XVIII Simpósio Brasileiro de Química Teórica – SBQT 2015
Pirenópolis – GO, 22-25 Novembro de 2015
On the Origin of the Negative Activation Energy in the OH+hbr Reaction Rate
Constant: Born-Oppenheimer Molecular Dynamic Study of Non-Arrhenius Behavior
Author name who will present the work a (PQ or PG), Second Authorb (PQ or PG)
a
b
Address 1
Address 2
Keywords: Ab Initio Molecular Dynamic, OH + HBr Reaction, Non-Arrhenius, Negative Activation
Energy
INTRODUCTION
In recent years several studies of gas-phase
reactions, which are believed to be elementary,
have been shown to present either curved
Arrhenius plots1 or even negative activation
energies.2 The reaction of the OH radical with HBr
molecule is a relevant example of an elementary
process presenting negative activation energy from
Arrhenius plots of the measured temperature
dependence. The apparent negative activation
energy of the reaction rate constants shows up
below 200K. For these reasons many theoretical
studies have been published to explain this unusual
behavior2 In this work we used Born-Oppenheimer
molecular dynamics for understanding and to
explain the appearance of a negative activation
energy for the OH+ + HBr reaction.
Accordingly, the manifestation of a negative
activation energy observed in this reaction is
attributed to the orientation between vdW complex
formation and the molecular orientation between
reactants. Conversely, at high energies the
molecules fail to orient themselves and disfavor the
formation of the complex.
Figure 1. Effective free-energy profile for the OH•
+ HBr reaction: a) lower temperature, b) higher
temperature.
METHODS
CONCLUSIONS
The Born-Oppenheimer molecular dynamics
method was used to study OH+ + HBr reaction.
Periodic boundary conditions, norm conserving
pseudopotentials of the Troullier_Martins and
functional by Perdew, Burke and Ernzerhof were
employed. The free energy profile was obtained
from ∆𝐺(𝑉) = −𝑘𝐵 𝑇𝑙𝑛[∫ 𝑃(𝑅, 𝑉)𝑑𝑅], where the
coordinates 𝑉 and 𝑅 were defined on the basis of
the Jacobi’s vector scheme.
The simulations performed in this work reveal
the origin of the negative curvature on the OH+ +
HBr reaction rate constant, in agreement with the
experiments. A key role is played by the vdW
complex formation, strongly dependent on the
effectiveness of molecular alignment. Therefore,
the dynamics presents two limiting pathways,
characterizing the low and high temperature
behavior of this reaction, which is influenced by
the effectiveness of the vdW complex.
RESULTS AND DISCUSSION
During the simulations, we found two reactions
pathways: (i) At lower energies, only the direct
hydrogen abstraction from HBr through Van der
Waals (vdW) complex and the transition state has
lower energy than the reactants (Fig.1a); (ii) At
higher energies, there is no formation of vdW
complex and the transition state energy has higher
energy than the reactants (Fig.1b). This result
seems to be consistent with recent crossed
molecular beam studies of the steric effect for this
reaction.3
ACKNOWLEDGMENTS
The authors are grateful for the support given
from the FAPEG, CAPES, CNPQ and FINATEC.
1
S. W.Benson and O. Dobis, J. Phys. Chem. A,102
, 5175, (1998).
2
J. Liu, Z.Li, C.Sun, J. Phys. Chem. A, 105, 7707,
(2001). 3Tsai, P.-Y.; Che, D.-C.;
3
M. Nakamura, K. C. Lin, T. Kasai, Phys. Chem.
Chem. Phys.,12, 2532,(2010).