RESUMO

The pressure dependence of the recombination reaction Cl + FC(O)O + M --> FC(O)OCl + M has been investigated at 296 K. FC(O)O radicals and Cl atoms were generated by laser flash photodissociation of FC(O)OO(O)CF at 193 nm in mixtures with Cl2 and He or SF6 over the total pressure range 8-645 Torr. The measured FC(O)O radical and F atom yields in the photolysis are 0.33 +/- 0.06 and 0.67 +/- 0.06. The reaction lies in the falloff range approaching the high-pressure limit. The extrapolations toward the limiting low- and high-pressure ranges were carried out using a reduced falloff curves formalism, which includes a recent implementation for the strong-collision broadening factors. The resulting values for the low-pressure rate coefficients are (2.2 +/- 0.4) x 10(-28)[He], (4.9 +/- 0.9) x 10(-28)[SF6], (1.9 +/- 0.3) x 10(-28)[Cl2] and (5.9 +/- 1.1) x 10(-28)[FC(O)OO(O)CF] cm3 molecule(-1) s(-1). The derived high-pressure rate coefficient is (4.4 +/- 0.8) x 10(-11) cm3 molecule(-1) s(-1). For the reaction Cl + FC(O)OCl --> Cl2 + FC(O)O a rate coefficient of (1.6 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) was determined. The high-pressure rate coefficient was theoretically interpreted using SACM/CT calculations on an ab initio electronic potential computed at the G3S level of theory. Standard heat of formation values of -99.9 and -102.5 kcal mol(-1) were computed at the G3//B3LYP/6-311++G(3df,3pd) level of theory for cis-FC(O)OCl and trans-FC(O)OCl, respectively. The computed electronic barrier for the conversion between the trans and cis conformers is 8.9 kcal mol(-1). On the basis of the present results, the above reactions are expected to have a negligible impact on stratospheric ozone levels.