Kinetics of the Gas-Phase Reactions of Cyclo-CF2 CFXCHXCHX-(X = H, F, Cl) with OH Radicals at 253-328 K

Rate constants were determined for the reactions of OH radicals with halogenated cyclobutanes cyclo-CF2CF2CHFCH2-(k(1)), trans-cyclo-CF2CF2CHClCHF-(k(2)), cyclo-CF2CFClCH2CH2-(k(3)), trans-cyclo-CF2CFClCHClCH2-(k(4)), and cis-cyclo-CF2CFClCHClCH2-(k(5)) by using a relative rate method. OH radicals were prepared by photolysis of ozone at a UV wavelength (254 nm) in 200 Torr of a sample reference H2O-O-3-O-2-He gas mixture in an 11.5-dm(3) temperature-control led reaction chamber. Rate constants of k(1) = (5.52 +/- 1.32) x 10(-13) exp[-(1050 +/- 70)/T], k(2) = (3.37 +/- 0.88) x 10(-13) exp[-(850 +/- 80)/T], k(3) = (9.54 +/- 4.34) x 10(-13) exp[-(1000 +/- 140)/T], k(4) = (5.47 +/- 0.90) x 10(-13) exp[-(720 +/- 50)/T], and k(5) = (5.21 +/- 0.88) x 10(-13) exp[-(630 +/- 50)/T] cm(3) molecule(-1) s(-1) were obtained at 253-328 K. The errors reported are +/- 2 standard deviations, and represent precision only. Potential systematic errors associated with uncertainties in the reference rate constants could add an additional 10%-15% uncertainty to the uncertainty of k(1)-k(5). The reactivity trends of these OH radical reactions were analyzed by using a collision theory-based kinetic equation. The rate constants k(1)-k(5) as well as those of related halogenated cyclobutane analogues were found to be strongly correlated with their C-H bond dissociation enthalpies. We consider the dominant tropospheric loss process for the halogenated cyclobutanes studied here to be by reaction with the OH radicals, and atmospheric lifetimes of 3.2, 2.5, 1.5, 0.9, and 0.7 years are calculated for cyclo-CF2CF2CHFCH2-, trans-cyclo-CF2CF2CHClCHF-, cyclo-CF2CFClCH2CH2-, trans-cyclo-CF2CFClCHClCH2-, and cis-cyclo-CF2CFClCHClCH2-, respectively, by scaling from the lifetime of CH3CCl3. (C) 2009 Wiley Periodicals, Inc. Int I Chem Kinet 41:532-542, 2009