An a b i n i t i o molecular orbital study of NaCN and KCN

Abstract

A b initio self-consistent-field calculations using large flexible Gaussian basis sets are reported for sodium and potassium cyanide. The potential energy curves calculated for the linear cyanide, linear isocyanide, and a T-shaped geometry in which Na+ or K+ approaches the center of mass of a rigid (CN)− indicate that the T- shaped minimum is lowest in energy and the linear cyanide structure highest. The fully optimized equilibrium (re) geometries determined for both complexes are significantly nonlinear. Nearly L-shaped structures KNC and NaNC lie ∼1.7 and ∼0.8 kcal/mol lower than the optimized linear isocyanide geometries. For KCN, the optimum geometry is calculated to be re(KC) = 2.850 Å, re(KN) = 2.616 Å, re(CN) = 1.153 Å, ϑe(KCN) = 66.6°, and ϑe(KNC) = 90.5° in good agreement with recent experimental determinations of the gas phase structure. The predicted geometry for NaCN is re(NaC) = 2.470 Å, re(NaN) = 2.195 Å, re(CN) = 1.152 Å, ϑe(NaCN) = 62.7°, and ϑe(NaNC) = 89.5°. The electronic distributions in the complexes are discussed in terms of Mulliken net charges and one-electron properties. The effects of including electron correlation have been estimated on the basis of the London dispersion energy formula which is valid at long range. It is probable that electron correlation corrections will not change the qualitative conclusion of this work that both KCN and NaCN are very floppy molecules which, however, do exhibit nonlinear equilibrium geometries.