An ab initio study of the transition state and forward and reverse rate constants for C2H5 reversible arrow H+C2H4

Different levels of ab initio theory are used to calculate geometries, vibrational frequencies, and energies for stationary points on the H + C2H4 reversible arrow C2H5 potential energy surface. Frequencies and geometries calculated for C2H4 at the QCISD/6-311G** and MRCI/cc-pVDZ levels of theory are in very good agreement with experiment. The ab initio normal modes for the H---C2H4 transition state are similar to those for C2H4, and ratios between the C2H4 experimental anharmonic frequencies and ab initio harmonic frequencies are used to estimate anharmonic frequencies for the transition state. Nine of the 10 frequencies assigned for C2H5 from experiment are consistent with the ab initio calculations. The CC stretch was apparently misassigned and reassigned here. The ab initio calculations do not give definitive values for the H + C2H4 --> C2H5 barrier height and heat of reaction. Using these two energy terms as adjustable parameters in concert with the above stationary point geometries and frequencies, transition state theory (TST) rate constants are calculated for H + C2H4 --> C2H5 recombination and C2H5 --> H + C2H4 dissociation in the high-pressure limit. A H + C2H4 --> C2H5 barrier height of 3.0-3.1 kcal/mol gives TST recombination rate constants in excellent agreement with experiment. Because of the uncertainty in the experimental C2H5 --> H + C2H4 rate constant, a definitive value could not be deduced for the dissociation barrier from the TST fits to the experimental dissociation rate constants. Some of the experimentally determined dissociation A factors are more than an order of magnitude smaller than the TST value.