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, E/sub a/ = 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, E/sub a/ = 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 E/sub a/ = 58.5 +- 2, log A = 14.1 +- 0.4 and E/sub a/ = 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