Investigation of the Reaction Mechanism and Kinetics of Iodic Acid

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Investigation of the Reaction Mechanism and Kinetics of Iodic Acid with OH radical using Quantum Chemistry Sarah Khanniche, Florent Louis, Laurent Cantrel, and Ivan Cernusak ACS Earth Space Chem., Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Earth and Space Chemistry

Investigation of the Reaction Mechanism and Kinetics of Iodic Acid with OH radical using Quantum Chemistry Sarah Khanniche, †, ‡ Florent Louis, *, †, ‡ Laurent Cantrel, §, ‡ Ivan Černušák# †

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 ‡

Laboratoire de Recherche Commun IRSN-CNRS-Lille1 "Cinétique Chimique, Combustion,

Réactivité" (C3R), Cadarache, St Paul Lez Durance, 13115, France #

Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius

University in Bratislava, Mlynská dolina CH1, 84215 Bratislava, Slovakia

Keywords: hydrated iodine oxides, iodic acid, reactivity, thermochemical properties, kinetic parameters

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ABSTRACT: In this paper, the mechanism of the HOIO2 + OH → products reaction is investigated using quantum chemistry tools. Two pathways are considered: HOIO2 + OH → OIO2 + H2O, and HOIO2 + OH → OIO + H2O2. The potential energy surfaces are calculated at the CCSD(T)/CBS(D,T)//B3LYP/aug-cc-pVTZ level of theory. OH radical was found to attack iodic acid either from the front side (TSHabs) or from the top side (TSOHabs) subsequently leading to H- or OH-transfer channels. Reaction enthalpies and standard Gibbs free reaction energies at 298 K are provided. The values indicate that only the HOIO2 + OH → OIO2 + H2O channel is exothermic and exergonic. Theoretical prediction of the kinetic parameters is performed within the framework of the TST and VTST theories. The rate constants (in cm3 molecule-1 s-1) for temperatures from 250 to 2500 K are: kHabs(T) = 1.76 ×10-22× T2.39 exp (-3.5(kJ mol-1)/RT) and kOHabs(T) = 3.16 ×10-21× T2.57 exp (-96.2(kJ mol-1)/RT). The HOIO2 + OH overall reaction is significantly dominated by the HOIO2 + OH → OIO2 + H2O channel for tropospheric temperatures. The main outcomes of this work are: (i) the lifetime of iodic acid towards its removal by OH radicals is extremely long (1336 years) enabling its transportation to different locations around the Earth (marine, polar, and continental) as it was confirmed by recent field measurements; other possible loss pathways under clear sky (gas phase) and cloudy conditions (aqueous phase) could reduce its atmospheric lifetime, (ii) the thermochemical properties of the OIO2 radical are provided: ∆fH°298K = 168.3 kJ mol-1, S°298K = 301.45 J mol-1 K-1, and Cp(300 K) = 65.82 J mol-1 K-1.

1. INTRODUCTION The presence of iodine in the atmosphere arises mainly from emissions of inorganic and organic compounds from microscopic (phytoplankton) and macroscopic marine plants.1,2 The mechanism of reactive iodine release into the atmosphere appears to be particularly important 2 ACS Paragon Plus Environment

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for the destruction of vast amounts of ozone.3-6 Volatile iodine will be emitted into the containment’s atmosphere of a nuclear power plant during a severe accident in which the normal core cooling is lost.7-9 Air radiolysis products such as ozone, hydroxyl and hydroperoxyl radicals would react with volatile iodine to form iodine oxides species of different size as represented in Figure 1. Small iodine oxides include IO, OIO, I2O2 and their hydrated counterparts (HOI, HOIO). Bigger iodine oxides can be present such as I2O3, I2O4, and finally I2O5. HOIO2 was identified as one of the product of the IO + HO2 reaction and it has been shown that iodic acid could be also formed by the simple addition of OH to OIO.10,11 Moreover, HOIO2 which corresponds to the hydrated from of I2O5, can be formed through the reaction of diiodine pentoxide with water (Figure 1).

