Study of the electronic structure of molecules. XVIII. Interaction between a lithium atom and a cyano group as an example of a polytopic bond

Abstract

We have reinvestigated the LiCN and LiNC molecules in the linear configurations. With very near to Hartree-Fock functions we obtain the following equilibrium distances: R(Li-C) =3.65±0.015 a.u., R(C-N) = 2.14±0.015 a.u. for LiCN; A(Li-N)=3.35±0.015 a.u. and A(C-N)=2.18±0.015 a.u. for LiNC. The Hartree-Fock energy (at equilibrium) are -99.8176 a.u. for LiCN and -99.8275 a.u. for LiNC. These data are corrected using a statistical model to compute the correlation energy. The resulting atomization energy (total binding relative to dissociation into ground state atoms) is 13.48 eV for LiCN and 13.78 eV for LiNC. (These data are, therefore, confirming previous computation by Bak, Clementi, and Kortzeborn, done with smaller basis set.) With a constant value for the C-N distance, we have mapped the energy surface for a number of geometries in nonlinear conformations, following the lowest energy path leading from LiCN to LiNC: no energy barrier was found between the LiCN and the LiNC linear configuration or between the linear and nonlinear configuration (if one remains in the vicinity of the bottom of the energy surface valley). Thus by thermal excitation (of about 0.3 eV or above) the lithium orbits around the CN radical, and no preferred structural formula can be drawn (in a traditional sense). It is expected that this somewhat untraditional type of bond is somewhat common especially for high temperature species, where thermal excitation is sufficient to compensate for law (if any) barriers between isomers. We have referred to this bond as a "polytopic bond" to stress its equality in energy for substantially different position, relative to the remainder of the molecule.