Abstract
Ab initio calculations of the vibronic potential surface of the 2 T 2 ground state of CH4 + have been carried out, using a contracted gaussian approximation to a double-zeta basis of Slater orbitals. The minimum energy is found for a tetragonal distortion to the point group D 2d , with two HCH angles of 141° and four of 96°, and all r ch equal. All calculations for distorted CH4 + were carried out with the mean r ch held at 1·147 å, which minimises the energy for Td symmetry. The distortion from Td symmetry lowers the electronic energy by 1·41 ev. Distortions from Td to C 3v lead to a maximum stabilization of 1·22 ev. Vibronic perturbation parameters derived from this surface are employed in discussions of the 2 T 2-1 A 1 photoelectron transition of CH4 and a possible 2 E2 B 2 absorption transition of CH4 +. A vibrational interval of ∼ 1200 cm-1 observed near the origin of the 2 T 2-1 A 1 photoelectron transition is assigned to one component of v 2 (calculated frequency, 1300 cm-1).

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