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Reactivity of Hydrogen Peroxide with Br and I Atoms Camille Fortin, Sarah Khanniche, Dorra Khiri, Valérie Fèvre-Nollet, Patrick Lebègue, Fréderic Cousin, Ivan Cernusak, and Florent Louis J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10318 • Publication Date (Web): 30 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018
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The Journal of Physical Chemistry
Reactivity of Hydrogen Peroxide with Br and I Atoms Camille Fortin,†,§,‡ Sarah Khanniche,†, ‡,£ Dorra Khiri,†, ‡ Valérie Fèvre-Nollet,†, ‡ Patrick Lebègue,†, ‡ Frédéric Cousin,§,‡ Ivan Černušák*,#, Florent Louis,* †, ‡
†
Univ. Lille, CNRS, UMR 8522 - PC2A - PhysicoChimie des Processus de Combustion et de
l'Atmosphère, F-59000 Lille, France §
Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES, Cadarache, St Paul Lez
Durance, 13115, France #
Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius
University in Bratislava, Ilkovičova 6, 84215 Bratislava, Slovakia ‡
Laboratoire de Recherche Commun IRSN-CNRS-Lille1 "Cinétique Chimique, Combustion,
Réactivité" (C3R), Cadarache, St Paul Lez Durance, 13115, France £
Present address: Department of Chemical Engineering, Massachusetts Institute of
Technology, Cambridge, MA 02139, United States
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Abstract The reaction mechanisms of Br and I atoms with H2O2 have been investigated using DFT and high-level ab initio calculations. The H-abstraction and OH-abstraction channels were highlighted. The geometries of the stationary points were optimized at the B3LYP/aug-ccpVTZ level of theory and the energetics were recalculated with the coupled cluster theory. Spin-orbit coupling for each halogenated species was also explicitly computed by employing the MRCI level of theory. Thermochemistry for HOBr and HOI has been revised and updated standard enthalpies of formation at 298 K for HOBr and HOI are the following: ∆fH°298K(HOBr) = (-66.2 ± 4.6) kJ mol-1 and ∆fH°298K(HOI) =(-66.8 ± 4.7) kJ mol-1. The rate constants have been estimated using both TST, CVT and CVT/SCT over a wide temperature range (250-2500 K). For the direct abstraction mechanism, the overall rate constant at 300 K was predicted to be 2.58 × 10-16 and 7.42 × 10-25 cm3 molecule-1s-1 for the Br + H2O2 and I + H2O2 reactions respectively. The modified Arrhenius parameters have been estimated for the overall reactions: kBr+H2O2(T) = 4.80 × 10-26 T4.31 exp(−5.51 (kJ mol-1) / RT) and kI+H2O2(T) = 3.41 × 10-23 T3.41 exp(−56.32 (kJ mol-1) / RT).
