Torsional coupling in pyruvic acid

New assignments of rotational transitions in the first excited state of the skeletal torsion were made and showed an unexpected interchange in the sequence of energy levels for the internal rotation of the methyl group. This fact is traced back to a strong coupling between the skeletal torsion and the methyl torsion. A two dimensional flexible model for torsional motion was developed which could account for the observations. The model includes structural relaxation and torsional potential energy coupling and allowed successful prediction and further assignments of transitions in the second excited state of the skeletal torsion.