Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2

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A Theoretical Study of the Reactions of H Atoms with CHI and CHI Dorra Khiri, Ivan Cernusak, and Florent Louis J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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The Journal of Physical Chemistry

A Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2 Dorra Khiri,† Ivan Černušák,# Florent Louis,†,*

†

Univ. Lille, CNRS, UMR 8522 - PC2A - PhysicoChimie des Processus de Combustion et de

l'Atmosphère, F-59000 Lille, France #

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

University in Bratislava, Ilkovičova 6, 84215 Bratislava, Slovakia

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Abstract High level ab initio methods have been used to provide reliable kinetic data for the H+ CH3I and H + CH2I2 gas phase reactions. The (H, I)- abstraction and I- substitution reaction pathways were identified. The structures were determined on the potential energy surface at the MP2/aug-cc-pVTZ level of theory. The energetics was then refined using the coupled cluster theory. For the iodinated species, the spin-orbit coupling was calculated using the MRCI approach. The core valence and the scalar relativistic corrections were considered. Thermal rate constants were reported using the canonical transition state theory (TST) and compared to computed values with the canonical variational transition state theory (CVT) using the zero curvature tunneling (ZCT) and the small curvature tunneling (SCT) corrections over a wide temperature range (250-2500 K) to show the importance of quantum tunneling effects at low temperatures. They are given by the following expressions for the overall reactions using CVT/SCT method: kH+CH3I(T) = 1.07 × 10-17 × T2.13exp(2.68 (kJ mol-1) / RT) and kH+CH2I2(T) = 5.73 × 10-21 × T2.97 exp(3.15 (kJ mol-1) / RT). The I- abstraction is predicted to be the major pathway for both H + CH3I and H + CH2I2 reactions. The obtained kinetic parameters for the H + CH3I reaction are in excellent agreement with their experimental counterparts over the temperature range 300 - 750 K. Based on our calculated reaction enthalpies, a new evaluation of the standard enthalpy of formation at 298 K of CH2I and CHI2 has been provided. Obtained values are ∆fH°298K (CH2I) = (219.5 kJ mol-1) and ∆fH°298K(CHI2) = (296.3 kJ mol-1). 1. Introduction Iodine chemistry has become an important issue, as documented in numerous experimental and theoretical studies1–3. Iodomethanes (e.g. CH3I and CH2I2) have been detected in the marine boundary layer, in the coastal water, or in the open ocean.4 They are produced by 2 ACS Paragon Plus Environment

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microalgae and phytoplankton in marine boundary and belong to the most abundant and reactive alkyl iodides in the troposphere and these species could lead to aerosol formation in coastalzones.5 Methyl iodide has an atmospheric lifetime of only a few days and is considered to as a very short-lived substance.6 CH3I plays an important role in atmospheric chemistry and is a significant carrier of iodine from the ocean to the atmosphere.7 The reactivity of H atoms towards CH3I has been the subject of numerous experimental and theoretical works.8–14 The rate constant for the reaction of H + CH3I have been measured using the discharge flow with mass spectrometry by Leipunskii et al.8 and using the flash-photolysis resonance technique by Levy and Simons9 and Yuan et al.11 and using pulse radiolysis combined with infrared diode laser spectroscopy by Sillesen et al.10 The computational studies of the reactions of CH3I with H and OH have been performed by Marshall et al.12 using theMP4/6-311G(d,p) level of theory, they used all-electron frozen-core computations without effective core potentials, spin-orbit and core-valence corrections. They showed that for the H + CH3I reaction, the favoured channel corresponds to I- abstraction, while the H- abstraction is the major pathway for OH + CH3I reaction. Their rate constants have been calculated using TST method without including variational and quantum tunneling effects. The reactivity of CH3I and CH2I2 with OH radicals have been also investigated recently by Louis et al.15 They reported also that the major pathway corresponds to the H- abstraction. Gas-phase reactions of CH3I and CH2I2 with O(3P) and Cl have been also studied16,17 and the favoured reaction pathway is predicted to be the I- abstraction for reactions with O(3P) and the H- abstraction for reactions with Cl. To the best of our knowledge, there is no theoretical nor experimental studies reported in the literature about thermochemical properties and kinetic parameters of H + CH2I2 reaction. The literature Rate Parameters of the H + CH3I reaction are reported in Table 1.



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This works aims to provide a systematic study of the thermochemistry and reactivity of hydrogen atoms with CH3I and CH2I2. Three different channels have been investigated for each reaction: CH3I + H→ HI + CH3

(R1a)

CH3I + H→ H2 + CH2I

(R1b)

CH3I + H→I + CH4

(R1c)

CH2I2 + H→ HI + CH2I

(R2a)

CH2I2 + H→ H2 + CHI2

(R2b)

CH2I2 + H→ I + CH3I

(R2c)

Transition state theory (TST) and canonical variational transition state theory (CVT) using the ZCT and the SCT corrections are employed to provide rate constants as a function of temperature. TST rate constants have been compared to CVT/SCT results in order to better understand the importance of quantum and variational effects. The kinetic parameters obtained in this paper will allow improved modeling of the combustion chemistry of methyl iodides. In Section 2 we describe the computational methods and in section 3 we present the results and the discussions.


