Determination of Arrhenius parameters for unimolecular reactions of chloroalkanes by IR laser pyrolysis

A simple and reliable method is elaborated for accurate measurements of thermal rate constants of homogeneous gas phase unimolecular reactions. A pulse of CO2 laser radiation was used to multiphoton excite SiF4 sensitizer molecules and consequently produce temperatures in the range 1100–1400 K. Expansion of the heated gas column quenches pyrolysis reactions on a 10 μs time scale. There are no hot surfaces to induce chemistry. HCl elimination from C2H5Cl, Ea=57.4 kcal/mol and log A(s−1)=13.8, was used as an internal temperature standard. For the molecular elimination CCl3CH3 → HCl+CCl2CH2, Ea=49.5±1.3 kcal/mol and log A(s−1)=13.1±0.3, were determined. In these experiments the major decomposition products of CHCl2CH2Cl are HCl and cis- or trans-CHClCHCl with Ea=58.5±2, log A=14.1±0.4 and Ea=59.5±2, log A=13.9±0.4, respectively. HCl elimination to give CCl2CH2 and C–Cl bond breaking to CHClCH2Cl radical have higher activation energies. The method is generally useful for kinetics at high temperature.