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, $k_2$ , and the initial repulsive energy in I–CH₂ compression, $P$ . By varying $k_2$ , the model spans the spectrum of possible direct interaction reaction dynamics from the limit of the adiabatic, very slow CsI–CH₂R separation (low $k_2$ limit) to the limit of impulsive release of the I–CH₂ compression (high $k_2$ 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 $k_2$ , near the impulsive limit, is used; (2) the potential surface is largely repulsive, with $\text{P\hspace{0.17em}=\hspace{0.17em}19.0}\mathrm{kcal}/\mathrm{mole}$ 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.