3-DIMENSIONAL VIBRATIONAL STUDY OF THE COUPLING BETWEEN THE METHYL TORSION AND THE MOLECULAR FRAME IN THE S-0 STATE OF ACETALDEHYDE

Hybrid free rotor-harmonic oscillator basis functions are used for the variational study of the vibrational coupling between the methyl tors ion and the aldehydic hydrogen wagging + CCO bending motions in the S- 0 state of acetaldehyde. The kinetic terms and the potential function are expressed as a three-dimensional symmetry-adapted Fourier + Taylor series expansions. The data for the derivation of the kinetic and pot ential functions were obtained from ab initio calculations at the MP2( full)/6-311G (d,p) level. The use of hybrid basis functions reduces th e size of the Hamiltonian matrices. Thus, sizes of 140, 135, and 250 w ere used for the a(1), a(2), and e representations of the nonrigid gro up of the molecule. A perturbation is found between the e components o f the fourth level of the torsional mode and the first quantum of bend ing. As a consequence, the a(1)-e unperturbed sequence for the two fir st torsional energy levels attached to the first quantum of bending is reversed. The separation between these energy levels increases from 0 .06 to 2.67 cm(-1).