Jens Sehested

Absolute rate constants for the reaction of NO with a series of peroxy radicals in the gas phase at 295 K

Abstract The rate constants for the reaction of NO with a series of peroxy radicals: CH 3 O 2 , C 2 H 5 O 2 , (CH 3 ) 3 CCH 2 O 2 , (CH 3 ) 3 CC(CH 3 ) 2 CH 2 O 2 , CH 2 FO 2 , CH 2 ClO 2 , CH 2 BrO 2 , CHF 2 O 2 , CF 2 ClO 2 , CHF 2 CF 2 O 2 , CF 3 CF 2 O 2 , CFCl 2 CH 2 O 2 and CF 2 ClCH 2 O 2 were measured at 298 K and a total pressure of 1 atm. The rate constants were obtained using the absolute technique of pulse radiolysis combined with time-resolved UVVIS spectroscopy. The results are discussed in terms of reactivity trends and the atmospheric chemistry of peroxy radicals.

Absolute rate constants for the reaction of CF3O2 and CF3O radicals with NO at 295 K

Abstract Using a pulse radiolysis UV absorption technique and subsequent simulations of experimental NO 2 and FNO absorption transients, rate constants for reaction between CF 3 O and CF 3 O 2 radicals with NO were determined, CF 3 O 2 + NO→CF 3 O + NO 2 (3), CF 3 O + NO→CF 2 O + FNO (5). k 3 was derived to be (1.68±0.26) × 10 −11 cm 3 molecule −1 s −1 , and k 5 = (5.2±2.7) × 10 −11 cm 3 molecule −1 , s −1 . Results are discussed in the context of the atmospheric chemistry of halocarbons.

Upper limits for the rate constants of the reactions of CF3O2 and CF3O radicals with ozone at 295 K

Abstract Using the pulse radiolysis UV absorption technique and subsequent simulations of experimental absorption transients at 254 and 276 nm, upper limits of the rate constants for the reactions of CF 3 O 2 and CF 3 O radicals with ozone were determined at 295 K, CF 3 0 2 +O 3 →CF 3 O+2O 2 (4), CF 3 O+O 3 →CF 3 O 2 +O 2 (5). The upper limits were derived as k 4 −14 cm 3 molecule −1 s −1 , and k 5 −13 cm 3 molecule −1 s −1 . Results are discussed in the context of the atmospheric chemistry and ozone depletion by hydrofluorocarbons.

UV absorption spectrum of CH3OCH2 radicals and kinetics of the reaction of CH3OCH2O2 radicals with NO and NO2 in the gas phase

Abstract Alkyl and alkylperoxy radicals originating from dimethyl ether have been studied in the gas phase at 296 K. A pulse radiolysis-UV absorption technique was used. Absorption cross-sections were quantified over the wavelength range 220–350 nm. At 230 nm, σCH3OCH2 = (4.2 ± 0.5) × 10−18 cm2 molecule−1 has been obtained. Rate constants for the reaction of the alkylperoxy radical with NO and NO2 were determined to be (9.1 ± 1.0) × 10−12 cm3 molecule−1 s−1 and (7.9 ± 0.4) × 10−12 cm3 molecule−1 s−1, respectively.

Kinetic study of the formation of isotopically substituted ozone in argon

The kinetics of the formation of ozone was studied by using pulse radiolysis coupled with time-resolved UV absorption at 275 nm and at T = 294.9±0.6 K. The rate constant for the formation of ozone 16O16O16O in argon was determined to be k3a = (3.38±0.04) × 10−34 cm6 molecule−2 s−1. The rate constants for the reactions 18O + 16O16O (k3b), 16O + 16O18O (k3c), 16O + 18O18O (k3d), 18O + 16O18O (k3e), and 18O + 18O18O (k3f) were studied, and the following parameters were determined: (k3b + k3d)/(2k3a) = (1.184±0.037), (k3c + k3e)/(2k3a) = (1.155±0.062), and k3f/k3a = (0.977±0.021). The values for (k3b + k3d)/(2k3a) and (k3c + k3e)/(2k3a) obtained here are equal to the values derived from the product studies and the recently reported relative rate study but higher than the reported values for (k3b + k3d)/(2k3a) and (k3c + k3e)/(2k3a) obtained by using CO2 as a third body. The parameter k3f/k3a = (0.977±0.021) is lower than the value of k3f/k3a obtained by using CO2 as a third body and the value derived from the product studies. These different values of k3f may be partly due to changes in third body efficiency or due to resonance interactions between the excited ozone molecules and the third body. The absolute measurements reported here together with literature data suggest that the nature of the third body is an important factor in controlling the enhancements of the rate constants for ozone formation and that asymmetry of neither ozone nor dioxygen ensure a fast ozone formation rate.

