NASA - National Aeronautics and Space Administration

Time-resolved UV–visible absorption spectroscopy has been coupled with UV laser flash photolysis of Cl2/ RI/N2/X mixtures (R ) CH3 or C2H5; X ) O2, NO, or NO2) to generate the RI-Cl radical adducts in the gas phase and study the spectroscopy and reaction kinetics of these species. Both adducts were found to absorb strongly over the wavelength range 310–500 nm. The spectra were very similar in wavelength dependence with λmax ≈ 315 nm for both adducts and σmax ) (3.5 ( 1.2) × 10-17 and (2.7 ( 1.0) × 10-17 cm2 molecule-1 (base e) for CH3I-Cl and C2H5I-Cl, respectively (uncertainties are estimates of accuracy at the 95% confidence level). Two weaker bands with λmax ∼ 350 and 420 nm were also observed. Over the wavelength range 405–500 nm, where adduct spectra are reported both in the literature and in this study, the absorption cross sections obtained in this study are a factor of ∼4 lower than those reported previously [Enami et al. J. Phys. Chem. A 2005, 109, 1587 and 6066]. Reactions of RI-Cl with O2 were not observed, and our data suggest that upper limit rate coefficients for these reactions at 250 K are 1.0 × 10-17 cm3 molecule-1 s-1 for R ) CH3 and 2.5 × 10-17 cm3 molecule-1 s-1 for R ) C2H5. Their lack of reactivity with O2 suggests that RI-Cl adducts are unlikely to play a significant role in atmospheric chemistry. Possible reactions of RI-Cl with RI could not be confirmed or ruled out, although our data suggest that upper limit rate coefficients for these reactions at 250 K are 3 × 10-13 cm3 molecule-1 s-1 for R ) CH3 and 5 × 10-13 cm3 molecule-1 s-1 for R ) C2H5. Rate coefficients for CH3I-Cl reactions with CH3I-Cl (k9), NO (k22), and NO2 (k24), and C2H5I-Cl reactions with C2H5I-Cl (k14), NO (k23), and NO2 (k25) were measured at 250 K. In units of 10-11 cm3 molecule-1 s-1, the rate coefficients were found to be 2k9 ) 35 ( 12, k22 ) 1.8 ( 0.4, k24 ) 3.3 ( 0.6, 2k14 ) 40 ( 16, k23 ) 1.8 ( 0.3, and k25 ) 4.0 ( 0.9, where the uncertainties are estimates of accuracy at the 95% confidence level.