1 Disproportionation Channel of Self-Reaction of Hydroxyl Radical, 2 OH + OH → H 2 O + O, Studied by Time-Resolved Oxygen Atom Trapping 3 Manuvesh Sangwan and Lev N. Krasnoperov* 4 Department of Chemistry and Environmental Science, New Jersey Institute of Technology University Heights, Newark, New Jersey 5 07102, United States 6 ABSTRACT: The disproportionation channel of the self-reaction of hydroxyl 7 radicals, OH + OH → H 2 O + O (1a) was studied using pulsed laser photolysis 8 coupled to transient UV-vis absorption spectroscopy over the 298-414 K tem- 9 perature and 3-10 bar pressure ranges (bath gas He). To distinguish channel 1a 10 from the recombination channel 1b, OH + OH → H 2 O 2 (1b), time-resolved 11 trapping of oxygen atoms, produced in channel 1a, was used. The ozone produced 12 in the reaction of oxygen atoms with molecular oxygen was measured using strong 13 UV absorption at 253.7 nm. The results of this study (k 1a = (1.38 ± 0.20) × 10 -12 14 (T/300) -0.76 confirm the IUPAC recommended value of Bedjanian et al. (J. Phys. 15 Chem. A 1999, 103, 7017-7025), as well as the negative temperature dependence 16 over the temperature range studied, and do not confirm the ca. 1.8 higher value 17 obtained in the most recent study of Bahng et al. (J. Phys. Chem. A 2007, 111, 18 3850-3861). The V-shaped temperature dependence of k 1a based on combined 19 current and previous studies in the temperature range of 233-2380 K is k 1a = (5.1 20 exp(-T/190 K) + 0.30(T/300 K) 1.73 ) × 10 -12 cm 3 molecule -1 s -1 . 21 ■ INTRODUCTION 22 Hydroxyl radical plays an important role both in atmospheric 3-9 23 and in combustion 10-16 chemistry. Self-reaction of hydroxyl 24 radical is very important in the laboratory kinetic studies as well 25 as for fundamental chemical kinetics: + → + OH OH HO O 2 26 (1a) + → OH OH HO 2 2 27 (1b) 28 In the reaction mechanisms used in the laboratory studies of 29 radical-radical reactions with the participation of hydroxyl 30 radical, reaction 1 not only serves as an additional sink for hy- 31 droxyl radicals but also initiates a sequence of secondary, highly 32 undesirable reactions of oxygen and hydrogen atoms, formed in 33 channel 1a, and their subsequent reactions. Therefore, accurate 34 values of the rate constant of reaction 1 as well as the branching 35 ratios for the disproportionation channel 1a and recombination 36 channel 1b are important. Kinetic behavior of reaction 1 is com- 37 plicated due to the existence of the two channels 1a and 1b. The 38 disproportionation channel 1a is assumed to be pressure in- 39 dependent; however, the recombination channel is pressure de- 40 pendent over the wide range of temperatures and pressures. In 41 addition, our recent study revealed a peculiar V-shaped tem- 42 perature dependence of the rate constant of channel 1a, with 43 negative temperature dependence at ambient and slightly elevated 44 temperatures and positive at temperatures higher than ca. 450 K. 17 45 Reaction 1 was exhaustively studied both experimentally 1,2,17-36 46 and theoretically. 37-46 Detailed summary of the previous works is 47 given in recent publications. 2,17 The temperature dependence of 48 channel 1a near ambient temperature was a subject of minor 49 controversy in the past. A positive temperature dependence was 50 measured in one experimental study, 17 although subsequent 51 studies resulted in a small negative temperature dependence. 30 It 52 appears that the negative temperature dependence of the dispro- 53 portionation channel 1a is well established. 54 The absolute value of the rate constant of the disproportio- 55 nation channel 1a is a subject of current controversy. Bedjanian 56 et al., using discharge flow combined with mass-spectrometric 57 detection, obtained a slightly negative temperature dependence 58 in the range 233 to 360 K at 1 Torr of He. 1 The measured ap- 59 parent activation energy was -1.7 kJ mol -1 , and the room tem- 60 perature rate constant was 1.43 × 10 -12 cm 3 molecule -1 s -1 . 61 Both the room temperature value of (1.4 ± 0.2) × 10 -12 cm 3 62 molecule -1 s -1 and the negative temperature dependence of 63 Bedjanian et al. are currently accepted in the IUPAC recom- 64 mendations. 47 However, the most recent direct measurements of 65 Bahng and Macdonald 2 using laser pulsed photolysis coupled to 66 time-resolved IR absorption are in significant, almost a factor 67 of 2, disagreement with this value (2.7 ± 0.9) × 10 -12 cm 3 68 molecule -1 s -1 ). 69 The purpose of this study was to resolve this discrepancy using 70 a completely different approach. Typically, separation of the 71 pressure independent channel from the pressure dependent 72 recombination channel is performed via kinetic measurements at 73 low pressures (Torr range) where the recombination channel can 74 be neglected. This leads to difficulties caused by high wall activity 75 toward heterogeneous reactions of a hydroxyl radical as well as 76 potential participation of excited species. The approach used in Received: September 6, 2012 Revised: November 2, 2012 Article pubs.acs.org/JPCA © XXXX American Chemical Society A dx.doi.org/10.1021/jp308885j | J. Phys. Chem. A XXXX, XXX, XXX-XXX dmadmin | MPSJCA | JCA10.0.1465/W Unicode | jp-2012-08885j.3d (R3.4.i1:3887 | 2.0 alpha 39) 2012/09/13 09:54:00 | PROD-JCA1 | rq_795363 | 11/08/2012 08:58:48 | 6