol^–1 for (NaCN)₂. Linear structures are substantially less stable. No non-planar isomers were found to be local energy minima.