Vibrational spectroscopic studies of internal rotation of symmetrical groups. Part I. Evidence for a high barrier to internal rotation in methylpentacarbonylmanganese, CH3Mn(CO)5

Abstract

Methylpentacarbonylmagnanese has been examined by i.r. spectroscopy in the vapour state, in solution, and as a solid in a potassium bromide disc. Particular attention has been paid to the antisymmetric methyl stretching vibration. Contrary to expectation on grounds of symmetry, which predicts a 12-fold barrier for internal rotation, there appears to be a high barrier of the methyl group estimated to be 2·7 ± 0·5 kcal. mole^–1 in C₂Cl₄ solution. Intermolecular forces may also contribute to the barrier in solution, but there are indications that this is high in the vapour state also.

Possible explanations for this unexpected result are considered in relation to the low-frequency vibrations of this molecule or in terms of d_π donation from metal atom to the methyl group.