Ab initio molecular orbital calculations on ion-molecule and ion pair-molecule complexes of the water-lithium cyanide system

Abstract

Ab initio molecular orbital (MO) calculations with the 3-21G and 6-31G basis sets were performed on a series of ion-molecule and ion pair-molecule complexes for the H₂O + LiCN system. Stabilisation energies (with counter-poise corrections), geometrical parameters, internal force constants and harmonic vibrational frequencies were evaluated for 16 structures of interest. Although the interaction energies are smaller, the geometries and relative stabilities of the monohydrated contact ion pair are reminiscent of those computed for the complexes of the individual ions. Thus, interaction of the oxygen lone pair with lithium leads to a highly stabilised C_2v structure, while the coordination of water to the cyanide ion involves a slightly non-linear hydrogen bond. Symmetrical bifurcated structures are computed to be saddle points on the potential energy surface, and to have an imaginary frequency for the rocking mode of the water molecule. On optimisation the geometries of the solvent shared ion pair structures (e.g. Li⁺·· OH₂···CN^-) revealed a proton transfer from the water molecule leading to hydrogen bonded forms such as Li-O-H···HCN. The variation in the force constants and harmonic frequencies in the various structures considered are discussed in terms of ion-molecular and ion pair-molecule interactions.