AB-INITIO RRKM APPROACH TOWARD THE UNDERSTANDING OF ETHYLENE PHOTODISSOCIATION/

The optimized structures and harmonic frequencies for the transition s tates and intermediates on the ground state potential energy surfaces of ethylenes, including C2H4, C2D4, D2CCH2, and cis- and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G(d,p) l evel. The coupled cluster method, CCSD(T)/6-311+G(3df,2p), was employe d to calculate the corresponding energies with the zero-point energy c orrections by the B3LYP/6-311G(d,p) approach. Ethylidene was found to be an intermediate in the 1,2-H-2 elimination channel. The barrier for the 1,1-H-2 elimination was computed to be the lowest (4.10-4.16 eV), while the 1,2-H-2 elimination and H loss channels have barriers of a similar height (4.70-4.80 eV). The rate constant for each elementary s tep of ethylene photodissociation at 193 and 157 nm was calculated acc ording to the RRKM theory based on the ab initio surfaces. The rate eq uations were subsequently solved, and thus the concentration of each s pecies was obtained as a function of time. The concentrations at t-->i nfinity were taken for calculating branching ratios or yields. In acco rd with previous experimental findings, the calculated branching ratio for the 1,1-H-2 elimination process is higher than that for the 1,2-H -2 elimination, and the atomic elimination channel is predicted to be favored at increasing excitation energy when competing with the molecu lar elimination. The significant discrepancies between theoretical and experimental results in the magnitude of the yields and their depende nce on the wavelength for the molecular elimination channels suggest t he dynamics of either 1,2-H-2, or 1,1-H-2 elimination, or both channel s may be nonstatistical in nature. (C) 1998 American Institute of Phys ics.