AN !SCF POTENTIAL ENERGY SURFACE FOR LlTHRlM CYANIDE

Recent microwave experiments [I] have shown that potassium cyamde has a triangular shape. This finding has been supported by ab initio SCF calculations [I-4]. Tbe ab initio calculations predict that sodium cyanide is also triangular [3,4] _ Lithium cyanide, on the other hand, is predicted to be linear and to have an isocyanide structure [5,6]. Numerous calculations [6-I I] agree that hydrogen cyanide is linear with the H atom bound to the carbon atom, and this is born out by experiment [ 1~1. These cyanides have in common that theu bond to the metal atom is highly ionic, which is a consequence of the large electron afftity (experimental value: 3.82 eV [13]) of the cyanide and the low ronizahon potential of the alkali-metal atoms. Hence, the Coulomb force dominates the bondmg, and as this force is isotropic the benchng force constants of the alkali cyanides are exceptionally low. It is therefore interestmg to study their rotational-vibrational spectra. Computations, yielding these spectra, have been performed for LICN 1141 and for KCN [ 14-171. Tbe LiCN work was based on a rather crude potential, containing parameters fitted to the ab initio points of ref. [S] _ Although many points of the surface of LiCN are