Figure 1. Schematic representation of gas phase chemistry for iodine-containing species. HOIO2 can be formed from IO + HO2 and OIO + OH reactions. HOIO2 is the hydrated form of I2O5. 3 ACS Paragon Plus Environment


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Little is known about the gas phase properties of iodic acid. Few theoretical studies of HOIO2 are available in the literature10-13 and experimental evidence of iodine oxide particle formation through addition of iodic acid has just been published. HIO3 vapor was observed at Mace Head (Ireland) with concentrations greater than 108 molecules cm-3.14 Elevated concentration of iodic acid was also observed in Greenland and HIO3 was measured above the limit detection more than 100 km distance from the Antarctic coast.14 Kürten et al.15 revealed also the presence of iodic acid in field campaign in Germany. Its presence indicates long-range transport of iodine-containing substances. Quantum chemistry calculations were performed by Drougas and Kosmas to characterize the products of the IO + HO2 reaction and to investigate the four (HIO3) isomers.10 They employed the B3LYP and MP2 methods in combination with 6311++G(3df,3dp), LANL2DZdp, LANL2DZspdf(+ ECP) basis sets for H, O, and I atoms, respectively. Single-point energy calculations were performed at the CCSD(T) level of theory. It was shown that HOIO2 was the most stable form. Standard enthalpies of formation at 0 and 298 K were calculated considering the IO + HO2 and OIO + OH dissociation pathways. HOIO2 was predicted to be the most stable isomer and that HOOOI and HOOIO were relatively close from each other. HIO3 was the less stable molecule. The kinetic behaviour of the HO2 + IO reaction was determined to proceed through the HOOIO (and possibly HOOOI) intermediate and then forming HOIO2 or dissociating to HOI + O2. The reaction of OIO with OH was explored11 by employing B3LYP method with 6311+G(2d,p) basis set for H and O atoms and the full electron basis set for iodine of Glukhovtsev et al.16 The rate was determined using quantum chemistry tools and Rice Rampsberger Kassel Marcus (RRKM) kinetic theory. It was found that the OIO + OH channel could directly lead to very stable HOIO2 molecule, whereas the bimolecular reaction channels to IO + HO2 and HOI + O2 were not probable because of significant energetic barriers (50 –


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90 kJ mol-1). The predicted geometry and stability of HOIO2 was in good agreement with the work of Drougas and Kosmas10. Recently, Khanniche and co-workers12 reinvestigated the structures and the thermodynamic properties of HIO3 isomers at the CCSD(T)/CBS(T,Q,5) level of theory with the B3LYP and MP2 optimized structures. The aug-cc-pVTZ basis sets were utilized for H, and O whereas the aug-cc-pVTZ-PP+ECP28 was selected for iodine. In our previous work, the spin orbit corrections (SOC) for each isomer have been calculated using the CASSCF/CASPT2/RASSI-SO scheme with the ANO-RCC-VQZP basis sets. New set of thermodynamic data (∆fH°298K, S°298K, and Cp(T)) were provided. The mechanism of the I2O5 + H2O = 2 HOIO2 gas-phase reaction has been also investigated and elucidated in our group.13 At atmospheric conditions, the gas phase reaction of hydration of I2O5 into HOIO2 was found to be too slow with respect to the nucleation process, whereas for severe accidents, a large excess of iodic acid should be present.13 The presence of daytime oxidants, such as for example OH, HO2, and Cl, determine the chemical removal of iodic acid in the atmosphere. HOIO2 can also be photolyzed after sunrise. During nighttime, the reactions with NO3 can become an important oxidation sink.17 Herein, we focus on the reactivity of iodine oxide with hydroxyl radical, which is the most potent oxidant. This paper seeks to explore the reaction mechanism of iodic acid (HOIO2) with OH radical and will contribute to better understand iodine oxide reactivity in presence of hydroxyl as encountered in severe nuclear accident as well as in atmospheric conditions. Two possible pathways were considered as follows: H-abstraction

HOIO2 + OH → OIO2 + H2O

(R1a)

O(H)-abstraction

HOIO2 + OH → OIO + H2O2

(R1b) 5

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In this work, electronic structure calculations, as well as classical transition state theory were employed to compute the kinetic parameters associated with the bimolecular reaction. The methodology employed is detailed in Section 2. Results are given and discussed in Section 3. This is the first time that the kinetics of the HOIO2 + OH → products reaction is reported.

