KINETICS OF THE SELF-REACTION OF NEOPENTYL RADICALS Ksenia A. Loginova and Vadim D. Knyazev Department of Chemistry, The Catholic University of America, Washington, DC, USA neo-C 5 H 11 + neo-C 5 H 11 → C 10 H 22 (1) Introduction Radical-radical reactions are among the very important elementary processes occurring in the oxidation and pyrolysis of hydrocarbons and substituted hydrocarbons. Usually, although not without exceptions, these reactions serve as chain termination pathways. In addition, radical-radical reactions represent pathways of molecular mass growth. Certain members of this class of reactions have been linked to formation of aromatic rings and polyaromatic hydrocarbons (PAH), which leads, in turn, to production of soot in combustion systems. In spite of the importance of these reactions, experimental information on them is rather sparse and, in many cases, controversial. This lack of data is primarily due to the difficulties encountered in experimental studies of reactions between radicals. Typical experimental difficulties encountered in studies of radical-radical reactions: Often, radical sources create reactive byproducts that interfere with the kinetics of the reaction under study. Exact knowledge of radical concentrations is needed for determination of rate constants. Errors in calibration of radical signal directly translate into errors in rate constants. Experimental Method Laser Photolysis / Photoionization Mass Spectrometry Radicals are created via 193 nm laser photolysis of oxalyl chloride 1 with subsequent fast conversion of the Cl atoms into neo-C 5 H 11 radicals: No photolysis byproducts interfere with the kinetics of the reaction under study. Kinetics of radical decay and growth of products are monitored in real time using photoionization mass spectrometry. Initial concentrations of radicals are obtained directly from real-time HCl signal profiles. Kinetic mechanism in the experimental system: Analytical solution for neopentyl radical signal: h (CClO) 2 (oxalyl chloride)  2 Cl + 2 CO Cl + neo-C 5 H 12  HCl + neo-C 5 H 11 Daly Detector PMT Signal Processing Exit Lens Target Computer Diffusion Pump Laser Photolysis / Photoionization Mass Spectrometry Apparatus Experimental Conditions: T = 300 – 500 K; [He] = (3 – 12)  ; [(CClO) 2 ] = (2.5 – 18)  molecule cm -3 ; [neo-C 5 H 12 ] = (2.4 – 8.7)  molecule cm -3 ; [neo-C 5 H 11 ] 0 = (2 – 14)  molecule cm -3. Typical ion signal (S C5H11 ), the corresponding reciprocal signal (1/S C5H11 ), and product (C 10 H 22 ) profiles. Curvature of the 1/S C5H11 plot indicates the presence of heterogeneous loss of radicals with the first-order constant k w. Values of the k 1 [C 5 H 11 ] 0 product and k w are obtained from fitting the S C5H11 vs time profiles. Data Analysis Kinetics of neo-C 5 H 11 decay is studied at varied initial radical concentrations. Values of the k 1 [neo-C 5 H 11 ] 0 product are obtained from fitting the [neo-C 5 H 11 ] vs time profiles. Initial concentrations of neo-C 5 H 11 ([neo-C 5 H 11 ] 0 ) are determined from the real-time HCl profiles. Fitted values of the k 1 [neo-C 5 H 11 ] 0 product are plotted as a function of [neo-C 5 H 11 ] 0 to ensure the absence of systematic deviations from linearity. The values of k 1 are obtained from the slopes of the k 1 [neo-C 5 H 11 ] 0 vs [neo-C 5 H 11 ] 0 dependences obtained at different temperatures. k 1 (T) = 3.1  10 ‑ 12 exp(+506 K/T) (  17 %) cm 3 molecule ‑ 1 s ‑ 1 Results The room-temperature value of the rate constant ((1.63  0.36)  10 ‑ 11 cm 3 molecule ‑ 1 s ‑ 1 ) is in agreement with the value of (2.1  0.3)  10 ‑ 11 cm 3 molecule ‑ 1 s ‑ 1 obtained by Nielsen et al. 2 The rate constant demonstrates strong negative temperature dependence. Comparison with data on other alkyl radical self-reactions: There are the only three alkyl radical self-reactions for which directly determined temperature-dependent rate constants are available in the literature: self-reactions of methyl, ethyl, and cyclohexyl radicals. Within the 300 – 500 K temperature range, the rate constant of neopentyl self-reaction decreases with temperature much faster than the rates of self- reactions of the two smaller radicals (CH 3 and C 2 H 5 ) but with a manner comparable with that of cyclohexyl radicals. The difference between the temperature dependences of the rate constants of the self reactions of neopentyl and cyclehexyl radocals and those of methyl and ethyl radical is in general agreement with the observation by Klippenstein et al. 3 that additional substituents at the radical center and increasing steric bulk of radicals result in stronger negative temperature dependences of their theoretically calculated recombination rate constants. References 1. Baklanov, A. V.; Krasnoperov, L. N. J. Phys. Chem. A 2001, 105, Nielsen, O. J.; Ellermann, T.; Wallington, T. J. Chem. Physl. Lett., 1993, 203, Klippenstein, S. J.; Georgievskii, Y.; Harding, L. B. Phys. Chem. Chem. Phys. 2006, 8, Acknowledgment This research was supported by U.S. National Science Foundation, Combustion, Fire, and Plasma Systems Program under Grant No CBET neo-C 5 H 11 + neo-C 5 H 11  C 10 H 22 (bineopentyl) (k 1 ) neo-C 5 H 11  wall decay (k w ) The k 1 [neo-C 5 H 11 ] 0 vs [neo-C 5 H 11 ] 0 dependences obtained at different temperatures. Circles: bath gas density of 12.0  molecule cm -3, squares: bath gas density of 3.0  molecule cm -3.