Ab initio study of the He(1S)+CH(X 2Π) interaction

Abstract

Potential energy surfaces for the 2A′ and 2A″ states of the He(1S)–CH(X 2Π) complex were calculated using supermolecular unrestricted Mo/ller–Plesset perturbation theory and analyzed via the relevant perturbation theory of intermolecular forces. It has been found that the two states are distinctly different. The potential energy surface (PES) of the A″ state has only a single and relatively deep minimum of De≊335 μEh for the T-shaped geometry, at R=5.0 a0 and Θ=100°. The position of this minimum is determined by the exchange repulsion which is substantially reduced at this geometry. The minimum is unusually deep for a complex of He, and it can be viewed as an example of an incipient chemical bond. In contrast, the A′ state’s PES represents a typical van der Waals interaction which is characterized by two similarly deep minima. The shape and location of these minima are determined primarily by the anisotropy of the dispersion component. The first minimum occurs for the collinear He–C–H arrangement, at R≊7.5 a0, and Θ=0°, and is 55 μEh deep. The second minimum has a troughlike form which joins the region between R=7.5 a0, Θ=140° and R=8.0 a0, Θ=180°. The lowest point is approximately 54 μEh deep and occurs at R=7.5 a0 and Θ=140°.