2-DIMENSIONAL TREATMENT OF THE METHYL AND ALDEHYDIC TORSIONS IN GAS-PHASE PROPANAL

The potential energy surface for the simultaneous methyl and asymmetri c aldehydic torsions in propanal has been determined at the restricted Hartree-Fock/second-order Moller-Plesset level using the 6-311(3df,p) basis set. The fit of the energy values to a symmetry adapted functio nal form was carried out in two ways: using all the 56 energy values o btained in the optimization procedure, or dropping out the energy valu es which do not respect the C-3 dynamical symmetry of the methyl group , i.e., using only 28 values. With these potentials, as well as with t he kinetic parameters and the electric dipole moment variations, the f ar infrared frequencies and intensities for the methyl and aldehyde to rsions were determined theoretically. It is found that the two-dimensi onal calculation for the cis conformer satisfactorily reproduces not o nly the methyl torsion and asymmetric aldehyde torsion spectrum, but f urnishes also methyl torsionally excited progressions for the aldehyde torsion modes. Most of these transitions occur in the region of 220-1 00 cm(-1) and are responsible for the complexity of the spectrum. From these theoretical results, a new assignment is proposed. The fit of t he energy values which respect to the C-3 dynamical symmetry seems to produce better results and also permits one to save computational time . (C) 1998 American Institute of Physics.