2. COMPUTATIONAL METHODS The structures of all stationary points were optimized by using Density Functional Theory (DFT). The Becke 3 parameters and Lee-Yang-Parr functional18 (B3LYP) was adopted to evaluate exchange and correlation energies. DFT calculations were performed with the augmented correlation-consistent polarized valence basis sets from Dunning of triple-zeta quality termed aug-cc-pVTZ.19 Scalar relativistic effect for iodine was treated with the smallcore pseudopotential (28 electrons) with the accompanying aug-cc-pVTZ-PP basis sets of Peterson20 for the outer-core 25 valence electrons (aug-cc-pVTZ-PP+ECP28). Such computational method has already given reliable results for the thermodynamics and kinetics of similar reacting systems.12,13,21,22 Vibrational frequency calculations were undertaken at the same level of theory with a scaling factor of 0.96823 allowing for partial compensation of the anharmonicity of the OH bond stretching mode. Transition state structures were characterized by a single imaginary vibrational frequency whereas all wavenumbers were real for molecular complexes. Minimum energy pathways (MEP) at the B3LYP/aug-cc-pVTZ were constructed using IRC calculations24-26 starting from TS structures. In a second step, the potential energies of all stationary points were recalculated by employing CCSD(T)27-30 electron correlation corrections within the semi-core approximation (4d5s5p electrons for iodine were correlated). The coupled-cluster (CC) wavefunction, , includes excitations between correlated electrons in an exponential way as follows: 6 ACS Paragon Plus Environment

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= ( )

(1)

where is the cluster operator acting on the single-determinant reference state Φ0. Usually, the cluster operator is partitioned into classes of all single (S), double (D), or triple (T) excitations. Since the triples contribution is computationally most demanding task, it is estimated by a perturbative treatment – hence the acronym CCSD(T)31-33CCSD(T) calculations were carried out with Dunning’s aug-cc-pVnZ basis set of double-zeta (n = 2) and triple-zeta (n = 3) qualities. Extrapolations to the complete basis set (CBS) limit from Dunning double to triple correlation consistent polarized basis sets were performed for total energies using Kim’s scheme.34 The same level of theory as used in this paper (CCSD(T)/CBS(D,T)) has already been employed to characterize the reactivity and kinetics of I2O5 + H2O reaction.13 Spin-orbit coupling (SOC), which can be mandatory for heavy-element compounds, is found to be close to zero for the stationary points along the reaction pathway. This was expected since the SOC value is -0.25, -0.83, and -0.93 kJ mol-1 for HOIO2,12 OH reactant,35 and OIO product,36 respectively. Negligible SOC was also reported for OIO by Peterson37. SOC38,39 calculations were performed with the Molcas software40 using the CASSCF/CASPT2/RASSISO scheme with the ANO-RCC-VQZP basis sets39,40. Different sizes of active space have been tested ranging from 11 to 17 valence electrons that did not impact on the non-significant results obtained. For instance, for OIO2 radical, the SOC value is -0.05 kJ mol-1. This multiconfigurational scheme has proven reliability and accuracy in previous works: the computed SOC values for I,41 HI,41 and IO21 were found to be in excellent agreement with their literature counterparts.42-44 This methodology has also been employed for intermediate species of several bimolecular reactions21,41,45,46 for which no literature data were available. The overall rate of the HOIO2 + OH reaction, r is written as:

= ( + )HOIO OH

(2) 7



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where kHabs and kOHabs are the rate constants for H-abstraction and O(H)-abstraction pathways.

kOHabs was computed using Transition State Theory (TST)47-49 with the partition functions Q of HOIO2, OH, and TSOHabs at the temperature T as: () = Γ() ×

×

!"#$%&' ()

($)*)+ (,)()$ (,)

