STUDY OF THE H + O2 REACTION BY MEANS OF QUANTUM MECHANICAL AND STATISTICAL APPROACHES P. Bargueño(1,2), T. González- Lezana(2), P. Larrégaray(3), L. Bonnet(3), J.–C. Rayez(3), M. Hankel(4), S.C. Smith(4) and A.J.H.M. Meijer(5) (1)Departamento de Química Física, Universidad de Salamanca, 37008 Salamanca (Spain); Instituto de Matemáticas y Física Fundamental, CSIC c/ Serrano 123, 28006, Madrid (Spain); (3) Laboratoire de Physicochimie Moleculaire, UMR 5803 du CNRS, Université Bordeaux 1, 33405 Talence Cedex (France); (4)CCMS, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072 (Australia); (5) Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom (2) → The recent development of the new XXZLG potential energy surface (PES)  for the H+O2 OH+O reaction has renewed the interest on this process [2-5]. One of the most investigated issues on the dynamics of this reaction regards the possible occurrence of a complex forming mechanism from reactants to products. Previous studies suggest the coexistence of a reaction pathway governed by the formation of a long-lived intermediate at the low energy regime and a direct process at a higher energy range . In this work, the dynamics of the H+O2 reaction is analysed by means of time dependent wave packet (TDWP) techniques [2,3], a statistical quantum method (SQM)  and a recently developed mean potential phase space theory (MPPST) . The calculations are performed using the XXZLG surface and the older DMBEIV PES . H+O 2(v =0, j =1) → HO+O H+O2(v =0, j =1) → HO+O 0.4 0.6 XXZLG PES TDWP SQM MPPST 2 Integral cross section [Å ] 0.2 0.1 0.0 0.7 0.8 0.9 1.0 1.1 1.2 1.3 0.7 Total energy [eV] 0.8 0.9 1.0 1.1 1.2 1.3 1.4 Total energy [eV] Ec=0.9 eV TDWP  0.3 0.2 0.8 1.0 1.4 1.6 The systematic overestimation observed for the opacity functions, specially for the larger J values, leads to statistical integral cross sections (ICS) clearly above the TDPW results. Only near to the thresholds for each PESs, the TDWP cross sections are in accord with the SQM and MPPST predictions. The dependence with respect to the energy of the statistical ICSs is, apart of a constant shift consistent with the different reaction thresholds, quite similar in both PESs. The SQM and MPPST ICSs, which seem to differ as the energy increases, do not improve the degree of accord found between the exact and the measured cross sections. XXZLG PES Ec = 0.8 eV H+O2(v , j =1) → HO+O 0.1 v=0 Etot = 0.7661 eV  C. Xu et al., J. Chem. Phys. 122, 244305 (2005); D. Xie et al. J. Chem. Phys. 126, 074315 (2007).  M. Hankel et al., J. Chem. Phys. 127, 064316 (2007).  P. Bargueño et al. Phys. Chem. Chem. Phys. 9, 127 (2007).  P. Honvault et al., J. Phys. Chem. A 111, 5349 (2007).  S. Y. Lin et al., J. Phys. Chem. A (in press 2007).  A.J.H.M. Meijer et al., J. Chem. Phys. 108, 5404 (1998) and references therein.  E.J. Rackham et al., Chem. Phys. Lett. 343, 356 (2001); E.J. Rackham et al., J. Chem. Phys. 119, 12895 (2003).  P. Larrégaray et al., J. Chem. Phys. 127, 084308 (2007).  M. R. Pastrana et al., J. Phys. Chem. 94, 8073 (1990).  M. A. Bajeh et al., J. Phys. Chem. A 105, 3359 (2001).  E. M. Goldfield et al., J. Chem. Phys. 113, 11055 (2000). Etot = 0.7661 eV v =1 0.2 v=3 Etot = 0.78735 eV 2 0.1 0.0 0 XXZLG PES 0.1 0.0 0.0 DCS [Å /sr] The statistical probabilities are quite similar in both cases. The comparison with the exact TDWP opacity functions reveal a significant overestimation, specially as J increases. The situation is still less favourable for a statistical description for the XXZLG PES. The centrifugal sudden (CS) approximation starts to diverge from the coupled-channel (CC) results above J ~ 20. 2 10 20 30 40 50 0 10 20 30 40 Total angular momentum, J Total angular momentum, J DCS [x10 Å /sr] 0 Etot = 0.7761 eV v=3 2 2 EQM  SQM MPPST 0.4 0.2 DCS [Å /sr] 2 DCS [x10 Å /sr] 0.8 0.0 1.2 E col [eV] H+O2(v =0, j =1) → HO+O DMBE IV PES Exp  0.4 0.0 0.6 0.2 TDWP SQM-CS SQM-CC MPPST XXZLG PES SQM MPPST TDW P  0.1 The TDWP reaction probability at zero total angular momentum J=0 is reproduced on average at low energy with the SQM and MPPST approaches, but the sudden increase above Etot ~ 1.25 eV seems not to have a statistical origin. On the XXZLG PES, the J=0 probability is sensibly lower and the statistical predictions remain slightly over the TDWP result. The reaction threshold for the probability on the XXZLG surface is shifted to a lower energy with respect to the result found on the DMBEIV PES. Probability DMBEIV PES SQM MPPST TDW P  TDW P  0.5 Probability 0.3 DMBEIV PES J=0 30 60 90 120 150 CM scattering angle [deg] 180 0.1 0.0 0 30 60 90 120 150 180 CM scattering angle [deg] Statistical differential cross sections (DCS) obtained on the XXZLG PES for different initial O2(v,j) states at energies close to the reaction threshold have been compared with recently reported time independent exact quantum mechanical (EQM) angular distributions . Although the agreement between the exact DCSs and the statistical predictions is certainly not perfect, the SQM and MPPST approaches seem to reproduce some of the dynamical features present in the EQM DCSs.