Florent Louis

Theoretical Study of the Gas-Phase Reactions of Iodine Atoms (2P3/2) with H2, H2O, HI, and OH

The rate constants of the reactions of iodine atoms with H(2), H(2)O, HI, and OH have been estimated using 39, 21, 13, and 39 different levels of theory, respectively, and have been compared to the available literature values over the temperature range of 250-2500 K. The aim of this methodological work is to demonstrate that standard theoretical methods are adequate to obtain quantitative rate constants for the reactions involving iodine-containing species. Geometry optimizations and vibrational frequency calculations are performed using three methods (MP2, MPW1K, and BHandHLYP) combined with three basis sets (cc-pVTZ, cc-pVQZ, and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVnZ (n = T, Q, and 5), aug-cc-pVnZ (n = T, Q, and 5), 6-311G(d,p), 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Canonical transition state theory with a simple Wigner tunneling correction is used to predict the rate constants as a function of temperature. CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory provide accurate kinetic rate constants when compared to available literature data. The use of the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ and CCSD(T)/6-311++G(3df,3pd) levels of theory allows one to obtain a better agreement with the literature data for all reactions with the exception of the I + H(2) reaction R(1) . This computational procedure has been also used to predict rate constants for some reactions where no available experimental data exist. The use of quantum chemistry tools could be therefore extended to other elements and next applied to develop kinetic networks involving various fission products, steam, and hydrogen in the absence of literature data. The final objective is to implement the kinetics of gaseous reactions in the ASTEC (Accident Source Term Evaluation Code) code to improve speciation of fission transport, which can be transported along the Reactor Coolant System (RCS) of a Pressurized Water Reactor (PWR) in case of a severe accident.

Kinetic study of the reactions of CF3O2 radicals with Cl and NO

Kinetic studies of the reactions CF3O2+Cl and CF3O2+NO were performed at room temperature in the gas phase using the discharge flow mass spectrometric technique (DFMS). The reactions were investigated under pseudo-first-order conditions with Cl or NO in large excess with respect to the CF3O2 radicals. The rate constant for the reaction CF3O2+NO was measured at 298 K and the value of (1.6±0.3)×10-11 cm3 molecule-1 s-1 is in very good agreement with all previous values. For the reaction CF3O2+Cl, we obtain a rate constant at 298 K of (4.2±0.8)×10-11 cm3 molecule-1 s-1 in excellent agreement with the only published value. Product analysis shows that this reaction occurs via the major reaction pathway CF3O2+Cl→CF3O+ClO at room temperature. In addition, an abinitio theoretical study was performed to gain insights on the different postulated reaction pathways. There is a significant disagreement between experimental and abinitio values recommended for the formation enthalpies of CF2O, CF3O and related molecules produced in this system. Consequently, we provide self-consistent values of enthalpies based on isodesmic reactions for the CF3O2+Cl reaction system using the G2, G2(MP2) and CBS-Q methods. These values are also compared with BAC-MP4 heats of formation calculated in this work.

Atmospheric Reactivity of CH2ICl with OH Radicals: High-Level OVOS CCSD(T) Calculations for the X-Abstraction Pathways (X = H, Cl, or I)

Reactants, weak molecular complexes, transition states, and products for the H-, Cl-, and I-abstraction channels in the reaction of OH radicals with chloroiodomethane CH(2)ICl as well as the energy profiles at 0 K have been determined using high-level all-electron ab initio methods. The results showed that all-electron DK-CCSD(T)/ANO-RCC approach performed very well in predicting the reactivity of iodine. In terms of activation enthalpy at 0 K, the energy profile for the Cl-abstraction showed that this reaction pathway is not energetically favorable in contrast to the two other channels (H- and I-abstractions), which are competitive. The H-abstraction was strongly exothermic (-87 kJ mol(-1)), while the I-abstraction was modestly endothermic (11.8 kJ mol(-1)). On the basis of our calculations including the following corrections to the potential energies: basis set saturation, valence and core-valence electron correlation, relativistic effects, spin-adaptation, vibration contributions, and tunneling corrections, rate constants were predicted using canonical transition state theory over the temperature range 250-500 K for each abstraction pathway. The overall rate constant at 298 K was estimated to be 4.29 × 10(-14) and 5.44 × 10(-14) cm(3) molecule(-1) s(-1) for complex and direct abstraction mechanisms, respectively. In addition, the overall rate constant computed at 277 K was used in the estimation of the atmospheric lifetime for CH(2)ICl. On the basis of our theoretical calculations, the atmospheric lifetime for the OH removal process is predicted to be close to 1 year. In terms of atmospheric lifetime, the OH reaction is not competitive with the Cl reaction and photolysis processes.