× exp 0−

2"#$%&' 32$)*)+ 32)$ 4

(3)

where Γ(T) is the tunneling correction at T, kB, and h are Boltzmann’s and Planck’s constants, respectively and EHOIO2, EOH, and ETSOHabs are the potential energies at 0 K including zeropoint vibrational energy (ZPE) correction. The reaction path degeneracy is not included in Equation 3 as rotational symmetry numbers are introduced in the calculation of Q.

kHabs was estimated using a two-steps kinetic scheme already detailed elsewhere22,50. Briefly, we assumed a fast pre-equilibrium between reactants and MCR whose equilibrium constant,

KMCR(T) is obtained by the following equation: 5678 () =

!9:;$%&' () !$)*) ()!)$ ()

2$)*) < 2)$ 329:;$%&' 4

× exp 0

(4)

where QMCRHabs(T) is the partition function of MCRHabs at the temperature T and EMCRHabs the potential energy at 0 K with ZPE correction. In the second step, variational TST (VTST)51-53 in a canonical ensemble was applied to calculate the VTST rate constant, kCVT(T,s) from MCRHabs to TSHabs as a function of the reaction coordinate s along the MEP with s = 0 at the saddle point:

7=, (, ?) = Γ() ×

×

!@"# (,,) !9:; ()

× exp 0−

2@"# ()329:;

4

(5)

where QGTS(T,s) is the partition function for the generalized transition state (GTS). GTS was obtained by selecting ten points from IRC calculations either on the MCRHabs’s side (s < 0) or on the MCPHabs’s side (s > 0) of the TSHabs saddle point with -0.20 ≤ s ≤ +0.20. The potential energies of the selected points at the CCSD(T)/CBS(D,T) level of theory were employed with B3LYP/aug-cc-pVTZ vibrational frequencies to construct the MEP, which presents a


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maximum that is shifted towards the MCPHabs at s = +0.12. The corresponding barrier height increases by 6.1 kJ mol-1. Finally kHabs was obtained as: () = 5678 () × k 7=, (, ?)

(6)

Tunneling effect was accounted using 1-D quantum mechanical treatment through an unsymmetrical Eckart potential energy barrier.54 The Eckart tunneling correction ΓEckart(T) is obtained by:

ΓCDEFG () =

H

I

∞

E

× exp 0 04 L0 p(I) exp 0− 4 NI B

(7)

where p(E) is the probability of transmission through the corresponding 1-D barrier at energy E. ΓEckart(T) was estimated at 5.4 and 1.6 at 300 K for kHabs and kOHabs, respectively. Intramolecular rotors are treated as sinusoidally hindered rotors with a potential energy curve:

P=

QR

(1 − cos WX)

(8)

where V0 is barrier height, n is the symmetry number of rotation, and θ is angle of rotation. Their partition functions were calculated by using Pitzer-Gwin approximation55 in the Gpop program,56 which was also employed to estimate the Eckart tunneling corrections and to calculate the equilibrium constant (KMCR) and rate constants (kOHabs and kCVT) over the temperature range of interest.

3. RESULTS 3.1. Structures

3.1.1. Reactants and products The optimized structures of the HOIO2 and OH reactants together with the products are sketched in Figure 2. OIO2 and H2O are produced from the H-transfer channel while OIO and



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H2O2 are the products of the O(H)-abstraction. Figure 3 displays the optimized geometries of transition states and molecular complexes. Tables 1 and 2 collect their rotational constants, vibrational wavenumbers, and ZPE.

HOIO2

OH

OIO2

H2O

OIO

H2O2

Figure 2. Optimized structures at the B3LYP/aug-cc-pVTZ level of theory for reactants, and products of the HOIO2 + OH reaction. Bond lengths are in Angstroms, valence, and dihedral angles are in degrees.


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Table 1. Rotational constants in GHz, unscaled vibrational frequencies in cm-1, Zero-Point Energy (ZPE) in kJ mol-1 (B3LYP/aug-cc-pVTZ), and SOC (RASSI/ANO-RCC-VQZP) in kJ mol-1 for reactants and products of the HOIO2 + OH reaction.

