THE H-2-]HD+D REACTION - QUASI-CLASSICAL TRAJECTORY STUDY OF CROSS-SECTIONS, RATE CONSTANTS, AND KINETIC ISOTOPE EFFECT(D)

The quasiclassical trajectory method has been applied to the calculati on of cross sections and rate constants for the H + D-2 --> HD +D reac tion on three ab initio potential energy surfaces. The results include state-selected cross sections for the reaction with D-2(v=0j=0-9) and D-2(v=1j=0) and thermal rate constants in the 200-1500 K temperature range. A global good agreement is found between the present results an d those from experiment and from approximate quantum mechanical calcul ations. This agreement is particularly good between 200 and 900 K. At higher temperatures, the quasiclassical rate constants deviate gradual ly toward lower values. A detailed comparison is performed between the reactivity of this isotopic variant and that of D + H-2. Special atte ntion is paid to the effect of rotational excitation on reactivity, wh ich is opposite for the two isotopomers, and to the microscopic dynami cs responsible for the observed ratio of thermal rate constants and cr oss sections. In particular, the larger reaction cross section of D H-2 as compared with H + D-2 is found to be caused by the more efficie nt transfer of collision energy from the heavier D atom to the molecul ar bond of the lighter H-2 molecule. These findings can be rationalize d with simple dynamical models.