A b i n i t i o SCF calculation on LinHm molecules and cations with four or less atoms

Abstract

All possible molecules and monopositive cations containing lithium and hydrogen, up to a total of four atoms, have been studied by ab initio calculations using SCF wave functions built from Gaussian-type orbitals. The triatomic molecules and ions Li2H, Li3, H+3 , LiH+2 , Li2H+, and Li+3 were all found to be stable, i.e., of lower energy than any possible dissociation product. The neutral molecules H3 and LiH2 were found to be unstable relative to H2+H and Li+H2, respectively. The ions H+3 and Li+3 were found to be equilateral triangles, Li2H+ was found to be a linear species, while all of the other stable triatomic species were found to be bent. The binding energies (relative to the most stable dissociation products) for the triatomic molecules and ions ranged from 4.53 eV for H+3 to 0.24 eV for LiH+2 . All of the ten possible tetra-atomic molecules and cations were found to be stable (except H4 neutral, for which no calculations were done). The lowest energy structures for LiH3 and LiH+3 consisted of planar structures with an H2 unit perpendicular to an LiH unit and having the H2 at the Li atom end. The lowest energy for Li2H2 was obtained for a planar rhombic structure, while for the corresponding cation it was obtained for a planar structure with an Li2 unit perpendicular to an H2 unit. Li3H and Li3H+ were found to have lowest energy for planar kite shaped structures with a lithium atom loosely bound to the base of a triangle formed by an Li–H–Li unit. The shapes of Li4 and Li+4 were respectively: a rhombic structure and a triangular Li+3 unit with the fourth lithium atom attached to a vertex to form a planar structure similar to H+4 . The predicted binding energies for the tetra-atomic molecules and ions ranged from 0.08 eV for LiH3 (least stable) to 1.20 eV for Li2H2 (most stable). The present work predicted for the first time a stable LiH3 neutral molecule, and stable Li2H+2 and Li3H+ cations. It also predicted for the first time that the most stable Li+4 ion corresponded to a C2v structure analogous to that of H+4 .