O. Motapon

Collisional-radiative model in air for earth re-entry problems

A nonlinear time-dependent two-temperature collisional-radiative model for air plasma has been developed for pressures between 1kPa and atmospheric pressure to be applied to the flow conditions of space vehicle re-entry into the Earth’s atmosphere. The model consists of 13 species: N2, O2, N, O, NO, N2+, O2+, N+, O+, NO+, O2−, O− in their ground state and major electronic excited states and of electrons. Many elementary processes are considered given the temperatures involved (up to 10 000K). Time scales to reach the final nonequilibrium or equilibrium steady states are derived. Then we apply our model to two typical re-entry situations and show that O2− and O− play an important role during the ionization phase. Finally, a comparison with existing reduced kinetic mechanisms puts forward significant discrepancies for high velocity flows when the flow is in chemical nonequilibrium and smaller discrepancies when the flow is close to chemical equilibrium. This comparison illustrates the interest of using a ti...

Solvation Energies of the Proton in Methanol

pKas, proton affinities, and proton dissociation free energies characterize numerous properties of drugs and the antioxidant activity of some chemical compounds. Even with a higher computational level of theory, the uncertainty in the proton solvation free energy limits the accuracy of these parameters. We investigated the thermochemistry of the solvation of the proton in methanol within the cluster-continuum model. The scheme used involves up to nine explicit methanol molecules, using the IEF-PCM and the strategy based on thermodynamic cycles. All computations were performed at B3LYP/6-31++G(dp) and M062X/6-31++G(dp) levels of theory. It comes out from our calculations that the functional M062X is better than B3LYP, on the evaluation of gas phase clustering energies of protonated methanol clusters, per methanol stabilization of neutral methanol clusters and solvation energies of the proton in methanol. The solvation free energy and enthalpy of the proton in methanol were obtained after converging the partial solvation free energy of the proton in methanol and the clustering free energy of protonated methanol clusters, as the cluster size increases. Finally, the recommended values for the solvation free energy and enthalpy of the proton in methanol are -257 and -252 kcal/mol, respectively.

Reactive collisions between electrons and NO+ ions: rate coefficient computations and relevance for the air plasma kinetics

Extensive calculations of the rate coefficients for dissociative recombination (DR), elastic collisions, inelastic collisions (ICs) and superelastic collisions of NO+ ions on initial vibrational levels, , with electrons of energy between 10−5 and 10 eV have been performed, with a method based on multichannel quantum defect theory. Comparisons of the DR rate coefficients with the plasma experimental results give a good agreement, confirming that the vibrationally excited NO+ ions recombine more slowly than those in the ground state. Also, our ground state IC rate coefficients are very similar to previously computed R-matrix data. The rate coefficients have been fitted to a modified Arrhenius law, and the corresponding parameters are given, in order to facilitate the use of the reaction data in kinetical plasma modelling.

Structures of protonated methanol clusters and temperature effects

The accurate evaluation of pKas, or solvation energies of the proton in methanol at a given temperature is subject to the determination of the most favored structures of various isomers of protonated (H(+)(MeOH)n) and neutral ((MeOH)n) methanol clusters in the gas phase and in methanol at that temperature. Solvation energies of the proton in a given medium, at a given temperature may help in the determination of proton affinities and proton dissociation energies related to the deprotonation process in that medium and at that temperature. pKas are related to numerous properties of drugs. In this work, we were interested in the determination of the most favored structures of various isomers of protonated methanol clusters in the gas phase and in methanol, at a given temperature. For this aim, the M062X/6-31++G(d,p) and B3LYP/6-31++G(d,p) levels of theory were used to perform geometries optimizations and frequency calculations on various isomers of (H(+)(MeOH)n) in both phases. Thermal effects were retrieved using our homemade FORTRAN code. Thus, we accessed the relative populations of various isomers of protonated methanol clusters, in both phases for temperatures ranging from 0 to 400 K. As results, in the gas phase, linear structures are entropically more favorable at high temperatures, while more compact ones are energetically more favorable at lower temperatures. The trend is somewhat different when bulk effects are taken into account. At high temperatures, the linear structure only dominates the population for n ≤ 6, while it is dominated by the cyclic structure for larger cluster sizes. At lower temperatures, compact structures still dominate the population, but with an order different from the one established in the gas phase. Hence, temperature effects dominate solvent effects in small cluster sizes (n ≤ 6), while the reverse trend is noted for larger cluster sizes.

