Journal of Chemical Physics, Vol.103, No.20, 8884-8890, 1995
Theoretical-Study of the Effect of Reagent Rotation on the Reaction of F+h-2(Nu=0,J)
Quasiclassical calculations on the Muckerman V potential energy surface were carried out on the reaction F+H-2(nu=0, J)-->FH+H at a relative energy of 2 kcal/mol for J=0 to 10. This surface is, to use the classification of Levine and co-workers, very oblate; for a given distance R(c.m.) from F to the center of mass of H-2 the potential energy is much lower for the collinear configuration (chi=0) than for the perpendicular configuration (chi=90 deg). The goal of the work was to understand the effect of molecular rotation on such an extremely oblate surface. It proved useful to decompose the reactive cross section Q(R)(J) into the product of a hitting cross section Q(hit)(not equal)(J) for F hitting H-2 times the probability P-R(J) of reaction occurring once F hits H-2. Both Q(hit)(not equal)(J) and P-R(J) go through minima at J approximate to 4-5. We determined that Q(hit)(not equal)(J=0) is increased by about a factor of 2 by "reorientation" of the H-2 molecule towards a linear configuration by the F atom as it approaches. For J>0 Q(hit)(not equal)(J) declines due both to loss of this reorientation effect as well as to the more oblique approach of the trajectory to the reactant valley. Many trajectories bounce off the repulsive wall near chi=90 deg before the F atom can hit H-2; this effect has been discussed by other authors. The initial decline of P-R(J) with J is due to a relatively unusual feature of the potential surface, whereby rotation of the H-2 molecule away from a linear F-H-H configuration can easily switch the system from the product region back to the reactant region of the system. Both Q(hit)(not equal)(J) and P-R(J) increase above J=5 because the H-2 molecule now has enough rotational energy to rotate through the barrier at chi=90 deg rather than bounce off it.
Keywords:POTENTIAL-ENERGY SURFACE;DIFFERENTIAL CROSS-SECTIONS;TRANSITION-STATE;F+H2 REACTION;+ H-2;DYNAMIC STEREOCHEMISTRY;CHEMICAL-REACTIVITY;VIBRATIONAL-ENERGY;EXCHANGE-REACTIONS;REACTANT ROTATION