Reactions of Hydrogen Atom with Hydrogen Peroxide

Abstract

Rate coefficients are calculated using canonical variational transition state theory with multidimensional\ntunneling (CVT/SCT) for the reactions H + H₂O₂ → H₂O + OH (1a) and H + H₂O₂ → HO₂ + H₂ (1b).\nReaction barrier heights are determined using two theoretical approaches: (i) comparison of parametrized\nrate coefficient calculations employing CVT/SCT to experiment and (ii) high-level <i>ab initio</i> methods. The\nevaluated experimental data reveal considerable variations of the barrier height for the first reaction: although\nthe zero-point-exclusive barrier for (1a) derived from the data by Klemm <i>et al.</i> (<i>First Int. Chem. Kinet.\n</i><i>Symposium</i> 1975, 61) is 4.6 kcal/mol, other available measurements result in a higher barrier of 6.2 kcal/mol.\nThe empirically derived zero-point-exclusive barrier for (1b) is 10.4 kcal/mol. The electronic structure of the\nsystem at transition state geometries in both reactions was found to have “multireference” character; therefore\nspecial care was taken when analyzing electronic structure calculations. Transition state geometries are\noptimized by multireference perturbation theory (MRMP2) with a variety of one-electron basis sets, and by\na multireference coupled cluster (MR-AQCCSD) method. A variety of single-reference benchmark-level\ncalculations have also been carried out; included among them are BMC-CCSD, G3SX(MP3), G3SX, G3,\nG2, MCG3, CBS-APNO, CBS-Q, CBS-QB3, and CCSD(T). Our data obtained at the MRMP2 level are the\nmost complete; the barrier height for (1a) using MRMP2 at the infinite basis set limit is 4.8 kcal/mol. Results\nare also obtained with midlevel single-reference multicoefficient correlation methods, such as MC3BB,\nMC3MPW, MC-QCISD/3, and MC-QCISD-MPWB, and with a variety of hybrid density functional methods,\nwhich are compared with high-level theory. On the basis of the evaluated experimental values and the\nbenchmark calculations, two possible recommended values are given for the rate coefficients.