Journal of Chemical Physics, Vol.105, No.21, 9525-9535, 1996
Ab-Initio Study of the He(S-1)+ch(X(2)Pi)-Interaction
Potential energy surfaces for the (2)A’ and (2)A" states of the He(S-1)-CH(X (2) Pi) complex were calculated using supermolecular unrestricted Moller-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 D-e approximate to 335 mu E(h) for the T-shaped geometry, at R=5.0 a(0) and Theta=100 degrees. 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 approximate to 7.5 a(0), and Theta=0 degrees, and is 55 mu E(h) deep. The second minimum has a troughlike form which joins the region between R=7.5 a(0), Theta=140 degrees and R=8.0 a(0), Theta=180 degrees. The lowest point is approximately 54 mu E(h) deep and occurs at R=7.5 a(0) and Theta=140 degrees.
Keywords:POTENTIAL-ENERGY SURFACES;DER-WAALS COMPLEXES;SET SUPERPOSITION ERROR;STRETCH-BEND LEVELS;PERTURBATION-THEORY;EXCITED-STATES;INTERMOLECULAR INTERACTIONS;ABINITIO CALCULATIONS;AR;SPECTROSCOPY