Magnitude and orientation of rotation in exchange reactions A + BC å AB + C.II.

A model study is reported of the kinematic (mass-dependent) and the dynamic (potential-energy-surface dependent) factors that play a part in determining the magnitude and polarisation of rotatioinal excitation in the product of exchange reaction. The “model” took the form of a 3D classical trajectory study of a hypothetical exothermic reaction A + BC å AB + C employing all combination of (a) a pair of contrasting potential-energy by hypersurfaces for reaction (one predominantly “repulsive” and one “attractive”), (b) three markedly different reagent mass-combinations (H + HL, L + LL and L + HH, where L  1 amu and H  80 amu), and (c) two different reagent energies (room temperature reagents, or 10 kcal mole^?1 collision energy). For H + HL under all conditions L ≈ J′ (i.e., the initial orbital angular momentum was converted to product rotation), for L + HH J ≈ L′, and for the “normal” intermediate case L + LL all three of J, L and R (the last being the repulsive energy-release) contributed to J′. The angle of orientation, x, of the product rotational vector J′ with respect to the direction of approach v_rel showed a maximum at x ≈ 90° for all conditions. This orientation became less marked along the series H + HL, L + LL, L + HH; i.e., as L played a diminishing part in governing J′. For L + LL the most oriented product AB (x ≈ 90°) was the most backward scattered (θ_at < 90°) on the repulsive surface, but not on the attractive surface. Since both x and θ_at are measurable, this correlation could assist in characterising the surface. For L + LL there was evidence that encounters with reagent rotational and orbital motions in approximately the same plane were the most conducive to reaction; i.e., 2D reaction was favoured. The corresponding degrees of freedom in the products were predominantly coplanar with opposed directions of motion. This led to a simplified picture of the angular motions in the course of reaction.