First direct kinetic study of isotopic enrichment of ozone

The formation kinetics of ozone has been studied using isotopes and pulse radiolysis combined with time-resolved UV absorption spectroscopy. An enhancement of (9.8±2.6)% was found for the rate constant for the reaction of 18O with 18O18O relative to that for the 16O+ 16O16O reaction. The average formation rate for unsymmetric 18O16O16O and 16O18O18O from O2 and 18O18O respectively was enhanced by (14.7±2.8)%. For the formation of a mixture of symmetric and unsymmetric ozone species from O18O the enhancement was (3.6±2.4)%. This leads to the conclusion that both mass and symmetry affect the rate constant for formation of isotopic ozone. The results are compared with recent enhancement studies from the literature, and an apparent conflict is discussed.

Mechanistic study of the gas-phase reaction of CH2FO2 radicals with HO2

Abstract Fourier transform infrared spectroscopy was used to identify CH 2 FOOH and HC(O)F as products of the gas-phase reaction of CH 2 FO 2 radicals with HO 2 radicals. At 700 Torr and 295 ± 2 K, branching ratios for the CH 2 FOOH forming channel, k 1a / k 1 =0.29 ± 0.08 and the HC(O)F channel, k 1b / k 1 =0.71 ± 0.11 were established. Quoted errors are 2 standard deviations together with our estimate of systematic uncertainties. This result is discussed with respect to previous literature data and to computer models of atmospheric chemistry. As part of this work, the reactivity of Cl atoms towards CH 2 FCl and CH 2 FOOH was investigated; rate constants of k (Cl + CH 2 FCl) = (1.1 ± 0.1) × 10 −13 and k (Cl + CH 2 FOOH) = (1.5 ± 0.5) × 10 −13 cm 3 molecule −1 s −1 were determined.

Hydrofluorocarbons and stratospheric ozone

Recognition of the adverse environmental impact of chlorofluorocarbons (CFCs)1 has led to an international agreement to cease their production. Hydrofluorocarbons (HFCs) are important CFC substitutes. An important question regarding HFCs is: what is their impact on stratospheric ozone? While it is well known that HFCs themselves do not react with ozone, questions have been raised regarding the possibility that species formed during the atmospheric oxidation of HFCs could deplete stratospheric ozone.

Atmospheric chemistry of acetone: Kinetic study of the CH3C(O)CH2O2+NO/NO2 reactions and decomposition of CH3C(O)CH2O2NO2

Pulse radiolysis was used to study the kinetics of the reactions of CH3C(O)CH2O2 radicals with NO and NO2 at 295 K. By monitoring the rate of formation and decay of NO2 using its absorption at 400 and 450 nm the rate constants k(CH3C(O)CH2O2+NO)=(8±2)×10−12 and k(CH3C(O)CH2O2+NO2)=(6.4±0.6)×10−12 cm3 molecule−1 s−1 were determined. Long path length Fourier transform infrared spectrometers were used to investigate the IR spectrum and thermal stability of the peroxynitrate, CH3C(O)CH2O2NO2. A value of k−6≈3 s−1 was determined for the rate of thermal decomposition of CH3C(O)CH2O2NO2 in 700 torr total pressure of O2 diluent at 295 K. When combined with lower temperature studies (250–275 K) a decomposition rate of k−6=1.9×1016 exp (−10830/T) s−1 is determined. Density functional theory was used to calculate the IR spectrum of CH3C(O)CH2O2NO2. Finally, the rate constants for reactions of the CH3C(O)CH2 radical with NO and NO2 were determined to be k(CH3C(O)CH2+NO)=(2.6±0.3)×10−11 and k(CH3C(O)CH2+NO2)=(1.6±0.4)×10−11 cm3 molecule−1 s−1. The results are discussed in the context of the atmospheric chemistry of acetone and the long range atmospheric transport of NOx.

Kinetics of the reaction of F atoms with O2 and UV spectrum of FO2 radicals in the gas phase at 295 K

Abstract The ultraviolet absorption spectrum of FO 2 radicals and the kinetics of the reaction of F atoms with O 2 have been studied in the gas phase at 295 K using pulse radiolysis combined with kinetic UV spectroscopy. At 230 nm, σ FO 2 =(5.08±0.70)×10 −18 cm 2 molecule −1 . The kinetics of the reaction F+O 2 +M→FO 2 +M (1), were investigated over the pressure range 200–1000 mbar of SF 6 diluent. At 1 atm total pressure the pseudo-second-order rate constant for reaction (1) was determined to be (1.9±0.3)×10 −13 cm 3 molecule −1 s −1 .