THEORETICAL CALCULATIONS OF THE THERMAL RATE CONSTANTS FOR THE GAS-PHASE CHEMICAL-REACTIONS H + NH3 REVERSIBLE H-2 + NH2 AND D + ND3 REVERSIBLE D-2 + ND2

Rate constants for the title reactions are computed by using variational transition-state theory with semiclassical ground-state adiabatic transmission coefficients for the temperature range from 200 to 2400 K. The rates are computed from selected information about the potential energy surface along the minimum energy path as parameters of the reaction path Hamiltonian. The potential information is obtained from ab initio electronic structure calculations with an empirical bond additivity correction. The accuracy of this semiempirical technique for obtaining the potential information is tested by comparing the results of the underlying ab initio calculations with higher quality multiconfiguration SCF and multireference CI calculations and by using increasingly higher quality ab initio electronic structure calculations before applying the bond additivity correction For the reactions H + NH3 → H2 + NH2, H2 + NH2 → H + NH3, and D + ND3 → D2 + ND2, the ultimate test is given by comparison with recent experimental results. Although the agreement is good in general, the comparisons of experiment and theory indicate that the computed barrier height is overestimated by about 1 kcal/mol. Quantitative agreement is found for all three reactions listed above by suitably scaling the potential information. The experimental results are extended down to 200 K and up to 2400 K for these three reactions, and predictions are provided for the reaction D2 + ND2 → D + ND3 for the temperature range from 200 to 2400 K. © 1990 American Chemical Society.