A theoretical study of the H-abstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms ( 2P3/2)

The rate constants of the reactions of HOI molecules with H, OH, O ((3)P), and I ((2)P(3/2)) atoms have been estimated over the temperature range 300-2500 K using four different levels of theory. Geometry optimizations and vibrational frequency calculations are performed using MP2 methods combined with two basis sets (cc-pVTZ and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVTZ, cc-pVQZ, 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Reaction enthalpies at 0 K were calculated at the CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory and compared to the experimental values taken from the literature. Canonical transition-state theory with an Eckart tunneling correction is used to predict the rate constants as a function of temperature. The computational procedure has been used to predict rate constants for H-abstraction elementary reactions because there are actually no literature data to which the calculated rate constants can be directly compared. The final objective is to implement kinetics of gaseous reactions in the ASTEC (accident source term evaluation code) program to improve speciation of fission products, which can be transported along the reactor coolant system (RCS) of a pressurized water reactor (PWR) in the case of a severe accident.

A theoretical study of the kinetics of the benzylperoxy radical isomerization.

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

Kinetics and products studies of reactions between fluorine atoms and CHF3, CHClF2, CHCl2F and CHCl3

The absolute rate coefficients for the hydrogen abstraction reactions from CHF3 (k1), CHClF2 (k2), CHCl2F (k3) and CHCl3 (k4) by fluorine atoms in the gas phase have been measured over the temperature range 297–421 K using the discharge flow/mass spectrometric technique (DF/MS). Experiments were performed under pseudo-first-order conditions with F atoms in large excess with respect to the halomethane. The temperature dependence of the bimolecular rate coefficients is expressed in the Arrhenius form: k1 = (2.1−0.7+1.1) × 10−11 exp[−(1470 ± 150)/T], k2 = (2.4−0.6+0.9) × 10−11 exp[−(1040 ± 100)/T], k3 = (2.6−0.7+1.0) × 10−11 exp[−(850 ± 120)/T] and k4 = (1.7−0.4+0.5) × 10−11 exp[−(550 ± 90)/T]. The units of the rate constants are cm3 molecule−1 s−1 and the quoted uncertainties are 2σ obtained from values incorporating systematic errors due to secondary reactions. Thanks to the use of mass spectrometry, CF4 and FCl as end products have been identified and quantified. The fast formation of CF4 observed for each reaction of the series F + CHCl3−xFx is interpreted by a sequence of rapid atom-switching reactions. The fast formation of FCl is explained by reactions between F and radicals of CCl3−xFx (x = 0, 1 and 2) type. A mechanism of secondary reactions is proposed to account for the experimental concentration profiles determined as a function of time for CF4 and FCl.

A Density Functional Theory and ab Initio Investigation of the Oxidation Reaction of CO by IO Radicals.

To get an insight into the possible reactivity between iodine oxides and CO, a first step was to study the thermochemical properties and kinetic parameters of the reaction between IO and CO using theoretical chemistry tools. All stationary points involved were optimized using the Beckes three-parameter hybrid exchange functional coupled with the Lee-Yang-Parr nonlocal correlation functional (B3LYP) and the Møller-Plesset second-order perturbation theory (MP2). Single-point energy calculations were performed using the coupled cluster theory with the iterative inclusion of singles and doubles and the perturbative estimation for triple excitations (CCSD(T)) and the aug-cc-pVnZ (n = T, Q, and 5) basis sets on geometries previously optimized at the aug-cc-pVTZ level. The energetics was then recalculated using the one-component DK-CCSD(T) approach with the relativistic ANO basis sets. The spin-orbit coupling for the iodine containing species was calculated a posteriori using the restricted active space state interaction method in conjunction with the multiconfigurational perturbation theory (CASPT2/RASSI) employing the complete active space (CASSCF) wave function as the reference. The CCSD(T) energies were also corrected for BSSE for molecular complexes and refined with the extrapolation to CBS limit while the DK-CCSD(T) values were refined with the extrapolation to FCI. The exploration of the potential energy surface revealed a two-steps mechanism with a trans and a cis pathway. The rate constants for the direct and complex mechanism were computed as a function of temperature (250-2500 K) using the canonical transition state theory. The three-parameter Arrhenius expressions obtained for the direct and indirect mechanism at the DK-CCSD(T)-cf level of theory is 1.49 × 10(-17) × T(1.77) exp(-47.4 (kJ mol(-1))/RT).

