Kinetics of the Unimolecular Decomposition of the 2-Chloroallyl Radical

The thermal decomposition of the 2-chloroallyl radical, CH2CClCH2 → CH2CCH2 + Cl (1), was studied using the laser photolysis/photoionization mass spectrometry technique. Rate constants were determined in time-resolved experiments as a function of temperature (720−840 K) and bath gas density ([He] = (3−12) × 1016, [N2] = 6 × 1016 molecule cm-3). C3H4 was observed as a primary product of reaction 1. The rate constants of reaction 1 are in the falloff, close to the low-pressure limit, under the conditions of the experiments. The potential energy surface (PES) of reaction 1 was studied using a variety of quantum chemical methods. The results of the study indicate that the minimum energy path of the CH2CClCH2 dissociation proceeds through a PES plateau corresponding to a weakly bound Cl−C3H4 complex; a PES saddle point exists between the equilibrium CH2CClCH2 structure and the Cl−C3H4 complex. The results of quantum chemical calculations, the rate constant values obtained in the experimental study, and literature data on the reverse reaction of addition of Cl to allene were used to create a model of reactions 1 and −1. The experimental dependences of the rate constants on temperature and pressure were reproduced in RRKM/master equation calculations. The reaction model provides expressions for the temperature dependences of the high-pressure-limit and the low-pressure-limit rate constants and the falloff broadening factors (at T = 300−1600 K): k ∞ 1 = 1.45 × 1020 T -1.75 exp(−19609 K/T) s-1, k ∞ - 1 = 8.94 × 10-10 T -0.40 exp(481 K/T) cm3 molecule-1 s-1, k 1 0(He) = 5.01 × 10-32 T -12.02 exp(−22788 K/T) cm3 molecule-1 s-1, k 1 0(N2) = 2.50 × 10-32 T -11.92 exp(−22756 K/T) cm3 molecule-1 s-1, F cent(He) = 0.46 exp(−T/1001 K) + 0.54 exp(−T/996 K) + exp(−4008 K/T), and F cent(N2) = 0.37 exp(−T/2017 K) + 0.63 exp(−T/142 K) + exp(−4812 K/T). The experimental data are not sufficient to specify all the parameters of the model; consequently, some of the model parameters were obtained from quantum chemical calculations and from analogy with other reactions of radical decomposition. Thus, the parametrization is most reliable under conditions close to those used in the experiments.