An ab initio calculation of the rotation and internal-rotation energy levels of the ethyl radical

By ab initio calculation at ten values of the internal-rotation angle we have determined the minimum energy path for internal-rotation in the ethyl radical C2H5. Bond-length and bond-angle relaxation are allowed via full optimization, using unrestricted second-order Møller–Plesset perturbation theory, for each choice of torsional dihedral angle. We find that the only significant relaxation is a wagging (or inversion) of the CH2 group through ±9° about a planar C–CH2 geometry as the internal-rotation proceeds. The torsional potential can be accurately fitted to a V6 function and the barrier to internal-rotation is calculated to be 26 cm−1. Using our general rotation–contortion Hamiltonian and computer program we have calculated the rotation–torsion energy levels of the molecule using this minimum-energy path. Brief comparison is made to recent experimental measurements of the rotational fine structure of the ν9=1 fundamental.