Theoretical calculations on the structure and barriers of internal rotation for the isopropyl radical

Abstract

The geometry of the isopropyl radical, C3H7, was optimized and found to have a Cs symmetry. The two methyl groups in the isopropyl system are symmetically located on the sides of the symmetry plane while the α-CH bond resides in the plane of symmetry. The barriers and minimum energy paths for the internal rotation of the methyl groups were investigated by computing the changes in total energy as a function of internal rotation of one methyl group and the in-phase and out-of-phase rotation of two methyl groups. For all of the internal rotations, potential functions were calculated for the rigid rotations and the minimum energy paths for the relaxed rotations. A threefold potential function was computed for the single methyl internal rotation with barrier heights of 1.08 and 0.8 kcal/mol for the rigid and relaxed motions. A threefold potential function was also found for the in-phase double internal rotation with barriers equal to 2.27 and 1.05 kcal/mol for the rigid and relaxed modes. The rigid out-of-phase double internal rotation has a threefold symmetry with a barrier height of 2.27 kcal/mol. The minimum energy path for the relaxed rotation, however, is accompanied by inversion of the radical center; in essence, the coupled internal rotation-inversion produces an energy path that has two barriers between 0° and 120°; the first is at 1.08 kcal/mol while the second is much lower at 0.30 kcal/mol. © 1987 American Chemical Society.