Vacuum–ultraviolet (147 nm) photodecomposition of 1,1,2-trichloro-2,2-difluoroethane

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Abstract
The 147 nm photolysis of CF2ClCHCl2 has been investigated at 25 °C as a function of reactant pressure, conversion, and nitric oxide as additive. In the absence of NO the observed reaction products are CF2CHCl, CF2CCl2, and the diastereomers of (CF2ClCHCl)2. At constant reactant pressure the quantum yields of the olefin decrease with increasing conversion and there is a corresponding increase in the quantum yield of the C4 product. For fixed values of conversion the olefin quantum yields decrease with increasing reactant pressure and approach limiting values at ∼100 Torr. The addition of NO completely suppresses the formation of the chlorofluorobutanes, while it enhances the olefin quantum yields at higher conversion. These observations are interpreted in terms of reactions of chlorine atoms which result either directly (by near simultaneous expulsion of two Cl atoms), or via the dissociation of an excited Cl2* molecule produced by molecular elimination in the primary process. Chlorine atoms abstract hydrogen from the parent or add to the product olefins. These processes provide the principal source of halo–ethyl radicals in the system. The addition reaction leads to chemically activated radicals with a mean lifetime of τ?0.8×10−8 sec which is commensurate with RRKM‐theory predictions. The addition of nitric oxide provides a competing channel for chlorine atom removal by way of their NO‐catalyzed recombination. The functional dependence of the olefin quantum yields with conversion in the absence and presence of NO suggests that the major fraction of the principal product, CF2CHCl, derives directly from a primary process, while CF2CCl2 is formed via both, the molecular elimination of HCl and from radical precursors. The limiting quantum yields of CF2CHCl and CF2CCl2 are found to be φ0?0.68 and φ0′?0.19, in the absence of NO, respectively, and φ0,NO?0.56 and φ′0,NO?0.087 in the presence of NO. The extinction coefficient for CF2ClCHCl2 at 147 nm and 25 °C has been determined: ε= (1/PL) ln(I0/It) =404±40 atm−1 cm−1.

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