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 NO x at 296 ± 2 K were investigated using three different FTIR smog chamber techniques. Reaction of Cl atoms with methyl formate proceeds 45 ± 7% via attack at the methyl group (forming HC(O)OCH 2 · radicals) and 55 ± 7% via attack at the formate group (forming ·C(O)OCH 3 radicals). The sole atmospheric fate of HC(O)OCH 2 · and ·C(O)OCH 3 radicals is addition of O 2 to give the corresponding peroxy radicals (HC(O)OCH 2 OO· and ·OOC(O)OCH 3 ). The peroxy radicals react with NO to give alkoxy radicals (HC(O)OCH 2 O· and ·OC(O)OCH 3 ). The atmospheric fate of ·OC(O)OCH 3 radicals is decomposition to give CH 3 O· and CO 2 and was unaffected by the method used to generate the ·OC(O)OCH 3 radicals (reaction of ·OOC(O)OCH 3 with either NO or with other peroxy radicals). There are two competing atmospheric loss mechanisms for HC(O)OCH 2 O· radicals; reaction with O 2 to give HC(O)OC(O)H (formic acid anhydride) and α-ester rearrangement to give HC(O)OH and HCO· radicals. It was found that α-ester rearrangement is more important when HC(O)OCH 2 O· radicals were produced via the HC(O)OCH 2 OO· + NO reaction than when they were produced via the self-reaction of peroxy radicals. We ascribe this observation to the formation of vibrationally excited HC(O)OCH 2 O· radicals in the HC(O)OCH 2 OO· + NO reaction. In 1 atm of air ([O 2 ] = 160 Torr) containing NO at 296 K, it can be calculated that 33 ± 5% of the HC(O)OCH 2 O· radicals undergo α-ester rearrangement, while 67 ± 12% react with O 2 . The infrared spectrum of the peroxynitrate CH 3 OC(O)OONO 2 was recorded, and absorption cross sections (base e) of (1.8 ± 0.1) × 10 -18 and (4.2 ± 0.2) × 10 -18 cm 2 molecule -1 at 1836 and 1236 cm -1, respectively, were determined. The chain chlorination of methyl formate was studied. Relative rate techniques were used to measure k (Cl +ClC(O)OCH 3 ) = (1.10 ± 0.23) × 10 -13, k (Cl + HC(O)OCH 2 Cl) = (2.5 ± 0.2) × 10 -13, and k (Cl + ClC(O)OCH 2 Cl) = (3.0 ± 0.4) × 10 -14 cm 3 molecule -1 s -1 . IR spectra of ClC(O)OCH 2 Cl, ClC(O)OCHCl 2, and ClC(O)OCCl 3 are presented. These results are discussed with respect to the atmospheric oxidation mechanism of methyl formate and other esters.