Molecular Orbital Energy Level Diagrams of HeH⁺

Abstract

HeH+ is the simplest heteronuclear diatomic molecule. Two molecular orbital energy level maps of HeH⁺ are shown in this note by using HF/6-311++G**. One is a covalent bond type map shown as He⁺ + H →HeH⁺. Another is a coordinate bond type map shown as He + H⁺→HeH⁺. The latter is stable. In the covalent bond type map, the orbital energy of HeH⁺ 1σ (-1.6288a.u.) is lower than that of H 1s (-0.4998a.u.) but higher than that of He+ 1s (-1.9983a.u.) because of destabilizing by electron repulsion. In the coordinate bond type map, the orbital energy of HeH⁺ 1σis lower than those of both H⁺ 1s (-0.4998a.u.) and He 1s (-0.9176a.u.) because of stabilization thanks to decreasing of the electron repulsion in He 1s.