Mechanism and Branching Ratios of Hydroxy Ethers + • OH Gas phase Reactions: Relevance of H Bond Interactions Annia Galano,* ,† J. Raul Alvarez-Idaboy,* ,‡ and Misaela Francisco-Ma ´rquez † 1 Departamento de Quı ´mica, UniVersidad Auto ´noma Metropolitana-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, C. P. 09340, Me ´xico D. F. Me ´xico and 2 Facultad de Quı ´mica, Departamento de Fı ´sica y Quı ´mica Teo ´rica, UniVersidad Nacional Auto ´noma de Me ´xico, Me ´xico DF 04510, Me ´xico ReceiVed: April 20, 2010; ReVised Manuscript ReceiVed: May 29, 2010 A theoretical study on the mechanism and branching ratios of the gas phase reactions of hydroxyl radicals with a series of hydroxy ethers is presented. This is the first report on branching ratios for these reactions. The studied hydroxy ethers are: methoxy-methanol (MM), ethoxy-methanol (EM), 1-methoxy-ethanol (1ME), 2-methoxy-ethanol (2ME), and 2-ethoxy-ethanol (2EE). All the possible H abstraction channels have been modeled, involving the rupture of C-H and O-H bonds. The H abstractions from the alcohol group were found to be almost negligible for all the studied systems. The role of H bond interactions in the transition states (TS) is discussed, as well as the importance of the location of the reaction site with respect to the alcohol and the ether functional groups. TSs with seven-member ring-like structures were found to lead to stronger H bond interactions than TSs with six- and five-member ring-like structures, with the latter leading to the weakest interactions. Kinetic calculations have been performed within the 250-440 K temperature range. Rate coefficients for the reactions of • OH with MM, EM, and 1ME are reported here for the first time. Nonlinear Arrhenius plots were found for all the overall reactions. Negative activation energies at room temperature are proposed for the • OH reactions with EM, 2ME, and 2EE. The excellent agreement with the scarce experimental data available supports the reliability of the data reported here for the first time. Introduction Volatile organic compounds (VOCs) are emitted to the atmosphere from anthropogenic and biogenic sources, 1–4 and may also be formed in situ. 5 The oxidation of VOCs leads to the formation of secondary products, which constitutes one of the largest unknowns in the quantitative prediction of the air quality. 6 In fact, it has been established that for modeling and controlling their impact, it is essential to understand the sources of VOC, their distribution, and the chemical transformations they undergo. 6 Hydroxy ethers are widely used solvents, therefore they are likely to be released into the atmosphere. Accordingly, they have been proposed to contribute to the formation of photochemical air pollution in urban and regional areas. 7 However, there is rather scarce information on the atmospheric chemistry of hydroxy ethers, and it all concerns glycol ethers. The most studied ones are 2-methoxy-ethanol (2ME) and 2-ethoxy-ethanol (2EE). Dagaut et al. 8 measured absolute rate constants for the gas- phase reactions of hydroxyl radicals with a series of glycol ethers by flash photolysis resonance fluorescence (FPRF) technique, at 298 K. They also studied the temperature dependence of the rate coefficient (k) of the • OH reaction with 2ME between 240 and 440 K, and propose the following Arrhenius expression: k ) (4.5 ( 1.4) × 10 -12 exp[(325 ( 100)/T] cm 3 molecule -1 s -1 . To the best of our knowledge this is the only previous report on the temperature dependence of a hydroxy ether reaction with • OH. The rate coefficients for the reactions of • OH with 2ME and 2EE were reported to be 1.25 × 10 -11 and 1.87 × 10 -11 cm 3 molecule -1 s -1 , respectively. On the basis of their k values, these authors concluded that the reaction with • OH is the most important sink for glycol ethers in the atmosphere, and that photolysis, reaction with NO 3 , and reaction with O 3 are negligibly slow by comparison. This conclusion is in agreement with the important role of hydroxyl radicals in the troposphere, 9–12 where it is considered as the dominant reactive species in the degradation of organic compounds during daylight hours. 5 Dagaut et al. 8 also proposed that the observed negative activation energy can be rationalized in terms of zero or near- zero activation energy associated with hydrogen atom abstraction from weak C-H bonds combined with the inverse temperature dependence of the pre-exponential factor in the Arrhenius expression. However, the alternative explanation of a complex mechanism via hydrogen bonded prereactant complex as proposed by Singleton and Cvetanovic, 13 has not been consid- ered. Later investigations from our group 14 support the second hypothesis for other VOCs. Moreover, the chemical structure of hydroxy ethers is very close to that of dieters, for which it has been proposed that hydrogen bonds in the transition states could be important in the explanation of negative activation energies and branching ratios. 15 H-bonded transition states drop the enthalpies of activation and decrease the entropy of the transition state, which increases the Gibbs free energy of activation. Therefore, the assumption of negative exponent for T in the rate equation is in disagreement with the role of H bonds in lowering the energy of transition states, which has been described for most of the • OH reactions with oxygenated VOCs. Stemmler et al. 16 also agree that the reaction of glycol ethers with • OH is expected to be their main removal process in the atmosphere. These authors measured the rate coefficients of the reactions of a series of glycol ethers with • OH, relative to those * To whom correspondence should be addressed. E-mail: agalano@ prodigy.net.mx (A. G.), jidaboy@unam.mx (J. R. A.-I.). † Universidad Auto ´noma Metropolitana-Iztapalapa. ‡ Universidad Nacional Auto ´noma de Me ´xico. J. Phys. Chem. A 2010, 114, 7525–7536 7525 10.1021/jp103575f  2010 American Chemical Society Published on Web 06/24/2010