Theoretical study of the intramolecular isotope effect in the reaction of F+HD.

Quasiclassical trajectory calculations on the Muckerman V potential energy surface were carried out for the reaction F+HD(υ=0, J)→FH+D or FD+H. The reactive cross sections for the two product channels as well as the isotope effect were calculated for J=0 over the relative collision energy range 0.8–900 kcal/mol and also for 0≤J≤10 at a collision energy of 2 kcal/mol. The results are compared to those for F+H2→FH+H. The goal of this work was to understand the remarkable isotope effect behavior in both sets of calculations. It proved useful to decompose the reactive cross sections QR(FH) and QR(FD) into the product of three factors; the cross sections for F hitting the H end and the D end of HD, Qhit(FHD) and Qhit(FDH); the probabilities of reacting to give either FH or FD once the H end or D end is hit, PR(FHD) and PR(FDH); and the corrections for knockout collisions at both ends of the molecule, CKO(FHD) and CKO(FDH). (A knockout collision is one where the F atom initially hits the H end but reacts to form FD or vice versa.) All three factors make important contributions to the isotope effect. At low collision energies for J=0 FD is favored over FH because F hits the D end of HD more often and because the overall reactivity is higher at the D end. Both effects become less important at higher energies, but above 30 kcal/mol knockout reactions strongly favor the production of FD. The isotope effect as a function of J is quite remarkable. Although FD is favored for J=0 at E=2 kcal/mol, by J=3 no trajectories hit the D end, and no DF is formed. But at J=7 DF product reappears, coinciding with the onset of knockout reactions and an increase in reactivity at both ends of HD. We attribute all of these effects to the ability of the system at high J to rotate through the barrier separating the reactant valleys at each end of HD. The results obtained in this paper are explained by reference to special features of the potential... [ABSTRACT FROM AUTHOR]