EFFECT OF SKELETAL RELAXATION ON THE METHYL TORSION POTENTIAL IN ACETALDEHYDE

Fully-relaxed model ab initio calculations at Hartree-Fock/6-31G(d,p) and Moller-Plesset (MP2)/6-31G(d,p) levels for acetaldehyde methyl conformers indicate significant skeletal flexing (e.g., the CH3-C bond length changes by 0.006 angstrom) and methyl hydrogen folding. Thirteen methyl conformer energies at 15-degrees intervals are used to assess the magnitudes of the torsional potential function expansion terms. Only two terms V3 = 373.8 and V6 = 3.4 cm-1 (both significantly different from those obtained from microwave and infrared analyses) are found to be important. These calculations clearly show that relaxation during methyl rotation (i.e., skeletal flexing and methyl hydrogen folding) is an important determinant of the torsional potential. Energy levels obtained from internal rotation potentials which include flexing simulate infrared torsional fundamental frequencies in CH3 CHO and CD3 CHO to within 1-2 cm-1 of the experimental values. In the absence of relaxation infrared torsional fundamental frequencies are poorly simulated by ab initio calculations.