Atmospheric Chemical Kinetics of Reactions of 2-butoxy and 3-pentoxy Radicals with NO and O 2 Wei Deng, Andrew J. Davis,

1. Atmospheric Chemical Kinetics of Reactions of 2-butoxy and 3-pentoxy Radicals with NO and O2 Wei Deng, Andrew J. Davis, Lei Zhang and Dr. Theodore S. Dibble Department of Chemistry, SUNY College of Environmental Science and Forestry, Syracuse, NY 13210 Atmospheric Importance Experiment Setup Data Analysis Kinetic results for 2-butoxy and 3-pentoxy + NO Alkoxy radicals are important intermediates in the atmospheric degradation of volatile organic compounds (VOC) as well as fascinating targets of basic research. The atmospheric chemistry of large alkoxy radicals is dominated by three reactions. Figure 6. Arrhenius plot showing the temperature dependence of the reaction of 2-butoxy with NO Figure 7. Arrhenius plot showing the temperature dependence of the reaction of 3-pentoxy with NO Figure 4. Typical LIF intensity versus time profiles for 3-pentoxy in the presence of different pressures of NO at 224 K (in 50 torr N2). Figure 5. Plot of pseudo-first-order rate constant for the reaction of 3-pentoxy + NO versus the NO concentration. The pseudo-first-order rate constant k1st can be obtained from the slope. The bimolecular rate constant (k) for the 3-pentoxy + NO reaction is obtained from the slope of the line in Figure 5. Table 1. Activation energy from direct kinetic studies of the reaction of alkoxy with NO where t is the delay time, k1st=k[NO], and k is the bimolecular rate constant. The delay time is the time between the time of the production of alkoxy radicals by the photolysis laser and the time alkoxy radicals are probed by the dye laser. where k1st=k[NO] k[NO]0 when [NO]>>[RO] Kinetic results for 2-butoxy and 3-pentoxy + O2 Negative Temperature Dependence Figure 1. Possible reaction pathways of 2-methyl-3-hexoxy radicals under atmospheric conditions. Reactions of alkoxy with NO The negative temperature dependence of the reaction of 2-butoxy and 3-pentoxy with NO suggests the barrierless radical-radical recombination reaction RO + NO  RONO. Each reaction pathway has different effects on the yield and spatial distribution of ozone formed during a smog episode. Therefore, understanding the alkoxy radical chemistry in the atmosphere is of crucial importance for modeling smog chemistry. However, the present understanding of large alkoxy radicals is primarily based on indirect studies or quantum calculations. Prior to this work, direct kinetic studies had only been carried for the unimolecular decomposition of tert-butoxy radicals and its reactions with NO and NO2. The reactions of alkoxy radicals with NO are not significant in the atmosphere, but the rate constants are valuable in interpreting other studies of alkoxy radicals. Figure 2. Laser Induced Fluorescence Experiment Setup Reactions of alkoxy with O2 Bofill et al. find the reaction of CH3O with O2 can proceed through a weakly bound prereactive complex as drawn in Figure 10. A negative temperature dependence in RO + O2 reactions might be rationalized if such a complex strongly influenced the reaction rate. LIF Spectra of 2-butoxy and 3-pentoxy Radicals Figure 9. Arrhenius plot showing the temperature dependence of the reaction of 3-pentoxy with O2 Figure 8. Arrhenius plot showing the temperature dependence of the reaction of 2-butoxy with O2 Methods Alkoxy production in the Laboratory Table 2. Activation energy from direct kinetic studies of the reaction of alkoxy with O2 Step 1. In a flask Figure 10. A possible reaction pathway for methoxy with O2 Bofill, J. M.; Olivella, S.; Solé, A.; Anglada, J. M. J. Am. Chem. Soc. 1999, 121, 1337 Step 2. Photolysis of RONO vapor Conclusion Direct kinetic studies of the reactions of 2-butoxy and 3-pentoxy radicals with NO and O2 are carried out for the first time by using Laser Induced Fluorescence (LIF) method to directly monitor the disappearance of large alkoxy radicals. Arrhenius expressions were obtained for all the reactions. The rate constants of 2-butoxy with NO are consistent with those previously observed in other alkoxy radicals, while the reaction of 3-pentoxy with NO has a more negative temperature dependence. The reactions of 3-pentoxy and 2-butoxy with O2 exhibit small negative temperature dependencies. This is interesting in light of the small-positive temperature dependencies observed for ethoxy and propoxy radicals. The surprising temperature dependencies observed for the reaction of NO with 3-pentoxy and the reactions of O2 with both 2-butoxy and 3-pentoxy suggest the need for direct kinetics studies of a much more diverse set of alkoxy radicals, not merely of those derived from linear alkanes. Further investigations of the pressure and temperature dependence of the rate of alkoxy with O2 reactions would be invaluable for illuminating the dynamics of this important class of reactions. Step 3. Laser Induced Fluorescence Figure 3. LIF Spectra of 2-butoxy and 3-pentoxy Radicals