Rate Coefficients for the Gas-Phase Reaction of the Hydroxyl Radical with...

Baasandorj, M., G. Knight, V. C. Papadimitriou, R. K. Talukdar, A. R. Ravishankara, and J. Burkholder (2010), Rate Coefficients for the Gas-Phase Reaction of the Hydroxyl Radical with CH2dCHF and CH2dCF2, J. Phys. Chem. A, 114, 4619-4633, doi:10.1021/jp100527z.

Rate coefficients, k, for the gas-phase reaction of the OH radical with CH2dCHF (k1) and CH2dCF2 (k2) were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH and laser-induced fluorescence (PLP-LIF) to detect it. Rate coefficients were measured over a range of temperature (220-373 K) and bath gas pressure (20-600 Torr; He, N2). The rate coefficients were found to be independent of pressure. The measured rate coefficient for reaction 1 at room temperature was k1(296 K) ) (5.18 ( 0.50) × 10-12 cm3 molecule-1 s-1, independent of pressure, and the temperature dependence is given by the Arrhenius expression k1(T) ) (1.75 ( 0.20) × 10-12 exp[(316 ( 25)/T] cm3 molecule-1 s-1; the rate coefficients for reaction 2 were k2(296 K) ) (2.79 ( 0.25) × 10-12 cm3 molecule-1 s-1 and k2(T) ) (1.75 ( 0.20) × 10-12 exp[(140 ( 20)/T] cm3 molecule-1 s-1. The quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. The fall-off parameters for reaction 2 of k∞ ) 3 × 10-12 cm3 molecule-1 s-1 and k0(296 K) ) 1.8 × 10-28 cm6 molecule-2 s-1 with Fc ) 0.6 reproduce the room temperature data obtained in this study combined with the low pressure rate coefficient data from Howard (J. Chem. Phys. 1976, 65, 4771). OH radical formation was observed for reactions 1 and 2 in the presence of O2, and the mechanism was investigated using 18OH and OD rate coefficient measurements with CH2dCHF and CH2dCF2 over a range of temperature (260-373 K) and pressure (20-100 Torr, He). Quantum chemical calculations using density functional theory (DFT) were used to determine the geometries and energies of the reactants and adducts formed in reactions 1 and 2 and the peroxy radicals formed following the addition of O2. The atmospheric lifetimes of CH2dCHF and CH2dCF2 due to loss by reaction with OH are approximately 2 and 4 days, respectively. Infrared absorption spectra of CH2dCHF and CH2dCF2 were measured, and global warming potentials (GWP) values of 0.7 for CH2dCHF and 0.9 for CH2dCF2 were obtained for the 100 year time horizon.

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Upper Atmosphere Research Program (UARP)