Harmonic Forces Linear Model for Reactions of Cs Atoms with Alkyl Iodides

Abstract

The Cs plus alkyl iodide reaction dynamics are investigated by means of a classical, linear, four-particle model, Cs–I–CH2–R, where R is H, CH3, C2H5, or C3H7, with linear harmonic restoring forces between adjacent particles. The full reaction exothermicity is initially partitioned between potential energy of Cs–I extension and I–CH2 compression, and the trajectories of the four particles are followed until the I–CH2 distance reaches a critical extension, at which point the reaction is assumed to be complete. The model is examined as a function of two parameters, the I–CH2 force constant, k2, and the initial repulsive energy in I–CH2 compression, P. By varying k2, the model spans the spectrum of possible direct interaction reaction dynamics from the limit of the adiabatic, very slow CsI–CH2R separation (low k2 limit) to the limit of impulsive release of the I–CH2 compression (high k2 limit). In contrast to previous calculations, we obtain a good qualitative fit to the experimentally reported product recoil energies if: (1) a relatively high value of k2, near the impulsive limit, is used; (2) the potential surface is largely repulsive, with P = 19.0 kcal/mole out of a total 28.7-kcal/mole reaction exothermicity; and (3) both the CsI and the CH2–R alkyl radical carry off substantial internal excitations.