DOES METHANE INVERT THROUGH SQUARE-PLANAR

MCSCF calculations using a triple-zeta basis set augmented with diffuse and polarization functions are used to probe that part of the singlet methane potential energy surface that pertains to the inversion of CH4 from one tetrahedral structure to another. The true inversion transition state is found to have a distorted C(s) structure, quite different both geometrically and energetically from the previously presumed square planar saddle point. At the second-order configuration interaction level of theory, the barrier to inversion is predicted to be just 7-8 kcal/mol higher in energy than the bond dissociation energy for the first C-H bond cleavage in methane.