Atmospheric Oxidation Mechanism of Methyl Formate

Abstract

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)OCH2· radicals) and 55 ± 7% via attack at the formate group (forming ·C(O)OCH3 radicals). The sole atmospheric fate of HC(O)OCH2· and ·C(O)OCH3 radicals is addition of O2 to give the corresponding peroxy radicals (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 fate 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 two competing atmospheric loss mechanisms for HC(O)OCH2O· radicals; reaction with O2 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)OCH2O· radicals were produced via the HC(O)OCH2OO· + 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)OCH2O· radicals in the HC(O)OCH2OO· + NO reaction. In 1 atm of air ([O2] = 160 Torr) containing NO at 296 K, it can be calculated that 33 ± 5% of the HC(O)OCH2O· radicals undergo α-ester rearrangement, while 67 ± 12% react with O2. The infrared spectrum of the peroxynitrate CH3OC(O)OONO2 was recorded, and absorption cross sections (base e) of (1.8 ± 0.1) × 10-18 and (4.2 ± 0.2) × 10-18 cm2 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)OCH3) = (1.10 ± 0.23) × 10-13, k(Cl + HC(O)OCH2Cl) = (2.5 ± 0.2) × 10-13, and k(Cl + ClC(O)OCH2Cl) = (3.0 ± 0.4) × 10-14 cm3 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.

The atmospheric oxidation of ethyl formate and ethyl acetate over a range of temperatures and oxygen partial pressures

“…The reported uncertainty in the rate coefÞcient ratio is an estimate and is dominated by possible systematic errors in our understanding of the chemistry rather than by precision errors in spectral analysis. The results indicate that the α-ester rearrangement, reaction (9), is about nine times more rapid than reaction (10) [5,7,9,11,[13][14][15].…”

“…Under these conditions, the radical product of reaction (1a) is converted quantitatively to a stable PAN-type compound [9,25], designated here as ethoxyPAN or EoPAN:…”

“…Spectra of the PAN species formed from methyl, isopropyl, and tbutyl formate oxidation in the presence of NO 2 are available from work in our laboratory [9,38], and all show integrated band strengths (for the band centered near 1830 cm −1 ) of (3.9 ± 0.8) × 10 −17 cm molecule −1 and peak cross sections for this band of (1.9 ± 0.4) × 10 −18 cm 2 molecule −1 . Use of these values for EoPAN quantiÞcation allows us to conclude that 18 ± 5% of the reaction of Cl-atoms with ethyl formate occurs via pathway (1a).…”

Characterization of a thermal decomposition chemical ionization mass spectrometer for the measurement of peroxy acyl nitrates (PANs) in the atmosphere

“…Typically, about 200 scans were taken and added to achieve the necessary sensitivity (acquisition time 3-4 min). The chamber and FTIR system have been used extensively for laboratory studies of the photochemical formation of PANs and other species as well as their chemical properties (Wallington et al, 2001;Orlando and Tyndall, 2002;Orlando et al, 2002) Quantification was achieved using absorption cross sections from the literature where available (PAN, (Tsalkani and Toupance, 1989); MPAN, (Orlando et al, 2002); APAN, (Orlando and Tyndall, 2002)). …”

“…(Tyndall et al, 2004;Wallington et al, 2001;Kirchner et al, 1997). Methyl formate is used primarily in the chemical industry to manufacture formamide, dimethylformamide and formic acid.…”