Atmospheric oxidation mechanism of methyl formate

The products following Cl atom initiated oxidation of methyl formate in 700 -760 Torr of air in the presence and absence of NOx at 296 +/- 2 K were inv estigated using three different FTIR smog chamber techniques. Reaction of C l atoms with methyl formate proceeds 45 +/- 7% via attack at the methyl gro up (forming HC(O)OCH2. radicals) and 55 +/- 7% via attack at the formate gr oup (forming .C(O)OCH3 radicals). The sole atmospheric fate of HC(O)OCH2. a nd .C(O)OCH3 radicals is addition of O-2 to give the corresponding peroxy r adicals (HC(O)OCH2OO . and . OOC(O)OCH3). The peroxy radicals react with NO to give alkoxy radicals (HC(O)OCH2O . and . OC(O)OCH3). The atmospheric fa te of . OC(O)OCH3 radicals is decomposition to give CH3O . and CO2 and was unaffected by the method used to generate the OC(O)OCH3 radicals (reaction of . OOC(O)OCH3 with either NO or with other peroxy radicals). There are tw o competing atmospheric loss mechanisms for HC(O)OCH2O . radicals; reaction with O-2 to give HC(O)OC(O)H (formic acid anhydride) and a-ester rearrange ment to give HC(O)OH and HCO . radicals. It was found that alpha -ester rea rrangement is more important when HC(O)OCH2O . radicals were produced via t he HC(O)OCH2OO . + NO reaction than when they were produced via the self-re action of peroxy radicals. We ascribe this observation to the formation of vibrationally excited HC(O)OCH2O . radicals in the HC(O)OCH2OO . + NO react ion. In 1 atm of air ([O-2] = 160 Torr) containing NO at 296 K, it can be c alculated that 33 +/- 5% of the HC(O)OCH2O . radicals undergo a-ester rearr angement, while 67 +/- 12% react with O-2. The infrared spectrum of the per oxynitrate CH3OC(O)OONO2 was recorded, and absorption cross sections (base e) of (1.8 +/- 0.1) x 10(-18) and (4.2 +/- 0.2) x 10(-18) cm(2) molecule(-1 ) at 1836 and 1236 cm(-1), respectively, were determined. The chain chlorin ation of methyl formate was studied. Relative rate techniques were used to measure k(Cl + CIC(O)OCH3) = (1.10 +/- 0.23) x 10(-13), k(Cl + HC(O)OCH2Cl) = (2.5 +/- 0.2) x 10(-13) and k(Cl + ClC(O)OCH2Cl) = (3.0 +/- 0.4) x 10(-1 4) cm(3) molecule(-1) s(-1). IR spectra of ClC(O)OCH2Cl, ClC(O)OCHCl2, and ClC(O)OCCl3 are presented. These results are discussed with respect to the atmospheric oxidation mechanism of methyl formate and other esters.