1. Introduction Because of their possible implications in the destruction of the stratospheric ozone layer1, attention has been paid to the reactions of Cl and Br with H2O22–10 (Table 1). Hydrogen peroxide is of atmospheric and combustion interests11 as it acts as a precursor for key species such as HO2 and OH radicals. For instance, the H- and O(H)- abstractions of hydrogen peroxide by halogen atoms (Cl, Br, I) produces hydroperoxyl (R1) and hydroxyl radicals (R2). X + H2O2 → HX + HO2
(R1) 2
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X + H2O2 → HOX + OH
(R2)
Extensive experimental studies of the Cl + H2O2→ HCl + HO2 reaction can be found in the literature (Table 1). Watson et al. 2and Michael et al.3 employed flash photolysis-resonance fluorescence to determine the reaction rate constant at 300 K and for temperatures ranging from 265-400 K, respectively. Leu and DeMore4 and Poulet et al.5 studied the kinetics of the reaction by discharge flow mass spectrometry at 295 and 298 K, respectively. Keyser6 determined by the discharge flow resonance fluorescence technique the rate constant at 298 K and the Arrhenius expression over the temperature range 298-424 K. Marouani et al.7 reported for the first time ab initio calculations of the Cl + H2O2 → HCl + HO2 and Cl + H2O2 → HOCl + OH reactions using the post Hartree–Fock MCQDPT2//CASSCF approach and the aug-cc-pVTZ basis set. The kinetics of both reactions was investigated using transition state theory over a wide temperature range (200-2500 K). The H-abstraction pathway dominates for temperatures below 1500 K while the OH-abstraction pathway significantly contributes for higher temperatures. The reaction of Br with H2O2 has also been studied previously over a narrow temperature range (Table 1) and yet large discrepancies remain. Leu8 and Posey et al.9 determined upper limits for the reaction rate constants over the temperature range 298-417 K and at 298 K, respectively. Heneghan and Benson10 employed a very low pressure reactor to investigate the rate constants and product formation of the Br + H2O2 reaction in the range of 300-350 K. They found that k (300 K) = (1.3 ± 0.45) × 10-14 cm3 molecule-1 s-1 and k (350 K) = (3.75 ± 1.1) × 10-14 cm3 molecule-1 s-1. The measured rate constants were found to be significantly faster than the upper limit values reported previously8,9 and Heneghan and Benson10 concluded that future studies should be carried out on the kinetics of the Br + H2O2 reaction. To the best of our knowledge, there is no experimental nor theoretical investigation of the I + H2O2 reaction system reported in the literature. However, besides its significant role in the 3 ACS Paragon Plus Environment
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depletion of ozone from the troposphere,12–14 reactivity of iodine is also relevant for nuclear safety.15–17 As a matter as fact, iodine species can be found inside the containment of a pressurized water reactor, in the case of a core melt accident. Iodine speciation and physical form are key issues to better consider radiological consequences of a nuclear accident. To accurately consider the high reactivity of iodine in the dispersion crisis tools, kinetic parameters are needed as input parameters in chemistrytransport models. Such data are relatively scarce for iodine-containing species and some of them exhibit large uncertainties. Quantum chemistry methods allow obtaining reliable quantitative data within chemical accuracy (± 4 kJ mol-1) as shown in our previous studies dealing with atmospheric iodine chemistry.18–24 In this article, we will present ab initio calculations of the reaction mechanisms of the H- and OH-abstraction of H2O2 by Br and I atoms. The rate constants in the temperature range of 250-2500 K were subsequently determined by the canonical variational transition state theory (CVT) with small curvature tunneling (SCT). This is the first time that the reactions of bromine and iodine with hydrogen peroxide are theoretically investigated. Details on the computational tools employed are provided in Section 2 and the results are exposed and discussed in Section 3.
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Table 1. Literature Rate Parameters for the X + H2O2 Reactions (X = Cl and Br)
Reactant
A 3
Ea -1 -1
(cm molecule s )
k -1
(kJ mol )
3
-13
(6.2 ± 1.5) × 10
-13
(3.14 ± 0.56) × 10 (1.24± 0.74) × 10-12
(4.0 ± 0.4) × 10
-13
(4.1 ± 0.2) × 10
300
experimenta
2
Watson et al., 1976
295
b
4
Leu et DeMore, 1976
c
experiment
3
Michael et al., 1977
298
experimentb
5
Poulet et al., 1978
298
d
experiment
6
Keyser, 1980
theorye
7
Marouani et al., 2009
experimentb
8
Leu, 1980
298
experimentb
9
Posey et al, 1981
300
f
298
experiment
298-424
8.16 3.16 × 10-14
298
-11
1500
-15
< 1.5 × 10
298
< 3.0 × 10-15
417
2.87 × 10 Br + H2O2
ref
265-400
3.19 -13
(1.05 ± 0.31) × 10-11
method
(cm molecule s ) (5.8 ± 2.0) × 10-13
Cl + H2O2
T (K) -1 -1
-15
