Z. Phys. Chem. 224 (2010) 1081–1093 . DOI 10.1524.zpch.2010.6142 © by Oldenbourg Wissenschaftsverlag, München Theoretical Study of the HOCH 2 OO • + HO 2 • Reaction: Detailed Molecular Mechanisms of the Three Reaction Channels By Thanh Lam Nguyen, Luc Vereecken, and Jozef Peeters * Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium Dedicated to Prof. Dr. Reinhard Zellner on the occasion of his 65 th birthday (Received December 10, 2009; accepted March 29, 2010) HOCH 2 OO • + HO 2 • Reaction . CBS-QB3 . CBS-QCI . APNO . CBS-Q(MPW1B95) . CI-PT2 The HO 2 • + HOCH 2 OO • reaction was theoretically investigated, using various high-level, single- reference Complete Basis Set methods including CBS-QB3, CBS-QCI.APNO and CBS- Q(MPW1B95) and a new multi-reference CI-PT2 approach. Three major product channels under atmospheric conditions were identified and their molecular mechanisms elucidated in great detail by Intrinsic Reaction Coordinate Analyses (IRC) at the B3LYP.6–311G(d,p) level: (i) Direct head-to-tail H-atom abstraction from the hydroperoxy radical by the alkylperoxy, occurring on the triplet Potential Energy Surface (PES) leading to HOCH 2 OOH + O 2 ; (ii) A two-step rearrangement of the initial singlet HOCH 2 OOOOH tetroxide complex to form HC(O)OH + • OH + HO 2 • ; (iii) A multi-step rearrangement of the initial HOCH 2 OOOOH tetrox- ide to yield HC(O)OH + O 2 ( 1 Δ)+H 2 O, about twice as fast as the former channel on the singlet- surface. The findings provide an explanation for the observed hydroxyl radical formation in this reaction (Jenkin et al., Phys. Chem. Chem. Phys. 9 (2007) 3149) and rationalize the high overall rate and its pronounced negative temperature dependence (Veyret et al., J. Phys. Chem. 93 (1989) 2368). 1. Introduction Formaldehyde (HCHO) is an ubiquitous key intermediate in the atmospheric photo-oxidation of methane (CH 4 ) and of non-methane volatile organic com- pounds (NMVOCs). These (oxygenated) hydrocarbon compounds originate ei- ther from the incomplete combustion of fossil fuels and biomass, evaporation of * Corresponding author. E-mail: Jozef.Peeters@chem.kuleuven.be