Environ. Sci. Technol. 1999, 33, 2885-2890
Products of the Gas-Phase Reaction of the OH Radical with the Dibasic Ester CH3OC(O)CH2CH2CH2C(O)OCH3 ERNESTO C. TUAZON,* SARA M. ASCHMANN, AND R O G E R A T K I N S O N * ,† Air Pollution Research Center, University of California, Riverside, California 92521
Dibasic esters are potentially important commercial solvents and their use will lead to possible release into the atmosphere, where they will react with the hydroxyl (OH) radical and contribute to the formation of photochemical air pollution in urban and regional areas. We have investigated the products formed from the gas-phase reaction of the OH radical with the dibasic ester dimethyl glutarate [CH3OC(O)CH2CH2CH2C(O)OCH3] in the presence of NO in large volume chambers with analysis by in situ Fourier transform infrared (FTIR) absorption spectroscopy, in situ atmospheric pressure ionization tandem mass spectrometry (API-MS), gas chromatography with flame ionization detection, and combined gas chromatography-mass spectrometry. No information was obtained concerning the reaction products using the gas chromatographic analyses. The in situ API-MS and API-MS/MS analyses indicated the formation of products with molecular weights 146, 174, and 221, and, based on plausible reaction schemes, these could be mono-methyl glutarate [CH3OC(O)CH2CH2CH2C(O)OH], dimethyl 1,3-acetonedicarboxylate [CH3OC(O)CH2C(O)CH2C(O)OCH3], and/or its isomers, and an organic nitrate(s), respectively. The in situ FT-IR spectroscopic analyses were consistent with these expectations, with the identification and quantification of organic nitrate(s), monomethyl glutarate, and dimethyl 1,3-acetonedicarboxylate with formation yields of 15 ( 6, 34 ( 16, and 28 ( 8%, respectively. These products therefore account for 77 ( 19% of the total reaction products and reaction pathways, recognizing that the quantifications of mono-methyl glutarate and dimethyl 1,3-acetonedicarboxylate are subject to significant uncertainties.
Introduction Dibasic esters are potentially important commercial solvents and their use will lead to possible release into the atmosphere, where they will react with the hydroxyl (OH) radical (1) and contribute to the formation of photochemical air pollution in urban and regional areas (2). We have recently measured rate constants for the gas-phase reactions of dimethyl succinate [CH3OC(O)CH2CH2C(O)OCH3; DBE-4], dimethyl glutarate [CH3OC(O)CH2CH2CH2C(O)OCH3; DBE-5], and dimethyl adipate [CH3OC(O)CH2CH2CH2CH2C(O)OCH3; DBE* To whom correspondence should be addressed. Phone: (909)7874191; fax: (909)787-5004; e-mail:
[email protected]. † Also Department of Environmental Sciences and Department of Chemistry, University of California, Riverside. 10.1021/es990142x CCC: $18.00 Published on Web 07/17/1999
1999 American Chemical Society
6] with the OH radical (1). These measured OH radical reaction rate constants, combined with an estimated tropospheric 24-h average OH radical concentration of 1.0 × 106 molecule cm-3 (3), lead to calculated lifetimes ranging from ∼1.4 days for DBE-6 to ∼8.3 days for DBE-4. Concurrent environmental chamber and computer modeling studies of ozone formation in irradiated NOx-volatile organic compound (VOC)-air mixtures in the presence of the dibasic esters showed that DBE-4 is less reactive with respect to ozone formation than is ethane (the criteria compound used by the U.S. Environmental Protection Agency to assess “reactive” versus “non-reactive” VOCs), DBE-5 is of comparable reactivity to ethane, and DBE-6 is more reactive than ethane (4). The precise reactivity of DBE-5 depends on the atmospheric degradation mechanism assumed and, because no product data are available, there is a significant uncertainty in the mechanism of the OH radicalinitiated reaction of DBE-5 and hence in its ozone-forming potential under atmospheric conditions (4). Accordingly, we have investigated the products of the reaction of the OH radical with DBE-5 in the presence of NO using a variety of analytical methods.