Species

Symmetry numbers

Electronic states

Rotational constants

Vibrational Frequencies

HOIO2

1

Cs - 1A'

5.70, 5.30, 3.59

45, 246, 253, 295, 565, 851, 876, 982, 3723

OIO2

3

C3v – 2A1

6.97, 4.89, 3.95

252, 252, 261, 711, 711, 771

OIO

2

C2v – 2B1

18.31, 7.02, 5.08

261, 791, 806

OH

1

C∞v – 2П

560.38

3693

H2O

2

C2v - 1A1

825.21, 430.15, 282.76

1627, 3798, 3900

H2O2

2

C2 - 1 A

303.47, 26.40, 25.56

375, 949, 1322, 1434, 3754, 3755

ZPE

SOC

45.4

-0.25

17.1

-0.05

10.8

-0.9336

21.4

-0.8335

54.0

0.0

67.1

0.0

11



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3.1.2. Transition states The structures of the transition states are displayed in Figure 3. Two transition structures were identified: TSHabs and TSOHabs. Indeed, hydroxyl radical can either attack the hydrogen of the OH fragment of iodic acid from the front side (TSHabs) or abstracts the whole OH group from the top side (TSOHabs). The bonds being broken and formed are sketched in Figure 3 in solid and dotted arrows, respectively. The OH breaking bond of iodic acid is stretched up to 1.2 Å in TSHabs and in TSOHabs while the IO cleaving bond is elongated to 2.3 Å. The forming OH bond of the water molecule in TSHabs is longer (1.147 Å) than the isolated product (0.962 Å in Figure 3). TSOHabs exhibits an O…O distance between the OH fragment of HOIO2 and the hydroxyl radical of 1.769 Å.

3.1.3. Molecular complexes IRC calculations revealed that both TS connect the HOIO2…OH complex to either OIO2…H2O or OIO…H2O2 complexes for TSHabs and TSOHabs, respectively. Both routes share similar prereactive complexes where the hydroxyl radical interacts with iodic acid through hydrogen bonds. The OH reactant is positioned perpendicular in between the OH group of HOIO2 and one of its oxygen atom with OOH…H(O)HOIO2 and HOH…OHOIO2 of 1.8 and 1.9 Å, respectively. Notice that OH and IO bond lengths of iodic acid involved in hydrogen interactions are slightly elongated in MCR (0.99 and 1.81 Å, respectively) with respect to their counterparts in the isolated HOIO2 reactant (Figure 3). In MCPHabs, the water molecule is positioned parallel to the (OIO) plane of the IO3 radical while interacting via hydrogen and halogen bonds. Similarly, both halogen and hydrogen interactions are present within MCPOHabs with IOIO…OH2O2 and OOIO…HH2O2 of 2.9 and 1.8 Å, respectively.


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MCRHabs

[TSHabs]≠

MCPHabs

MCROHabs

[TSOHabs]≠

MCPOHabs

Figure 3. Optimized structures at the B3LYP/aug-cc-pVTZ level of theory for molecular complexes and transition states of the H-abstraction (top) and O(H)-abstraction (bottom) of the HOIO2 + OH reaction. Bond lengths are in Angstroms, valence and dihedral angles are in degrees. 13



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Table 2. Rotational constants in GHz, unscaled vibrational frequencies in cm-1 and Zero-Point Energy (ZPE) in kJ mol-1 for TSs and molecular complexes involved in the HOIO2 + OH reaction calculated at the B3LYP/aug-cc-pVTZ level of theory

