Microhydration of caesium compounds Cs, CsOH, CsI and Cs2I 2 complexes with one to three H2O molecules of nuclear safety interest

International audienceStructure and thermodynamic properties (standard enthalpies of formation and Gibbs free energies) of hydrated caesium species of nuclear safety interest, Cs, CsOH, CsI and its dimer Cs2I2, with one up to three water molecules, are calculated to assess their possible existence in severe accident occurring to a pressurized water reactor. The calculations were performed using the coupled cluster theory including single, double and non-iterative triple substitutions (CCSD(T)) in conjunction with the basis sets (ANO-RCC) developed for scalar relativistic calculations. The second-order spin-free Douglas-Kroll-Hess Hamiltonian was used to account for the scalar relativistic effects. Thermodynamic properties obtained by these correlated ab initio calculations (entropies and thermal capacities at constant pressure as a function of temperature) are used in nuclear accident simulations using ASTEC/SOPHAEROS software. Interaction energies, standard enthalpies and Gibbs free energies of successive water molecules addition determine the ordering of the complexes. CsOH forms the most hydrated stable complexes followed by CsI, Cs2I2, and Cs. CsOH still exists in steamatmosphere even at quite high temperature, up to around 1100 K. © Springer-Verlag 2014

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

International audienceTo 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 Becke's 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 × T1.77 exp(-47.4 (kJ mol-1)/RT). © 2016 American Chemical Society

Computational study of the I2O5 + H2O = 2 HOIO2 gas-phase reaction

International audienceThis paper presents the mechanism and the kinetics of the I2O5 (g) + H2O (g) = 2 HOIO2 (g) reaction. The potential energy surface was explored with the B3LYP and MP2 methods with the aug-cc-pVTZ basis set. The rate constants were computed as a function of temperature (250–750 K) using transition state theory. At the CCSD(T)/CBS level, the rate constants were estimated to be (k in cm3 molecule−1 s−1) kforward(T) = 3.61 × 10−22 × T2.05 exp (−32.3 (kJ mol−1)/RT) and kreverse (T) = 6.73 × 10−27 × T2.90 exp (−24.5 (kJ mol−1)/RT). Implications for atmospheric chemistry and nuclear safety issues are discussed. © 201

Thermochemistry of HIO2 Species and Reactivity of Iodous Acid with OH Radical A Computational Study

International audienceThis paper reports computations of thermochemical properties [δfH298 K°, S298 K°, and Cp = f(T)] of HIO2 isomers (HOOI and HOIO) together with the kinetic parameters of the gas-phase HOIO + OH reaction. The calculations are performed using the CCSD(T) method on structures previously optimized at the B3LYP/aug-cc-pVTZ level of theory. Spin-orbit coupling is computed by employing the CASSCF/CASPT2/RASSI scheme. The standard enthalpies of formation at 298 K are equal to -24.8 ± 0.9 and 18.7 ± 0.4 kJ mol-1 for HOIO and HOOI, respectively. The H- and O(H)-abstraction pathways are considered for the reaction of iodous acid (HOIO) with OH. Our calculations reveal that the H-abstraction channel was exergonic by -135 kJ mol-1 at 298 K. The rate constants were computed as a function of the temperature (250-2500 K) using classical and variational transition state theory. At the CCSD(T)/CBS level, the rate constants are (in cm3 molecule-1 s-1) kHabs(T) = 2.22 × 10-19T1.83 exp[-4.9 (kJ mol-1)/RT] and kOHabs(T) = 7.73 × 10-21T2.66 exp[-70.2 (kJ mol-1)/RT]. The HOIO + OH overall reaction is significantly dominated by the HOIO + OH → OIO + H2O channel for tropospheric temperatures. The atmospheric lifetime of HOIO is estimated to be approximately 64 years. Thermochemical properties and kinetic parameters derived from this study can be used as input parameters in chemistry-transport models, for which iodine is important in the case of radioactive airbone species released for a severe accident occurring to nuclear power plants. © 2017 American Chemical Society

Molecular structures and thermodynamic properties of monohydrated gaseous iodine compounds Modelling for severe accident simulation

