Theoretical study of the gas-phase reactions of iodine atoms ( 2P3/2) with H2, H2O, HI, and OH

cited By 31The rate constants of the reactions of iodine atoms with H2, H2O, 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 2502500 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 + H2 reaction R1. 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. © 2010 American Chemical Society

Theoretical Study of the Oxidation Mechanisms of Naphthalene Initiated by Hydroxyl Radicals: The OH-Addition Pathway

The oxidation mechanisms of naphthalene by OH radicals under inert (He) conditions have been studied using density functional theory along with various exchange–correlation functionals. Comparison has been made with benchmark CBS-QB3 theoretical results. Kinetic rate constants were correspondingly estimated by means of transition state theory and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Comparison with experiment confirms that, on the OH-addition reaction pathway leading to 1-naphthol, the first bimolecular reaction step has an effective negative activation energy around −1.5 kcal mol^–1, whereas this step is characterized by an activation energy around 1 kcal mol^–1 on the OH-addition reaction pathway leading to 2-naphthol. Effective rate constants have been calculated according to a steady state analysis upon a two-step model reaction mechanism. In line with experiment, the correspondingly obtained branching ratios indicate that, at temperatures lower than 410 K, the most abundant product resulting from the oxidation of naphthalene by OH radicals must be 1-naphthol. The regioselectivity of the OH^•-addition onto naphthalene decreases with increasing temperatures and decreasing pressures. Because of slightly positive or even negative activation energies, the RRKM calculations demonstrate that the transition state approximation breaks down at ambient pressure (1 bar) for the first bimolecular reaction steps. Overwhelmingly high pressures, larger than 10⁵ bar, would be required for restoring to some extent (within ∼5% accuracy) the validity of this approximation for all the reaction channels that are involved in the OH-addition pathway. Analysis of the computed structures, bond orders, and free energy profiles demonstrate that all reaction steps involved in the oxidation of naphthalene by OH radicals satisfy Leffler–Hammond’s principle. Nucleus independent chemical shift indices and natural bond orbital analysis also show that the computed activation and reaction energies are largely dictated by alterations of aromaticity, and, to a lesser extent, by anomeric and hyperconjugative effects

Atmospheric degradation of two short-lived brominated hydrocarbons (CHBr 3 and CH 2 Br 2 )

International audienceTwo brominated VSLS, bromoform (CHBr 3) and dibromomethane (CH 2 Br 2), which have natural sources in coastal regions, have the potential to transport reactive bromine into the stratosphere and to contribute to the bromine budget. In order to better evaluate the impact of these two species, chemical schemes for their atmospheric degradation have been developed from a detailed kinetic and mechanistic analysis of all the gas phase reactions involved, specially of the peroxy radicals. The most likely pathways for the reactions of HO 2 with brominated peroxy radicals RO 2 (with R = CH 2 Br, CHBr 2 and CBr 3) have been established using ab initio calculations. The Henry's law constants of the brominated organics products have been also estimated using empirical methods. Using these constants, the less soluble species formed from the brominated VSLS degradation are found to be CBr 2 O, CHBrO, CBr 3 O 2 NO 2 , CHBr 2 O 2 NO 2 , BrO, BrONO 2 and HOBr. In the presence of deep convection, these species could be transported into the TTL (tropical tropopause layer). Then, these data have been implemented in a meteorological/tracer transport model (CATT-BRAMS), including a simplified chemistry of other atmospheric species The full degradation schemes have been run under realistic conditions of " clean " and moderately NO x-polluted atmospheres, which are representative of tropical coastal regions. The influence of the reactions of the RO 2 radicals with HO 2 , CH 3 O 2 and NO 2 on the nature and abundance of the stable intermediate and end-products has been tested. In the case of CHBr 3 degradation, it results that the reactions of RO 2 with NO 2 have no impact, and that the inclusion of the reactions of RO 2 with CH 3 O 2 and with HO 2 (with " new " branching ratios) leads to a slight decrease of the bromine potentially able to reach the TTL. In contrast to CHBr 3 , the CH 2 Br 2 degradation leads to a negligible production of organic species. Finally, for both bromoform and dibromomethane degradation, the effect of a moderate NO x pollution significantly increases the production of the less soluble species and thus approximately doubles the bromine potentially able to reach the TTL. By taking into account the results of these analysis, simplified degradation schemes for CHBr 3 and CH 2 Br 2 are proposed

Photoabsorption measurements and theoretical calculations of the electronic state spectroscopy of propionic, butyric, and valeric acids

Absolute photoabsorption cross sections of propionic (C2H5COOH), butyric (C3H7COOH), and valeric (C4H9COOH) acids have been measured from the dissociative * nO transition (beginning around 5.0 eV) up to 10.7 eV. This constitutes the first study of the neutral electronic states of propionic and butyric acids at energies above the * nO band, while no previous spectroscopic data is available for valeric acid in the present range. The present assignments are supported by the first theoretical calculations of electronic transition energies and oscillator strengths for these organic acids. In addition, the excitation energies of the vibrational modes of propionic acid in its neutral electronic ground state and the vertical ionisation energies of all three molecules have been calculated for the first time. The He(I) photoelectron spectroscopy of propionic acid has been measured from 10 to 16 eV, revealing new fine structure in the first ionic band

Ab initio calculations and iodine kinetic modeling in the reactor coolant system of a pressurized water reactor in case of severe nuclear accident

International audienceA kinetic model for iodine, oxygen and hydrogen is proposed in order to simulate transport of iodine along the reactor coolant system (RCS) of a Pressurized Water Reactor (PWR) in case of a severe accident. This kinetic mechanism is composed of 33 reactions. Due to lack of data in the literature for many gaseous iodine reactions, ab initio methods have been employed to determine rate constants as it was already the case in our previous works (Canneaux et al. J. Phys. Chem. A 114 (2010) 9270; Hammaecher et al. J. Phys. Chem. A 115 (2011) 6664). These computational studies have been completed here with calculations for the I/H substitution in HOI and iodine abstraction from hypoiodous acid by H and I ( 2P 3/2) atoms. The rate constants have been estimated using the same methodology over the temperature range (300-2500K). After being implemented in the ASTEC software (Accident Source Term Evaluation Code), simple computations were performed to assess time needed to reach equilibrium at three different temperatures 1000, 1200, and 1400K in a batch reactor at a constant pressure of 2×10 5Pa. The influence of gas composition has been also examined using four different gas compositions pure steam, pure hydrogen, and two hydrogen/steam intermediate mixtures containing in weighted percentages either 20/80 or 80/20% w. The PHEBUS-FPT1 experimental test was re-interpreted in activating with the kinetics of the I-O-H reaction system instead of thermodynamic equilibrium assumption. As expected, the kinetics promoted the persistence of gaseous iodine fraction at low temperature (428K) as observed in the PHEBUS tests and thermodynamic equilibrium assumption appeared to be too simplistic to get accurate iodine speciation at the break and thus reliable source term1Types and amounts of radioactive or hazardous material released to the environment following an accident. 1 estimations. © 2012 Elsevier B.V