Combining transition state calculations with quasiclassical trajectory calculations: II. Collinear collisions involving vibrationally excited reagents

Conventional quasiclassical trajectory simulations of collinear reactive collisions, A + BC (v = 0, 1 and 2) → AB + C, have been compared with trajectories integrated forward and backward in time from points along a dividing line S^* in the strong interaction zone. Calculations have been performed on three different potential energy surfaces, ranging from a strongly attractive surface with a 1 kcal mol⁻¹ barrier to a strongly repulsive surface with a 10 kcal mol⁻¹ barrier, and for all combinations of light (1 amu) and heavy (35 amu) atoms. Two methods of selecting S^* have been examined. The first, based on defining vibrationally adiabatic states orthogonal to the minimum energy path by an approximate analysis, works well for many combinations of potential energy surface and atomic masses. However, a better method is to use pods (periodic orbiting dividing surfaces) for which the action over one cycle of the pods motion is equal to (v + 1/2)h. In only a few cases, where the pods cross the minimum energy path after substantial curvature in the latter, is the agreement between the two sets of calculations less than very good. The results confirm that reagent vibrational motion is in many cases strongly adiabatic up to S^* (i.e. the transition state), and suggest that similar combined calculations on three-dimensional systems should provide a substantial saving in computer effort compared with conventional quasiclassical trajectory methods.