NASA - National Aeronautics and Space Administration

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the Cl(2PJ) + C2Cl4 association reaction as a function of temperature (231-390 K) and pressure (3-700 Torr) in nitrogen buffer gas. The reaction is found to be in the falloff regime between third and second order over the range of conditions investigated, although the second-order limit is approached at the highest pressures and lowest temperatures. At temperatures below 300 K, the association reaction is found to be irreversible on the experimental time scale of ∼20 ms. The kinetic data at T < 300 K have been employed to obtain falloff parameters in a convenient format for atmospheric modeling. At temperatures above 330 K, reversible addition is observed, thus allowing equilibrium constants for C2Cl5 formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction: ∆H°298 ) -18.1 ( 1.3 kcal mol-1, ∆H°0 ) -17.6 ( 1.3 kcal mol-1, and ∆S°298 ) -27.7 ( 3.0 cal mol-1 K-1. In conjunction with the well-known heats of formation of Cl(2PJ) and C2Cl4, the above ∆H values lead to the following heats of formation for C2Cl5 at 298 and 0 K: ∆H°f,298 ) 8.0 ( 1.3 kcal mol-1 and ∆H°f,0 ) 8.1 ( 1.5 kcal mol-1. The kinetic and thermochemical parameters reported above are compared with other reported values, and the significance of reported association rate coefficients for understanding tropospheric chlorine chemistry is discussed.