Theoretical studies on the barriers for internal rotation of the methyl groups in the tert-butyl radical

Restricted open-shell Hartree-Fock calculations are used to investigate the potential functions for internal rotation of the methyl groups in the tert-butyl radical. The optimized paths for three types of rotations are reported: Two are the A2 and E rotations permitted by the C3v symmetry of the radical; the third involves rotation of a single methyl group. The calculations reveal that the A2 mode is a sixfold potential function with a 1.69 kcal/mol barrier height, and that the E motion is also described by a sixfold potential function with two different minima one of which is very shallow. The barrier height that must be surmounted for the E mode to go between the well-defined minima, which are separated by a 120° rotation, is 1.86 kcal/mol A threefold potential function with a 1.51 kcal/mol barrier height is computed for the single methyl rotation. In essence all of the studies indicate that a reasonable value for the barrier height for internal rotation of methyl groups is ∼1.5 kcal/mol. This is true regardless of the manner in which the methyl groups are rotated as long as all of the geometrical parameters are optimized as a function of the internal rotation. © 1986 American Chemical Society.