Theoretical investigation of rotationally inelastic collisions of the methyl radical with helium

Rotationally inelastic collisions of the CH3 molecule in its ground (X) over tilde (2)A ''(2) electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH3, frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RCCSD(T)]. The anisotropy of the PES is dominated by repulsion of the helium by the hydrogen atoms. The dissociation energy D-e was computed to equal 27.0 cm(-1). At the global minimum, the helium atom lies in the CH3 plane between two C-H bonds at an atom-molecule separation R = 6.52 bohr. Cross sections for collision- induced rotational transitions have been determined through quantum scattering calculations for both nuclear spin modifications. Rotationally inelastic collisions can cause a change in the rotational angular momentum n and its body-frame projection k. Because of the anisotropy of the PES due to the hydrogen atoms, there is a strong propensity for Delta k=+/- 3 transitions. Thermal rate constants for state-specific total collisional removal have also been determined. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624525]