Experimental Methods Experiments were carried out at 298 ( 2 K and 740 Torr total pressure of air in a 5870 L evacuable, Teflon-coated chamber containing an in situ multiple-reflection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FTIR) absorption spectrometer, with irradiation provided by a 24kW xenon arc filtered through a 0.25 in. thick Pyrex pane (to remove wavelengths 300 nm (5,6), and NO was added to the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals (5). Experiments carried out with analyses of irradiated CH3ONO-NO-DBE-5-air mixtures using gas chromatography with flame ionization detection (GC-FID) and combined gas chromatography-mass spectrometry (GCMS) showed the absence of any GC peaks attributable to reaction products formed from DBE-5. Furthermore, attempts to gas chromatograph the potential reaction product dimethyl 1,3-acetonedicarboxylate [CH3OC(O)CH2C(O)CH2C(O)OCH3] (vide infra) were unsuccessful. Accordingly, analyses of reaction products were carried out by in situ FTIR absorption spectroscopy and in situ API-MS. Teflon Chamber with Analysis by API-MS. In the experiments with API-MS analyses, the chamber contents were sampled through a 25-mm diameter x 75-cm length Pyrex tube at ∼20 L min-1 directly into the API-MS source. The operation of the API-MS in the MS (scanning) and MS/MS [with collision activated dissociation (CAD)] modes has been described elsewhere (7). Use of the MS/MS mode with CAD allows the “daughter ion” or “parent ion” spectrum of a given ion peak observed in the MS scanning mode to be obtained (7). The positive ion mode was used in these API-MS and API-MS/MS analyses, with protonated water hydrates (H3O+(H2O)n) generated by the corona discharge in the chamber diluent gas being responsible for the protonation of analytes. VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Products Formed from the Gas-Phase Reactions of the OH Radical with DBE-5 in the Presence of NO, As Observed by API-MS and API-MS/MS Analyses product or reactant DBE-5 MDBE
(MW 160)
Product #1 M1
(MW 146)
Product #2 M2
(MW 174)
Product #3 M3
(MW 221)
API-MS data
other evidence
MDBE + H ) 161 MDBE + H2O + H ) 179 MDBE + MDBE + H ) 321 MDBE + MDBE + H + H2O ) 339 M1 + H ) 147 M1 + MDBE + H ) 307 M1 + MDBE + H2O + H ) 325
other parents of 161 u ion peak observed at 307 [MDBE +M1 + H], 335 [MDBE + M2 + H], and 353 [MDBE + M2 + H2O + H] u (Figure 1) MS/MS of 147 u and 307 u ion peaks; parents of 147 u ion peak observed at 293 [M1 + M1 + H], 307, 321 [M1 + M2 + H], 325, 339 [M1 + M2 + H2O + H] and 368 [M1 + M3 + H] u (Figure 2A) MS/MS of 175 u ion peak similar to that of authentic standard (Figure 3); MS/MS of 335 u and 353 u ion peaks; parents of 175 u ion peak observed at 321 [M2 + M1 + H], 335, 349 [M2 + M2 + H], 353 and 367 [M2 + M2 + H2O + H] u (Figure 2B) parent of 147 u ion peak observed at 368 [M3 + M1 + H] u (Figure 2A)
M2 + H ) 175 M2 + MDBE + H ) 335 M2 + MDBE + H + H2O ) 353
Ions are drawn by an electric potential from the ion source through the sampling orifice into the mass-analyzing first quadrupole or third quadrupole. For these experiments, the API-MS instrument was operated under conditions that favored the formation of dimer ions in the ion source region (7). Neutral molecules and particles are prevented from entering the orifice by a flow of high-purity nitrogen (“curtain” gas), and as a result of the declustering action of the curtain gas on the hydrated ions, the ions that are mass-analyzed are mainly protonated molecular ions ([M+H]+) and their protonated homo- and hetero-dimers (7). The initial concentrations of CH3ONO, NO, and DBE-5 were ∼4.8 × 1013 molecule cm-3 each, and irradiations were carried out for 30 min at 20% of the maximum light intensity. Evacuable Chamber Experiments with FTIR Analysis. For the experiments carried out in the 5870 L evacuable, Teflon-coated chamber (at
