RATE COEFFICIENTS FOR THE H + NH3 REACTION OVER A WIDE TEMPERATURE-RANGE

The temperature dependence of the rate coefficients for the H + NH3 → NH2 + H2 reaction is measured by the high-temperature photochemistry (HTP) technique. Technique improvements are discussed. For the 490-960 K range, we find k1(T) = 1.21 × 10-10 exp(-6920 K/T) cm3 molecule-1 s-1. When combined with two other sets of direct measurements, a best fit gives k1(T) = 9.0 × 10-19 (T/K)2.40 exp(-4991 K/T) cm3 molecule-1 s-1 for the 490-1780 K range. It is shown that these data can be more closely fitted by a calculation based on conventional transition-state theory plus an Eckart tunneling model than with other current transition-state models considered. Using the potential energy surface for the H + NH3 reaction, several tunneling models fit the results for the D + ND3 reaction. The kinetic isotope effect for these two reactions is compared to results from different theoretical models, with the Eckart model giving the best approximation. © 1990 American Chemical Society.