DEFINITION OF A PERIODIC METHYL TORSIONAL COORDINATE IN THE S-0 STATEOF ACETALDEHYDE

Torsional energy levels in acetaldehyde have been calculated at the MP 2(full)/6-311G(d,p) level with full molecular relaxation for analyzing the loss of symmetry of the methyl group, It is shown that the usual definition for the rotational angle using a dihedral angle is an appro ximation, The use of asymmetric models for torsion fails to reproduce the degeneracy of the torsional levels and it is necessary to define a new three-fold torsional coordinate. Two approaches have been used. F irst, the use of selected conformations or energy averages which lead to energy levels within 6 cm(-1) of the experimental results, Second, the generation of a three-fold potential function considering the coup ling with the remaining vibrations, This method produces energy levels in very good agreement with the experimental values.