Infrared chemiluminescence studies of the reactions of H atoms with CCl₃, CF₂Cl, and CH₂CH₂Cl radicals at 300 and 475 K: recombination-elimination vs. abstraction mechanisms

Abstract

The reactions of H atoms with CCl₃, CF₂Cl, and CH₂CH₂Cl radicals have been studied in a flow reactor at 300 and 475 K by observation of the infrared emission from the HCl and HF products. These reactions were observed as secondary reactions from the H + CCl₃Br, CF₂ClBr, and CH₂Cl-CH₂I chemical systems. The conditions in the flow reactor were controlled so that the nascent vibrational distributions of HCl and HF were recorded. The pattern of vibrational energy disposal to HCl was used to differentiate between Cl atom abstraction and recombination-elimination mechanisms. The H atom reactions with CCl₃ and CF₂Cl radicals occur only via a recombination-elimination mechanism and give HCl(Nu) or HF(Nu) in a unimolecular step. Thus, the Cl atom abstraction reactions must have ≥ 3.0 kcal mol^-1 higher activation energy than the recombination reaction. From observation of the ratio of the HCl and HF products from CHF₂Cl^∗, the difference in threshold energies for HF and HCl elimination was determined to be ~13 kcal mol^-1. On the other hand, Cl atom abstraction does compete with recombination-elimination in the H + CH₂CH₂Cl reaction, the branching fraction is ~0.3 at 300 K and ~0.6 at 475 K.