Atmospheric Chemical Kinetics of Reactions of 2-butoxy and 3-pentoxy Radicals with NO and O 2 Wei Deng, Andrew J. Davis, Lei Zhang and Dr. Theodore S. Dibble Department of Chemistry, SUNY College of Environmental Science and Forestry, Syracuse, NY Atmospheric Importance 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. 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 NO 2. 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 1. Possible reaction pathways of 2-methyl-3-hexoxy radicals under atmospheric conditions. Figure 2. Laser Induced Fluorescence Experiment Setup Direct kinetic studies of the reactions of 2-butoxy and 3-pentoxy radicals with NO and O 2 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 O 2 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 O 2 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 O 2 reactions would be invaluable for illuminating the dynamics of this important class of reactions. Figure 3. LIF Spectra of 2-butoxy and 3-pentoxy Radicals 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 N 2 ). Figure 5. Plot of pseudo-first-order rate constant for the reaction of 3-pentoxy + NO versus the NO concentration. The bimolecular rate constant (k) for the 3-pentoxy + NO reaction is obtained from the slope of the line in Figure 5. where t is the delay time, k 1st =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 k 1st =k[NO]  k[NO] 0 when [NO]>>[RO] 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 Table 1. Activation energy from direct kinetic studies of the reaction of alkoxy with NO RadicalC2H5OC2H5O1-C 3 H 7 O2-C 3 H 7 Otert-C 4 H 9 O2-C 4 H 9 O3-pentoxy Ea (kJ mol -1 ) Figure 9. Arrhenius plot showing the temperature dependence of the reaction of 3-pentoxy with O 2 Figure 8. Arrhenius plot showing the temperature dependence of the reaction of 2- butoxy with O 2 Methods Alkoxy production in the Laboratory Step 2. Photolysis of RONO vapor Experiment Setup LIF Spectra of 2-butoxy and 3-pentoxy Radicals The pseudo-first-order rate constant k 1st can be obtained from the slope. Table 2. Activation energy from direct kinetic studies of the reaction of alkoxy with O 2 RadicalCH 3 OC2H5OC2H5O1-C 3 H 7 O2-C 3 H 7 O2-C 4 H 9 O3-pentoxy Ea (kJ mol -1 ) Step 1. In a flask Step 3. Laser Induced Fluorescence Kinetic results for 2-butoxy and 3-pentoxy + NO Conclusion Data Analysis Figure 10. A possible reaction pathway for methoxy with O 2 Reactions of alkoxy with O 2 Bofill et al. find the reaction of CH 3 O with O 2 can proceed through a weakly bound prereactive complex as drawn in Figure 10. A negative temperature dependence in RO + O 2 reactions might be rationalized if such a complex strongly influenced the reaction rate. Kinetic results for 2-butoxy and 3-pentoxy + O 2 Negative Temperature Dependence 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. Bofill, J. M.; Olivella, S.; Solé, A.; Anglada, J. M. J. Am. Chem. Soc. 1999, 121, 1337