A Theoretical Study of the X-Abstraction Reactions (X = H, Br, or I) from CH2IBr by OH Radicals: Implications for Atmospheric Chemistry

Abstract We report the calculation of the H-, Br-, and I-abstraction channels in the reaction of OH radicals with bromoiodomethane CH2IBr. The resulting energy profiles at 0 K were obtained by high-level all-electron ab initio methods including valence and core-valence electron correlation, scalar relativistic effects, spin-orbit coupling, spin-adaptation, vibration contributions, and tunneling corrections. In terms of activation enthalpy at 0 K, the energy profile for the Br-abstraction showed that this reaction pathway is not energetically favorable in contrast to the two other channels (H- and I-abstractions), which are competitive. The H-abstraction was strongly exothermic (−84.4 kJ mol–1), while the I-abstraction was modestly endothermic (16.5 kJ mol–1). On the basis of our calculations, we predicted the rate constants using canonical transition state theory over the temperature range 250–500K for each abstraction pathway. The overall rate constant at 298 K was estimated to be 3.40 × 10−14 and 4.22 × 10−14 cm3 molecule–1 s–1 for complex and direct abstraction mechanisms, respectively. In addition, the overall rate constant computed at 277 K was used in the estimation of the atmospheric lifetime for CH2IBr. On the basis of our theoretical calculations, the atmospheric lifetime for the OH removal process is predicted to be close to 1 year. In terms of atmospheric lifetime, the OH reaction is not competitive with the Cl reaction and photolysis processes.

Thermochemistry of small iodine species

We present a systematic study of the thermochemistry for a set of iodine species relevant to atmospheric chemistry. The reactions include H, O and I atoms and H2, OH, HI, I2, iodine monoxide, hypoiodous acid (HOI) and H2O species. The calculations presented were performed using completely renormalized coupled cluster theory including single, double and non-iterative triple substitutions in conjunction with the ANO-RCC basis sets developed for scalar relativistic calculations. The second-order spin-free Douglas–Kroll–Hess Hamiltonian was used to account for the scalar relativistic effects. The calculations also included spin–orbit corrections and semi-core correlation contributions. The resulting reaction enthalpies and Gibbs energies at 298 K have been compared with the experimental data. On the basis of a set of selected reactions we suggest an updated value for Δ f H298K° of HOI based on the set of isogyric reactions: −69.0 ± 3.7 kJ mol−1.

Theoretical study of H-abstraction reactions from CH3Cl and CH3Br molecules by ClO and BrO radicals.

The rate constants of the H-abstraction reactions from CH(3)Cl and CH(3)Br molecules by ClO and BrO radicals have been estimated over the temperature range of 300-2500 K using four different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using B3LYP and MP2 methods combined with the cc-pVTZ basis set. Single-point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using the cc-pVTZ and cc-pVQZ basis sets. Canonical transition-state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature. In order to choose the appropriate levels of theory with chlorine- and bromine-containing species, the reference reaction Cl ((2)P(3/2)) + CH(3)Cl → HCl + CH(2)Cl (R(ref)) was first theoretically studied because its kinetic parameters are well-established from numerous experiments, evaluation data, and theoretical studies. The kinetic parameters of the reaction R(ref) have been determined accurately using the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory. This level of theory has been used for the rate constant estimation of the reactions ClO + CH(3)Cl (R(1)), ClO + CH(3)Br (R(2)), BrO + CH(3)Cl (R(3)), and BrO + CH(3)Br (R(4)). Six-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of these four reactions (including cis and trans pathways) over the temperature range of 300-2500 K.