A simple model for product rovibrational distributions in elementary chemical reactions

We explore the application of a simple model of collisional processes, deve loped initially for inelastic collisions, to the analysis of product rovibr ational states in elementary chemical reactions. The model depicts collisio nal transfer as a process of momentum exchange (predominantly linear-to-ang ular momentum) and is modified to take account of change in center-of-mass and enthalpy change that accompany reaction. The kinematics of center-of-ma ss shift derived by Elsum and Gordon [J. Chem. Phys. 76, 3009 (1982)] lead to two limiting cases based on the parameter beta. The kinematic extremes a lternatively may be specified in terms of the molecular torque arm about wh ich interconversion of linear and angular momentum is effected. This torque arm length approximates to the product bond length when beta similar or eq ual to 0 and the reactant bond length when beta similar or equal to 90 degr ees. Our approach shares elements in common with the classical kinematic mo del of Elsum and Gordon but is somewhat simpler and more transparent. The m ethod is shown to give accurate peak values of v, j states of the products of a wide range of elementary reactions for which experimental data is avai lable. Monte Carlo trajectory calculations based on the physical principles described here give excellent fits to experimental v, j distributions in F +I-2--> IF+I, H+D-2--> HD+D, and Cl+H-2--> HCl+H using input data consistin g of atomic radii, atomic masses, velocities, and reaction enthalpies. (C) 2000 American Institute of Physics. [S0021-9606(00)00409-8].