Symmetry Electronic numbers states H-abstraction: HOIO2 + OH → OIO2 + H2O

Rotational constants

Species

Vibrational Frequencies

ZPE

MCRHabs

1

C1 - 2A

4.54, 1.91, 1.74

48, 165, 203, 235, 278, 304, 358, 524, 600, 814, 876, 1196, 3290, 3415

75.28

TSHabs

1

C1 - 2A

4.89, 2.07, 1.85

1472i, 62, 162, 222, 269, 301, 466, 531, 572, 799, 832, 869, 1224, 1570, 3407

65.35

MCPHabs

1

C1 - 2 A

4.78, 2.22, 1.90

35, 65, 138, 167, 254, 256, 266, 355, 374, 656, 758, 775, 1629, 3765, 3863

77.32

O(H)-abstraction: HOIO2 + OH → OIO + H2O2 MCROHabs

1

C1 - 2A

4.56, 1.91, 1.74

47, 165, 203, 235, 278, 303, 355, 520, 599, 696, 814, 876, 1195, 3290, 3417

75.24

TSOHabs

1

C1 - 2A

4.89, 2.07, 1.85

648i, 43, 71, 155, 187, 209, 272, 284, 338, 814, 843, 956, 1086, 3722, 3756

73.76

MCPOHabs

1

C1 - 2A

4.77, 2.09, 1.90

45, 76, 102, 149, 225, 307, 331, 654, 771, 788, 947, 1333, 1527, 3442, 3753

83.67

14


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3.2. Energetics 3.2.1. Thermodynamics and relative enthalpies Table 3 gathers the predicted values for ∆rHo298K and ∆rG°298K for the HOIO2 + OH reaction. The H-transfer channel is exothermic by 16 kJ mol-1 and exergonic by 10 kJ mol-1 at 298 K. The positive values for the OH-abstraction indicates that the channel is endothermic and non spontaneous at 298 K. Table 3. Reaction enthalpies at 298 K, ∆rH°298K, and standard Gibbs free reaction energies ∆rG°298K for the HOIO2 + OH reaction calculated in kJ mol-1 at the CCSD(T)/CBS(D,T) level of theory including ZPE and SOC. H-abstraction

O(H)-abstraction

OIO2 + H2O

OIO + H2O2

∆rHo298K

-16.2

30.9

∆rGo298K

-10.4

20.3

If one considers the HOIO2 + OH → OIO2 + H2O, standard enthalpies of formation for OIO2 radical can be predicted based on the corresponding ∆rH°298K value in combination with ∆fH°298K for HOIO2,12 OH,57 and H2O57 (-95.4 ± 0.3, 37.492 ± 0.0276, and -241.831 ± 0.026 kJ mol-1, respectively). It follows that ∆fH°298K (OIO2) = 168.3 kJ mol-1. To the best of our knowledge, there is no available value in the literature. Our standard enthalpy of formation at 298K for OIO2 is consistent with the one obtained by Karton et al.58 for OClO2 using W4 level of theory (185.4 ± 1.7 kJ mol-1). There is also an estimated value taken from the NISTJANAF thermochemical tables59 for the OBrO2 (221 ± 50 kJ mol-1). In addition Table 4 provides the standard molar entropy at 298 K (S°298K) and heat capacities at constant pressure [Cp(T)] for (OIO2).



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Table 4. Calculated standard molar enthalpies at 298 K (S°298K) in J mol-1 K-1 and heat capacities at constant pressure [Cp(T)] in J mol-1 K-1 for OIO2 at the B3LYP/aug-cc-pVTZ level of theory.

S°298K

Cp (200)

Cp (300)

Cp (400)

Cp (500)

Cp (600)

Cp (800)

Cp (1000)

Cp (1500)

301.45

56.61

65.82

71.55 75.03

77.21

79.63

80.84

82.09

Table 5 collects the enthalpies at 0 K of the stationary points relative to HOIO2 + OH and the corresponding reaction profile is represented in Figure 4 at the CCSD(T)/CBS(D,T) level of theory on DFT structures.

Table 5. Enthalpies at 0 K in kJ mol-1 relative to HOIO2 + OH calculated at the CCSD(T)/CBS(D,T)//B3LYP/aug-cc-pVTZ level of theory including ZPE and SOC.

H-abstraction

O(H)-abstraction

OIO + H2O

IO + H2O2

MCR

-34.5

-38.5

TS

12.1

102.5

MCP

-40.4

-8.2

Products

-14.1

41.4

Species

The prereactive complexes are not very sensitive to the size of the basis sets. There is only 0.5 kJ mol-1 difference between the aug-cc-pVTZ basis sets and the CBS(D,T) level of theory for MCRHabs and MCROHabs. This difference slightly increases to 1.3 kJ mol-1 for TSHabs. Increasing the size from double to triple-zeta decreases the energetic barrier by 3 kJ mol-1 for the H-abstraction channel. This difference increases to 6 kJ mol-1 for MCPHabs and the 16 ACS Paragon Plus Environment

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corresponding products. Increasing the size of the basis set tends to raise the barrier height and destabilize slightly MCPOHabs.

