TIME-RESOLVED PHOTOTHERMAL GRATING CALORIMETRY FOR INVESTIGATING KINETICS OF FREE-RADICAL CHAIN-REACTIONS

Diffraction from photothermal gratings is monitored on a nanosecond time scale to determine the rates of a propagating free-radical chain reaction. The kinetics of photochlorination of chloroform, initiated by 355 nn laser radiation, are investigated. Monitoring these reactions on a nanosecond time scale avoids chain termination and enables the time-resolved diffraction transients to be interpreted using a simple kinetic model for the two propagation steps of the chain reaction. From the pseudo-first-order rate parameter of this simple kinetic model, individual rate constants for the steps that propagate the chain are determined from Stern-Volmer relationships. The rate constant for hydrogen abstraction from chloroform by a chlorine atom in carbon tetrachloride is determined, k(1) = (7.8 +/- 0.6) x 10(6) M(-1) s(-1), and the chlorine atom transfer rate constant is found, k(2) = (4.6 +/- 0.4) x 10(8) M(-1) s(-1). The reported rate constants do not depend on the concentration of radicals generated in the laser pulse. This direct approach is an attractive alternative to stationary-state techniques that generally only yield composite rate constants.