Kinetics of the R+Cl2 (R=CH2Cl, CHBrCl, CCl3 and CH3CCl2) reactions. An abinitio study of the transition states

Abstract

The kinetics of the reactions of CH₂Cl, CHBrCl, CCl₃ and CH₃CCl₂ radicals with molecular chlorine were investigated in a heatable tubular reactor coupled to a photoionization mass spectrometer. The reactions were studied under pseudo-first-order conditions. The radicals were photogenerated at 248 nm. The pressure-independent rate constants determined were fitted to the following Kooij and Arrhenius expressions (units in cm³ molecule^-1 s^-1): k(CH₂Cl)=7.56×10^-17(T)^1.45 exp(-350 J mol^-1/RT), k(CHBrCl)=5.83×10^-20(T)^2.3 exp(-300 J mol^-1/RT), k(CCl₃)=(8.4±2.9)×10^-13 exp[-(25±9) kJ mol^-1/RT] and k(CH₃CCl₂)=1.10×10^-26(T)^4.3 exp(+15000 J mol^-1/RT). The Arrhenius rate expression for the Cl+CCl₄ reaction was determined to be k(Cl+CCl₄)=(3.9±3.2)×10^-13 exp[-(71±9) kJ mol^-1/RT] using the kinetics measured and the thermochemistry of the CCl₃ radical. Errors for the Kooij expressions were estimated to be 25% overall, and for the Arrhenius expressions they were calculated to be 1σ+Student's t values. The transition states of the measured R+Cl₂ and four other similar reactions were localized and fully optimized at the MP2/6-31G(d,p) level of theory by abinitio methods. The energetics of the reactions were considered by determining thermochemical and activation parameters of the reactions. The reactivity differences of the radicals studied were explained by a free-energy correlation using an electronegativity difference scale.