3.2.2. Reaction mechanism The pre-reactive complexes are highly stable as they lie about 35 kJ mol-1 below reactants. Once the small barrier of 12 kJ mol-1 is crossed, hydrogen transfer between iodic acid and hydroxyl radical takes place leading to the H-bonded product complex (Figure 4). MCPHabs is approximately 26 kJ mol-1 more stable than the isolated OIO2 and water. Another pathway corresponds to the I-O(H)HOIO2 bond dissociation with the abstraction of the OH fragment from iodic acid to hydroxyl radical to produce hydrogen peroxide as shown in Figure 4. The system must overcome a higher energetic barrier of about 100 kJ mol-1 for the top side attack from OH radical to occur. The OIO…H2O2 post-reactive complex is extremely stable, by 50 kJ mol-1, when compared to the corresponding products. Notice a significant difference in the magnitude of the barrier (90 kJ mol-1) between the frontside and the topside TSs.



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Figure 4. Reaction profiles at 0 K of the HOIO2 + OH reaction including ZPE and SOC calculated at the CCSD(T)/CBS(D,T)//B3LYP/aug-ccpVTZ level of theory.

18


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3.3. Kinetics

3.3.1. Rate constants High-pressure limit rate constants for R1a and R1b were computed using TST and VTST theories in combination with the Eckart tunneling correction and hindered rotor treatment. The values for kHabs and kOHabs are listed in Table 6 together with the overall rate constant (kHOIO2+OH). The results are shown for temperatures of atmospheric (T ≤ 300 K) and nuclear (400 ≤ T (K) ≤ 1500) interests. At the temperature of the containment building (400 K), the reaction rate is of the order of 10−17 cm3 molecule−1 s−1. In the case of a core melt accident where the temperature can reach 1000 −2500 K, k increases up to 10−15−10−14 cm3 molecule−1 s−1. Table 6. H- and O(H)-abstraction rate constants in cm3 molecule-1 s-1.

Temperature (K) 250

300

400

1000

1500

2500

kHabs

1.62 ×10-17

3.43 ×10-17 1.02 ×10-16 1.65 ×10-15 4.85 ×10-15

2.13 ×10-14

kOHabs

3.50 ×10-35

1.27 ×10-31 4.24 ×10-27 1.52 ×10-18 2.02 ×10-16

1.71 ×10-14

kHOIO2+OH

1.62 ×10-17

3.43 ×10-17 1.02 ×10-16 1.65 ×10-15 5.05 ×10-15

3.84 ×10-14

Clearly, below 1500 K, the H2O elimination dominates the reactivity of iodic acid towards OH because of its low barrier height. Above 1500 K, R1b becomes competitive with the proton transfer channel. Interestingly, this trend is in line with the HOIO + OH reaction,22 where the overall reaction was found to be significantly dominated by the H-abstraction channel that leads to water elimination. As a side note, kHOIO2+OH is slightly lower than

kHOIO+OH. For instance, at 300 K, kHOIO+OH = 6.20 ×10-16 cm3 molecule-1 s-1.22



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3.3.2. Arrhenius Parameters The Arrhenius parameters include the activation energy, Ea, the pre-exponential factor, B, and unitless n that characterize the extended Arrhenius expression as:

() = Y × Z exp (−

2%

[

)

(9)

where R is the universal molar gas constant and T is the temperature. The Arrhenius parameters adjusted to Equation 9 for the HOIO2 + OH → OIO2 + H2O and the HOIO2 + OH

→ OIO + H2O2 channels are given in Table 7.

Table 7. Fitted Arrhenius parameters for R1a and R1b within 250 −2500 K.

