Reactions of Hydrogen Atom with Hydrogen Peroxide Benjamin A. Ellingson, Daniel P. Theis, ² Oksana Tishchenko, Jingjing Zheng, and Donald G. Truhlar* Department of Chemistry and Supercomputing Institute, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431 ReceiVed: September 13, 2007; In Final Form: October 12, 2007 Rate coefficients are calculated using canonical variational transition state theory with multidimensional tunneling (CVT/SCT) for the reactions H + H 2 O 2 f H 2 O + OH (1a) and H + H 2 O 2 f HO 2 + H 2 (1b). Reaction barrier heights are determined using two theoretical approaches: (i) comparison of parametrized rate coefficient calculations employing CVT/SCT to experiment and (ii) high-level ab initio methods. The evaluated experimental data reveal considerable variations of the barrier height for the first reaction: although the zero-point-exclusive barrier for (1a) derived from the data by Klemm et al. (First Int. Chem. Kinet. Symposium 1975, 61) is 4.6 kcal/mol, other available measurements result in a higher barrier of 6.2 kcal/mol. The empirically derived zero-point-exclusive barrier for (1b) is 10.4 kcal/mol. The electronic structure of the system at transition state geometries in both reactions was found to have “multireference” character; therefore special care was taken when analyzing electronic structure calculations. Transition state geometries are optimized by multireference perturbation theory (MRMP2) with a variety of one-electron basis sets, and by a multireference coupled cluster (MR-AQCCSD) method. A variety of single-reference benchmark-level calculations have also been carried out; included among them are BMC-CCSD, G3SX(MP3), G3SX, G3, G2, MCG3, CBS-APNO, CBS-Q, CBS-QB3, and CCSD(T). Our data obtained at the MRMP2 level are the most complete; the barrier height for (1a) using MRMP2 at the infinite basis set limit is 4.8 kcal/mol. Results are also obtained with midlevel single-reference multicoefficient correlation methods, such as MC3BB, MC3MPW, MC-QCISD/3, and MC-QCISD-MPWB, and with a variety of hybrid density functional methods, which are compared with high-level theory. On the basis of the evaluated experimental values and the benchmark calculations, two possible recommended values are given for the rate coefficients. 1. Introduction Due to the rising cost of gasoline and growing concern about the rapid rate of oil consumption, a significant amount of research has been performed to identify alternate sources of energy. 1-3 One alternative fuel that is being considered is hydrogen gas. Hydrogen gas offers a clean source of fuel that can produce a reasonable amount of energy and can be chemically synthesized from renewable resources at an afford- able cost. 1 These characteristics have led several researchers to study the feasibility of developing a combustion engine that uses H 2 for fuel. 3 This in turn has resulted in a renewed interest in the details of H 2 /O 2 combustion. 4 In addition, it has long been known that the oxidation of H 2 makes a significant contribution to the later stages of hydrocarbon oxidation. 5,6 Two reactions that play an important role in the high- temperature, high-pressure behavior of the H 2 /O 2 combustion system are 5-14 These reactions influence the dependence of the second explo- sion limit 8 on temperature and reactant concentration and dependence of the maximum rate of non-explosive oxidation on the pressure, temperature, and reactant concentration. 7,9 A recent study also found that for experimental conditions (pressures, temperatures, etc.) above the third explosion limit these reactions affect the length of time it takes to autocata- lytically induce an explosion. 4 Unfortunately, low-temperature measurements of the rate coefficients k 1a and k 1b of reactions 1a and 1b and of the sum of these two rate coefficients (which is denoted k 1 ) have shown significant variations both in the absolute magnitude of total rate coefficient k 1 and in the branching ratio R, defined as the rate of reaction 1a relative to the rate of reaction 1b, k 1a /k 1b . 15-18 These reactions have been difficult to study because they involve the same reactants, because OH can react with H 2 O 2 as a second route to producing H 2 O, and because HO 2 can react with H to produce either H 2 or H 2 O. Reaction 1b has also been studied recently using single-reference and multireference perturbation theories and using density functional theory; these calculations led to an estimate of 8.1-9.3 kcal/mol for the barrier height of this reaction. 19,20 In this Article, two complementary theoretical approaches have been employed to estimate the reaction barriers for reactions 1a and 1b. These approaches are (i) comparison of parametrized rate coefficient calculations employing canonical variational theory with small-curvature tunneling 21-25 (CVT/ SCT) to experiments and (ii) high-level electronic structure calculations, such as the benchmark-level multicoefficient correlation methods and the multireference correlation methods * Corresponding author. E-mail: truhlar@umn.edu. ² Current address: Chemistry Department, University of North Dakota, 151 Cornell St., Stop 9024, Grand Forks, ND 58202. H + H 2 O 2 f H 2 O + OH (1a) H + H 2 O 2 f HO 2 + H 2 (1b) 13554 J. Phys. Chem. A 2007, 111, 13554-13566 10.1021/jp077379x CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2007