Isotope effects in linear dihydrogen bonded complexes containing LiH

Abstract

An ab initio study of linear dihydrogen-bonded complexes containing LiH (acting as a proton acceptor) was undertaken. The complexes studied were LiH⋯HF, LiH⋯HCN, and LiH⋯HCCH. Equilibrium geometries and harmonic vibrational frequencies were computed at various levels of theory using a 6-31++G(d,p) basis set. It was found that the LiH⋯HF complex was not stable at all levels of theory. The relative stabilities of H-bonded and D-bonded isotopomers of LiH⋯HCN and LiH⋯HCCH were determined by differences in zero-point vibrational frequencies. For LiH⋯HCN isotopomers, it was found that the LiD⋯HCN was favored over LiH⋯DCN with a relative stability of 54 cm−1 calculated at the Quadratic Configuration Interaction—Singles and Doubles level. Similarly, LiD⋯HCCH is favored over LiH⋯DCCH, with a lower value for the relative stability. The relative stabilities of the H-bonded and D-bonded species LiH⋯HCCD and LiH⋯DCCH indicated that the D-bonded complex was energetically favored, in accordance with the Buckingham–Liu theory.