STUDY OF THE H + O2 REACTION BY MEANS OF - IFF-CSIC

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)
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The recent development of the new XXZLG potential energy surface (PES) [1] 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 [6]. 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) [7] and a
recently developed mean potential phase space theory (MPPST) [8]. The calculations are performed using the XXZLG surface and the older DMBEIV PES [9].
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 [5]
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
[1] C. Xu et al., J. Chem. Phys. 122, 244305 (2005); D. Xie et al. J. Chem. Phys. 126, 074315
(2007).
[2] M. Hankel et al., J. Chem. Phys. 127, 064316 (2007).
[3] P. Bargueño et al. Phys. Chem. Chem. Phys. 9, 127 (2007).
[4] P. Honvault et al., J. Phys. Chem. A 111, 5349 (2007).
[5] S. Y. Lin et al., J. Phys. Chem. A (in press 2007).
[6] A.J.H.M. Meijer et al., J. Chem. Phys. 108, 5404 (1998) and references therein.
[7] E.J. Rackham et al., Chem. Phys. Lett. 343, 356 (2001); E.J. Rackham et al., J. Chem. Phys.
119, 12895 (2003).
[8] P. Larrégaray et al., J. Chem. Phys. 127, 084308 (2007).
[9] M. R. Pastrana et al., J. Phys. Chem. 94, 8073 (1990).
[10] M. A. Bajeh et al., J. Phys. Chem. A 105, 3359 (2001).
[11] 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 [4]
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 [10]
0.4
0.0
0.6
0.2
TDWP
SQM-CS
SQM-CC
MPPST
XXZLG PES
SQM
MPPST
TDW P [5]
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 [3]
TDW P [11]
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 [4]. 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.