Interaction between overall and internal rotation below, near, and above the summit of a torsional barrier: 1nπ* methylglyoxal

Abstract

The effects of rotation–torsion interactions on the rotational structure of torsional vibronic bands in an electronic spectrum are examined, and a computer program to calculate the relevant energies and transition intensities is described. Rotationally resolved type C bands in the 1nπ* spectrum of methylglyoxal are reported and interpreted. Since the torsional conformation of methylglyoxal differs in the two electronic states, a long progression of bands is observed, involving levels well below, near, and above the 190 cm−1 S1 torsional barrier. The rotation–torsion interaction increases rapidly towards the free rotor limit as the barrier is approached. Observed effects include energy shifts and the appearance of transitions forbidden by rigid rotor or torsional vibronic selection rules.