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Table 1. Kinetic Parameters Available in Literature for the H + CH3I Reaction

A 3

Ea -1 -1

(cm molecule s )

6.30 × 10-11

T (K)

k -1

(kJ mol )

3

method

reference

-1 -1

(cm molecule s ) 4.00 × 10-12

298

experimenta Leipunskii et al.,19718

9.63 × 10-12

293

experimentb Levy and Simons, 19759

6.14 × 10-12

298

experimentc Sillesen et al., 199210

8.36 × 10-12

298

experimentb Yuan et al., 199711

297-575

5.00 -11

1.00 × 10

298

2.14 × 10-15

2.49

250-2500

4.15 × 10-12

20.90

1250-1550

theoryd

Marshall et al., 199712

experimente Hynes et al., 200013

1.25 × 10-10 5.00 1500-2000 experimentf Yang et al., 200914 a discharge flow/mass spectrometry. bflash photolysis/resonance fluorescence. cinfrared diode laser spectroscopy. dMP4/6-311G(d,p)//MP2/631G(d) level of theory associated to conventional transition state theory. ereflected shock waves in a single-pulse shock tube. fincident shock waves in a diaphragmless shock tube.

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2. Computational methods All stationary points structures found on the potential energy surface (PES) were fully optimized using the Møller−Plesset second-order perturbation theory (MP2)18,19 with the augmented correlation consistent polarized triple zeta20(aug-cc-pVTZ) basis set for C and H atoms and with the aug-cc-pVTZ-PP basis set of Peterson et al.21 for the 25 valence electrons of the iodine atom (in the rest of the paper, this basis set will be written without the PP term) while the pseudopotential ECP28MDF was used to describe the core electrons. The wavenumbers and the zero-point energies (ZPE) were computed at the MP2/aug-cc-pVTZ level of theory using the suitable scaling factor22 of 0.953. Transition states structures corresponding to (H, I)- abstraction and I- substitution pathways have been obtained. Intrinsic reaction coordinate (IRC)23–25 calculations have been performed to connect the transition state to corresponding molecular complexes. Gaussian 1626 has been used in this part of calculations.

To obtain more accurate potential energies, various single-point energy calculations were carried out using the Molpro 2015 program package.27 Coupled cluster theory28 calculations were performed using the MP2/aug-cc-pVTZ geometries and the weighted core-valence basis sets:29,30 aug-cc-pwCVnZ (n = T, Q, 5) for C, and H atoms and aug-cc-pwCVnZ-PP (n = T, Q, 5) for the iodine atom.21 The CCSD(T) energies were extrapolated to the complete basis set (CBS) limit using the mixed gaussian/exponential formula31 = + exp− − 1 + exp−n − 1

(1)

with ECBS is the estimated CBS limit as n→∞ and n is the cardinal number of the basis set (3 = awCVTZ, 4 = awCVQZ, and 5= awCV5Z). Additional calculations were carried out to consider different corrections applied to the CCSD(T)/CBS energies. The core-valence (CV) correction32 was performed at the 6 ACS Paragon Plus Environment

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CCSD(T)/aug-cc-pwCVTZ level of theory. The scalar relativistic corrections (SR) were also performed using the CISD/aug-cc-pVTZ level of theory. For the iodine-containing species, the scalar relativistic correction on I atom is already considered by using the pseudopotential. The SR correction was assessed from the expectation values for the two dominant terms in the Breit-Pauli Hamiltonian (the mass-velocity and one-electron Darwin33 (MVD) corrections). The double counting of the relativistic effect on iodine is small as shown by Dixon et al.34,35 in their previous works on iodine species. Due to the presence of the iodine atom the spin-orbit coupling (SO) is playing an important role in the determination of the total energies. For the iodine-containing species, the SO36,37 calculations were performed at the MRCI/aug-cc-pVTZ//CASSCF/aug-cc-pVTZ level of theory. For all intermediate species, the chosen active spaces are (15,12) and (21,15) for H + CH3I and H + CH2I2 reactions, respectively. The choice of active electrons is similar to the one used in our recent study.38 Different energy contributions have been included in the determination of the enthalpy at 0 K as follows: H0K= ECBS+ EZPE + ECV + ESR + ESO

(2)

Canonical variational transition state theory (CVT)39–41 with the zero curvature tunneling (ZCT)42 and the small curvature tunneling (SCT)39 have been used to calculate the rate constants over the temperature range (250 - 2500 K) using the POLYRATE (2017-B) program.43 The TST44–46 and CVT rate constants were compared to the values estimated by both CVT/SCT and CVT/ZCT calculations. Similar theoretical studies of abstraction reactions for species of combustion interest have been reported in the literature.47,48 In the CVT/SCT approach, the calculation of the energy along the minimum energy path (MEP), the gradients, and the Hessians at numerous points along the MEP are required. Truhlar and co-workers



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studies42,49,50 show that the zero-curvature tunneling approximation underestimates tunneling corrections to rate constants because it neglect the corner-cutting effect, which corresponds to the coupling between the reaction coordinate and orthogonal vibrational modes. This effect is taken into account in the SCT approximation. This increases the tunneling and reduces the tunneling path.42 The SCT calculations are based on the centrifugal-dominant small curvature semi classical ground state (CD-SCSAG).39,40 The transmission probability at energy E is given as follows: =

(3)

θ(E) corresponds to = ħ

-. !" {2&'' ()* " -/

− }/

(4)

where s0 and s1 are the reaction coordinates of the classical tunneling points in the reactant and product valleys, respectively, µeff is the effective mass of the tunneling motion along the reaction coordinate,41 and VaG is the vibrationally adiabatic ground-state potential energy: VaG (s) = VMEP (s) + ZPE(s)

(5)

VMEP represents the classical potential energy of the minimum energy path (MEP).

The TST rate constants have been computed using the following expression: 0 11 2 =

34 1

678

5 69 6:9

exp

∆

Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2

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