Photodissociation of 1,2-chloroiodoethane at 248 and 266 nm: The enthalpy of formation of CH2ClCH2I

Abstract

The technique of photofragmentation translational spectroscopy has been utilized to study the photodissociation of CH2ClCH2I at excitation wavelengths of 248 and 266 nm. The primary photofragments are C2H4Cl and either I*(2P1/2) or I(2P3/2). A fraction of the chloroethyl radical product contains enough internal excitation after the primary process to undergo secondary dissociation into C2H4 and Cl. By observing the threshold for this secondary process, the reaction enthalpy for CH2ClCH2I→C2H4+Cl+I was determined, which leads to: ΔH0f,0 (CH2ClCH2I)=−7.8±1 kcal/mol. The c. m. translational energy distribution of the photofragments was derived for both the I* and I channels, yielding I*/I branching ratios of 1.5 at 248 nm and 3 at 266 nm. The translational energy distribution also revealed that about 50% of the excess energy went into translation. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state iodine channels at both wavelengths proceeds via a parallel transition, i.e., the transition moment must be nearly parallel to the C–I bond.