EFFECT OF MOLECULAR-GEOMETRY RELAXATION ON THE POTENTIAL-ENERGY FUNCTION OF INTERNAL-ROTATION

Theoretical internal rotation potential energy (IRPE) curves for glyoxal were obtained by complete optimization of its geometrical parameters at the RHF/6-31G, RHF/6-31G*, RHF/6-31G**, and MP2/6-31G* levels (14 to 16 points along each curve). The corresponding IRPE functions, 2V = SIGMA(n = 1)6(V(n)(1 - cos n-phi)), where phi is the torsional angle, were calculated for each set of points and refined using experimental torsional transition frequencies in the trans and cis wells. This work corroborates the previous assignments of the torsional 0-11, 0-12, 0-13, and 0-14 transitions in the experimental spectrum of the trans well. The energy difference (DELTA-H-0, corrected for the zero-point energy of the torsional transition) between the planar cis and trans conformers, determined from the refined IRPE functions, decreases regularly from 1658 to 1599 cm-1 with increasing level of theory. These values are all lower than the value, 1670 cm-1, obtained from the pure experimental internal rotation potential in the literature. The DELTA-H-0 value calculated from the refined MP2 IRPE function approaches the experimental energy difference. The coefficients for the refined MP2 IRPE function are V1 = 1635.2, V2 = 1116.1, V3 = -36.5, V4 = -97.1, V5 = 17.7, and V6 = 6.9 cm-1. From this function the barriers (at 107-degrees from the bottom of the trans well) to trans --> cis and cis --> trans rotations are predicted to be 2063 and 446 cm-1 from the botton of each well, respectively. The necessity of accounting for molecular geometry relaxation when deriving IRPE functions is stressed.