Ba

n

Eab

kHabs

1.76 ×10-22

2.39

3.50

kOHabs

3.16 ×10-21

2.57

96.23

kHOIO2+OH

1.40 ×10-24

3.04

0.50

a

in cm3 molecule-1 s-1, b in kJ mol-1

3.3.3. HOIO2 lifetime towards OH removal ^] The lifetime of iodic acid towards its consumption by OH radical (\]^_^ ) was estimated at ^] 1336 years. This value was determined by scaling to that of methyl chloroform (\] ),60 àab `

^] whose lifetime towards OH is better known (\] = 5.9 years61) and by using a ratio of the àab `

^] rate constants of CH3CCl3 + OH and HOIO2 + OH at 277 K (] = 6.79 × 10-15 cm3 àab `

^] molecule-1 s-1, 57 and ]^_^ = 3.00 ×10-17 cm3 molecule-1 s-1). Possible loss mechanisms with

other photo-oxidants (e.g. HO2, Cl, NO3) and photolysis under clear sky and multiphase chemistry taking into account iodic acid solubility and its mass transfer to the cloud droplets may impact and reduce the iodic acid atmospheric lifetime. The field measurements at


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different locations14,15 far from marine environments could suggest however that iodic acid has enough time to be transported over the Earth.

4. CONCLUSION This work presents computations of the mechanism and kinetics of the reaction of iodic acid with OH radical. Two possible pathways were investigated that are: HOIO2 + OH → OIO2 + H2O and HOIO2 + OH → OIO + H2O2. The structures along the reaction path were optimized at the B3LYP/aug-cc-pVTZ level. Coupled-cluster theory was then employed to compute the potential energies. Spin-orbit effect which is only of minor importance for the reactants and products was found to be close to zero for the structures involved in the reaction. The Habstraction channel, only, that leads to water elimination was found to be exothermic and exergonic at 298 K. Two TS structures, characteristic of a frontside and a topside attack by OH radical, were identified. Clearly, the H-transfer channel dominates the reactivity at atmospheric temperatures. It is worth noting that similar trend was observed with the recently published HOIO + OH reaction.22 High-pressure limit rate constants were computed using TST and VTST theories in combination with Eckart tunneling correction and hindered rotor treatment. The modified Arrhenius expressions within 250-2500 K for H- and O(H)abstraction channels are:

kHabs (cm3 molecule-1 s-1) = 1.76 ×10-22× T2.39exp (-3.5(kJ mol-1)/RT) kOHabs (cm3 molecule-1 s-1) = 3.16 ×10-21× T2.57 exp (-96.2(kJ mol-1)/RT) The overall reaction rate, kHOIO2+OH, can be written as:

kHOIO2+OH (cm3 molecule-1 s-1) = 1.40 ×10-24 × T3.04 exp (-0.5(kJ mol-1)/RT) This paper provides the first set of kinetic parameters for the gas-phase reactivity of iodic acid towards OH radicals. Our results indicate that its lifetime towards OH atmospheric removal is



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extremely long The thermochemical properties for the OIO2 radical are also reported in this work: ∆fH°298K = 168.3 kJ mol-1, S°298K = 301.45 J mol-1 K-1, and Cp (300 K) = 65.82 J mol-1 K-1. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Computer time for part of the theoretical calculations was kindly provided by the Centre de Ressources Informatiques (CRI) of the University of Lille1 and the Centre Régional Informatique et d'Applications Numériques de Normandie (CRIANN). Part of the calculations was also performed within the Research & Development Operational Programme funded by ERDF (CGreen-I 26240120001 and CGreen-II 26240120025). This work was part of the MiRE project (Mitigation of outside releases in case of nuclear accident), which is funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract "ANR-11-RSNR-0013-01". We appreciate also the support from PIA managed by the ANR under grant agreement "ANR-11-LABX-0005-01" called CaPPA (Chemical and Physical Properties of the Atmosphere), and, also supported by the Regional Council "Nord-Pas de Calais" and the "European Funds for Regional Economic Development". We thank Slovak Grant Agencies VEGA (Grant 1/0092/14) and APVV


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(Project APVV-15-0105) for support. This work was performed in the frame of the international collaboration agreement between IRSN, Comenius, Lille 1, and CNRS.

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Investigation of the Reaction Mechanism and Kinetics of Iodic Acid

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