PROPERTIES OF ACTIVATION BARRIERS FROM THERMAL RATE DATA ON FORWARD AND REVERSE REACTIONS - OH+H2, NH2+H2, CH3+H2, AND CH3+HCL

Experimental data on the temperature dependence of forward and reverse transfers of hydrogen atoms have been assembled. The following transition-state theory (TST) expressions, incorporating a factor, κ, for tunneling through a one-dimensional Eckart barrier of effective forward and reverse heights Ef and Er and of characteristic tunneling temperature, T*, were fitted by least-squares methods to the data for each reaction: kf = κBf(T) Qv(T,wB) exp(-Ef/RT); kr = κBr(T) Qv(T,wB) exp(-Er/RT). Here Ef, T*, wB, and Ef - Er were adjustable parameters; wB is the geometric mean of the low-frequency vibrational term values of the transition species; and Qv(T,wB) is the corresponding partition function. Bf(T) and Br(T) incorporate other TST factors, calculated from spectroscopic properties of reactants and semiempirical internuclear distances of the transition species. For the reactions OH + H2 ⇌ H2O + H and NH2 + H2 ⇌ NH3 + H, most standard deviations are from 1% to 3% of the parameter values and the parameters agree well with independent quantum chemical or thermochemical estimates. The second reaction is 13 kJ mol-1 exothermic at 0 K. For the reactions CH3 + H2 ⇌ CH4 + H, Ef - Er disagrees with the values from the thermochemical tables, suggesting that there are errors in these tables or errors of 1 order of magnitude in the low-temperature rate constants. The latter possibility seems more likely, as we do find agreement between the established thermochemistry and recent kinetic data for the reactions CH3 + HCl ⇌ CH4 + Cl. The above TST expressions have also been fit to the results of variational TST calculations, incorporating more sophisticated tunneling corrections for the first and third reactions. The fitted parameters agree well with the properties of the tops of the vibrationally adiabatic barriers used in the variational calculations. © 1990 American Chemical Society.