But these distributions are different from each other. The OH fragment on the 3'A" state is rotationally excited stronger than that on the 3'A' state. This difference is due to the topographic difference between the 3'A' state and the 3'"' state. The energy ratio for the 3'A' state is E,/E,, = 0.07, and the total available energy hardly transmits to the rotational energy, similar to the results we discussed above. Thus, the high rotational excitation is not expected in the photodissociation process: HOCl(1 lA') + hv + HO('II) + C1(2P).In the previous work, we stated, only from the ab initio potential 1992, 96, 2111-2114 2111 energy surfaces, that the rotational excitation of the OH fragment may be strongly dependent on the excited state. The actual results of the trajectory study are state-dependent as expected, but most of the total available energy transmits to the relative translational energy during the photodissociation process.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 = Ct=l(Vn(l -cos n$)), where $ 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 0-1 1,0-12, 0-13, and 0-14 transitions in the experimental spectrum of the trans well. The energy difference (AH,, 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-' with increasing level of theory. These values are all lower than the value, 1670 cm-I, obtained from the pure experimental internal rotation potential in the literature. The AH, value calculated from the refined MP2 IRPE function approaches the experimental energy difference. The coefficients for the refined MP2 IRPE function are VI = 1635.2, V, = 1116.1, V, = -36.5, V, = -97.1, V, = 17.7, and V, = 6.9 cm-I. From this function the barriers (at 107O from the bottom of the trans well) to trans -cis and cis -trans rotations are predicted to be 2063 and 446 cm-' from the bottom of each well, respectively. The necessity of accounting for molecular geometry relaxation when deriving IRPE functions is stressed.
