Experiments and Theory on the Thermal Decomposition of CHCl₃ and the Reactions of CCl₂

Abstract

Rate constants for the thermal decomposition of CHCl3 in Kr diuent have been measured by the laser schlieren density gradient method. The only decomposition process indicated is molecular elimination giving the singlet carbene, CCl2, and HCl. Rate constants are determined under different conditions of density over the temperature range 1282−1878 K, giving k(±15%) = 4.26 × 1016 exp(−22 516 K/T) cm3 mol-1 s-1. Electronic structure calculations have provided models for both the transition state and molecule. With these models, both semiempirical Troe and Rice−Ramsperger−Kassel−Marcus unimolecular theoretical calculations are carried out. The experimental results agree with theory provided E0 = 56.0 kcal mol-1 and 〈ΔE〉down = (820 ± 30) cm-1, suggesting that the barrier for back reaction is 3.8 kcal mol-1. Cl-atom atomic resonance absorption spectrometric (ARAS) experiments, also in Kr diluent, are then carried out, confirming that atom formation is entirely due to the thermal reactivity of CCl2. On the basis of Cl-atom yield measurements, a mechanism for Cl-atom formation is devised. Chemical simulations of the absolute Cl-atom profile data then provide estimates of the temperature dependences for the rate constants used in the mechanism. These results are discussed in terms of unimolecular reaction rate theory suggesting that the heat of formation for CCl radicals is 100 ± 4 kcal mol-1 at 0 K. Our calculated results (R-CCSD(T)) extrapolated to the complete basis set limit give values of Δf = 53.0 and Δf = 102.5 kcal mol-1 and are consistent with the experimental results reported herein. Additionally, the results suggest that CCl2 undergoes dissociative recombination with a substantial activation energy.