A STUDY OF QUADRATIC PLUS GAUSSIAN PLUS COSINE FUNCTIONS FOR THE SIMULTANEOUS DESCRIPTION OF INTRAMOLECULAR TORSION AND INVERSION

The use of more realistic potential energy functions than the usual do uble Fourier series expansion is investigated for the description of c oupled inversion and torsional motion. A quadratic-gaussian expression is used for the description of the inversion motion whereas a trigono metric expansion is used for the torsion. The coupling between the two motions is introduced into the trigonometric function as a phase fact or that depends on the inversion angle. The treatment is applied to th e carbonyl hydrogen wagging and methyl torsion in the first excited el ectronic state of acetaldehyde. The use of experimental results in con junction with ab initio data for the determination of the potential fu nction is also tested.