The atmospheric degradation of HFC-134a (CF3CFH2) proceeds via the formation of CF3CFHO radicals. Long path length FTIR environmental chamber techniques were used to study the atmospheric fate of CF3CFHO radicals. Two competing reaction pathways were identified for CF3CFHO radicals:  reaction with O2, CF3CFHO + O2 → CF3C(O)F + HO2, and decomposition via C−C bond scission, CF3CFHO + M → CF3 + HC(O)F + M. CF3CFHO radicals were produced by two different reactions:  either via the self-reaction of CF3CFHO2 radicals or via the CF3CFHO2 + NO reaction. It was found that decomposition was much more important when CF3CFHO radicals were produced via the CF3CFHO2 + NO reaction than when they were produced via the self-reaction of CF3CFHO2 radicals. We ascribe this observation to the formation of vibrationally excited CF3CFHO* radicals in the CF3CFHO2 + NO reaction. Rapid decomposition of CF3CFHO* radicals limits the formation of CF3C(O)F and hence CF3COOH in the atmospheric degradation of HFC-134a. We estimate that the CF3COOH yield from atmospheric oxidation of HFC-134a is 7−20%. Vibrationally excited alkoxy radicals may play an important role in the atmospheric chemistry of other organic compounds.

The title reaction was studied at 296 K and 0.38-940 Torr total pressure using a FTIR smog chamber technique. The overall rate constant for reaction of CH 3 OCH 2 radicals with O 2 may be written, k 1 ) k RO2 + k prod , where k RO2 is the rate constant for peroxy radical production and k prod is the rate constant for the production of other species from reaction 1. k 1 was measured relative to the pressure independent reaction of CH 3 OCH 2 radicals with Cl 2 (k 4 ). There was no discernible effect of pressure on k 1 in the range 200-940 Torr. Between 200 and 2 Torr total pressure k 1 decreased by approximately a factor of 2. For pressures below 2 Torr k 1 was again independent of pressure. The reaction proceeds via the formation of an activated complex, CH 3 OCH 2 O 2 # , that is either collisionally stabilized to form the peroxy radical, CH 3 OCH 2 O 2 , or undergoes intramolecular H-atom abstraction followed by decomposition to give two formaldehyde molecules and an OH radical: H # f 2HCHO + OH. The products from this reaction were studied as a function of total pressure. The molar yield of formaldehyde increased from <2% at 700 Torr total pressure to ∼200% at 0.38 Torr total pressure, while the combined yield of methyl formate and methoxy methylhydroperoxide decreased from ∼100% to 4% over the same pressure range. Fitting the product yields and relative rate data using a modified Lindemann expression gave the following rate constants: k RO2,0 /k 4 ) (1.97 ( 0.28) × 10 -19 cm 3 molecule -1 , k RO2,∞ /k 4 ) 0.108 ( 0.004, and k prod,0 /k 4 ) (6.3 ( 0.5) × 10 -2 where k RO2,0 and k RO2,∞ are the overall termolecular and bimolecular rate constants for formation of the CH 3 OCH 2 O 2 radical and k prod,0 represents the bimolecular rate constant for the reaction of CH 3 OCH 2 radicals with O 2 to yield formaldehyde in the limit of low pressure. These data and absolute rate data from the literature were used to derive a rate constant for the reaction of CH 3 OCH 2 radicals with Cl 2 of (1.00 ( 0.16) × 10 -10 cm 3 molecule -1 s -1 . The results are discussed in the context of the use of dimethyl ether as an alternative diesel fuel.Dimethyl Ether Oxidation

A flash photolysis-resonance fluorescence technique was used to study the rate constant for the reaction of OH radicals with dimethyl carbonate over the temperature range 252-370 K. The rate constant exhibited a weak temperature dependence, increasing at both low and high temperature from a minimum value of approximately 3.1 × 10 -13 cm 3 molecule -1 s -1 near room temperature. Pulse radiolysis/transient UV absorption techniques were used to study the ultraviolet absorption spectra and kinetics of CH 3 OC(O)OCH 2 and CH 3 OC(O)CH 2 O 2 radicals at 296 K. Absorption cross sections of CH 3 OC(O)OCH 2 and CH 3 OC(O)OCH 2 O 2 at 250 nm were (3.16 ( 0.34) × 10 -18 and (3.04 ( 0.43) × 10 -18 cm 2 molecule -1 , respectively. Rate constants measured for the self-reactions of CH 3 OC(O)OCH 2 and CH 3 OC(O)OCH 2 O 2 radicals and reactions of CH 3 OC(O)OCH 2 O 2 radicals with NO and NO 2 were (5.6 ( 1.1) × 10 -11 , (1.27 ( 0.21) × 10 -11 , (1.2 ( 0.2) × 10 -11 , and (1.2 ( 0.2) × 10 -11 cm 3 molecule -1 s -1 , respectively. The rate constant for reaction of F atoms with dimethyl carbonate was determined by a pulse radiolysis absolute rate technique to be (6.1 ( 0.9) × 10 -11 cm 3 molecule -1 s -1 . A FTIR smog chamber system was used to show that, in 760 Torr of air at 296 K, CH 3 OC(O)OCH 2 O radicals are lost via three competing processes: 42 ( 15% via reaction with O 2 , 14 ( 2% via H atom elimination, and 44 ( 10% via decomposition and/or isomerization. Relative rate techniques were used to measure rate constants for the reactions of F atoms with CH 3 OC(O)OCH 3 , (6.4 ( 1.4) × 10 -11 cm 3 molecule -1 s -1 , and Cl atoms with CH 3 OC(O)OCH 3 , CH 3 OC(O)OCH 2 Cl, CH 3 OC(O)-OCHO, and HC(O)OC(O)OCHO, (2.3 ( 0.8) × 10 -12 , (4.6 ( 2.8) × 10 -13 , (1.7 ( 0.1) × 10 -13 , and (1.7 ( 0.1) × 10 -14 cm 3 molecule -1 s -1 , respectively. Results are discussed in the context of the atmospheric chemistry of CH 3 OC(O)OCH 3 .

A pulse-radiolysis system was used to measure absolute rate constants for the reactions of CH 3 C(O)O 2 radicals with NO and NO 2 at 295 K and 1000 mbar total pressure of SF 6 . When the rate of formation and decay of NO 2 using its absorption at 400.5 and 452 nm were monitored, the rate constants2) × 10 -11 cm 3 molecule -1 s -1 were determined. Long path-length Fourier transform infrared spectrometers were used to study the rate-constant ratioin 6-700 Torr total pressure of N 2 diluent at 243-295 K. At 295 K in 700 Torr of N 2 diluent k(CH 3 C(O)O 2 + NO)/k(CH 3 C(O)O 2 + NO 2 ) ) 2.07 ( 0.21. The results are discussed in the context of the atmospheric chemistry of acetylperoxy radicals.

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