Exact quantum mechanical reaction probabilities and rate constants for the isotopic collinear H + H₂ reactions

Quantum mechanical calculations on three of the collinear H + H₂ reactions involving D-substitutions are presented and compared with each other and with previous calculations on the H + H₂ reaction itself. The energy at which the reaction probability becomes appreciable is well predicted by the vibrationally adiabatic model. The reaction probabilities at low energies ("tunneling") are larger than predicted by tunneling through one-dimensional barriers for motion along the reaction coordinate. The deviations of the exact rates from transition state theory with unit transmission coefficient and with transmission coefficients corrected for tunneling and nonclassical reflection are examined. Transition state theory including tunneling is usually very accurate (correct within 20% for rate constants); but the errors are much larger at temperatures below 300°K. Although the main use of the present results is for testing approximate models of reaction, not for comparison with laboratory experiments, it is interesting to note that the isotope effects are in rough agreement with the (noncollinear) experimental ones. The results are used to examine the general validity of treatments of the dynamics which separate effects due to the different modes of motion.