International audienceMonohydrated complexes of iodine species (I, I2, HI, and HOI) have been studied by correlated ab initio calculations. The standard enthalpies of formation, Gibbs free energy and the temperature dependence of the heat capacities at constant pressure were calculated. The values obtained have been implemented in ASTEC nuclear accident simulation software to check the thermodynamic stability of hydrated iodine compounds in the reactor coolant system and in the nuclear containment building of a pressurised water reactor during a severe accident. It can be concluded that iodine complexes are thermodynamically unstable by means of positive Gibbs free energies and would be represented by trace level concentrations in severe accident conditions; thus it is well justified to only consider pure iodine species and not hydrated forms.© 2013 Elsevier B.V. All rights reserved

Structural changes in the series of boron-carbon mixed clusters C xB10-x-(x = 3-10) upon substitution of boron by carbon

We report a theoretical investigation on the ten-atom boron-carbon mixed clusters CxB10-x - (x = 3-10), revealing a molecular wheel to monocyclic ring and linear species structural change as a function of x upon increasing the number of carbon atoms in the studied series. The unbiased searches for the global minimum structures of the clusters with x ranging from 3 to 9 were conducted using the Coalescence Kick program for different spin multiplicities. Subsequent geometry optimizations with follow-up frequency calculations at the hybrid density functional B3LYP/6-311+G(d) level of theory along with the single point coupled-cluster calculations (UCCSD(T)/aug-cc-pVTZ// B3LYP/6- 311+G(d) and RCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d)) revealed that the C3B7 - and C4B6 - clusters possess planar distorted wheel-type structures with a single inner boron atom, similar to the recently reported CB9 - and C2B8 -. Going from C 5B5 - to C9B- inclusive, monocyclic and ringlike structures are observed as the most stable ones on the PES. The first linear species in the presented series is found for the C10 - cluster, which is almost isoenergetic with the one possessing a monocyclic geometry. The classical 2c-2e σ bonds are responsible for the peripheral bonding in both carbon- and boron-rich clusters, whereas multicenter σ bonding (nc-2e bonds with n > 2) on the inner fragments in boron-rich clusters is found to be the effective tool to describe their chemical bonding nature. It was shown that the structural transitions in the CxB 10-x - series occur in part due to the preference of carbon to form localized bonds, which are found on the periphery of the clusters. Chemical bonding picture of C10 - is explained on the basis of the geometrical structures of the C10 and C102- clusters and their chemical bonding analyses. © 2013 AIP Publishing LLC

Structural changes in the series of boron-carbon mixed clusters C xB10-x-(x = 3-10) upon substitution of boron by carbon

We report a theoretical investigation on the ten-atom boron-carbon mixed clusters CxB10-x - (x = 3-10), revealing a molecular wheel to monocyclic ring and linear species structural change as a function of x upon increasing the number of carbon atoms in the studied series. The unbiased searches for the global minimum structures of the clusters with x ranging from 3 to 9 were conducted using the Coalescence Kick program for different spin multiplicities. Subsequent geometry optimizations with follow-up frequency calculations at the hybrid density functional B3LYP/6-311+G(d) level of theory along with the single point coupled-cluster calculations (UCCSD(T)/aug-cc-pVTZ// B3LYP/6- 311+G(d) and RCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d)) revealed that the C3B7 - and C4B6 - clusters possess planar distorted wheel-type structures with a single inner boron atom, similar to the recently reported CB9 - and C2B8 -. Going from C 5B5 - to C9B- inclusive, monocyclic and ringlike structures are observed as the most stable ones on the PES. The first linear species in the presented series is found for the C10 - cluster, which is almost isoenergetic with the one possessing a monocyclic geometry. The classical 2c-2e σ bonds are responsible for the peripheral bonding in both carbon- and boron-rich clusters, whereas multicenter σ bonding (nc-2e bonds with n > 2) on the inner fragments in boron-rich clusters is found to be the effective tool to describe their chemical bonding nature. It was shown that the structural transitions in the CxB 10-x - series occur in part due to the preference of carbon to form localized bonds, which are found on the periphery of the clusters. Chemical bonding picture of C10 - is explained on the basis of the geometrical structures of the C10 and C102- clusters and their chemical bonding analyses. © 2013 AIP Publishing LLC