Ozone photolysis: Strong isotopologue/isotopomer selectivity in the stratosphere

Author Institution: CTMM Institut Charles Gerhardt UMR-CNRS 5253; University of Montpellier, France; Univ. Grenoble 1 / CNRS, LIPhy UMR 5588, Grenoble, F-38041, France; Department of Information Technology, University of Debrecen, P.O. Box 12, H-4010 Debrecen, Hungary

DFT study of the effect of solvent on the H-atom transfer involved in the scavenging of the free radicals ●HO2 and ●O2 − by caffeic acid phenethyl ester and some of its derivatives

AbstractH-atom transfer from caffeic acid phenethyl ester (CAPE), MBC (3-methyl-2-butenyl caffeate), BC (benzoic caffeate), P3HC (phenethyl-3-hydroxycinnamate), and P4HC (phenethyl-4-hydroxycinnamate) to the selected free radicals ●HO2 and ●O2− was studied. Such a transfer can proceed in three different ways: concerted proton-coupled electron transfer (CPCET), electron transfer followed by proton transfer (ET-PT), and proton transfer followed by electron transfer (PT-ET). The latter pathway is sometimes competitive with SPLET (sequential proton loss electron transfer) in polar media. Analyzing the thermodynamic descriptors of the reactions of CAPE and its derivatives with co-reactive species—in particular, the free energies of reactions, the activation barrier to the CPCET mechanism, and their rate constants—appears to be the most realistic method of investigating the H-atom transfers of interest. These analyses were performed via DFT calculations, which agree well with the data acquired from experimental studies (IC50) and from CBS calculations. The CPCM solvation model was used throughout the work, while the SMD model—employed as a reference—was used only for CAPE. The main conclusion drawn from the analysis was that SPLET is the mechanism that governs the reaction of phenolic acids with ●HO2, while PT-ET governs the reaction of phenols with ●O2−. In kinetic investigations of the CPCET process, the rate constant decreases as the solvent polarity increases, so the reaction velocity slows down. Graphical Abstract3D RPES of the reaction of CAPE with HO_2 radical (left), and free energies of the reaction of CAPE with O_2 radical in various media (right)

Electronic structure of a cylindrically confined hydrogen atom by the B-splines method: energy levels and dipole polarizability

The non-relativistic static and dynamic dipole polarizabilities of a hydrogen atom experiencing a cylindrical confinement are investigated. Two methods based on B-splines are used for computations of energies and wavefunctions. The first method is a variational-based method while the second one proceeds by a fit of the non-separable Coulomb potential in the product form. The computed energies compare very well with previous computations. They converge, as well as the dipole polarizability, to the exact unconfined free atom values with increasing basis size. The fit approach is found to be advantageous, as it helps to reduce the computational time without loss of accuracy.

State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions

We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and nonreactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2 = 1, 2), while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ toward the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2 = 2 might be significant in the photon-dominated region of NGC 7027.

Dissociative recombination and vibrational excitation of CO+: model calculations and comparison with experiment

The latest molecular data—potential energy curves and Rydberg/valence interactions—characterizing the super-excited electronic states of CO are reviewed, in order to provide inputs for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms for CO+ dissociative recombination are analyzed; its cross sections are computed using a method based on multichannel quantum defect theory. Convoluted cross sections, giving both isotropic and anisotropic Maxwellian rate coefficients, are compared with merged-beam and storage-ring xperimental results. The calculated cross sections underestimate the measured ones by a factor of two, but display a very similar resonant shape. These facts confirm the quality of our approach for the dynamics, and call for more accurate and more extensive molecular structure calculations. Keywords: dissociative recombination, electron impact vibrational excitation, vibrationally excited, multichannel quantum defect theory (Some figures may appear in colour only in the online journal)

Radiological Hazards in Soil from the Bauxite Deposits Sites in Dschang Region of Cameroon

This work evaluates the radiological health risk from NORM exposure in bauxite deposition sites of West Region in Cameroon. In-situ and laboratory measurements were performed using dose rate survey meter and Broad Energy Germanium (BEGe) detector. Radiometric analysis of 226Ra, 232Th and 40K in the soil samples from Fongo-Tongo and Mini-Matap were done with average activity concentration of 108.91 Bq/kg, 117.79 Bq/kg and 143.07 Bq/kg and, 113.15Bq/kg, 196.14 Bq/kg and zero were determined respectively. In-situ measurement of dose rate at 1 m above the ground and the annual effective dose values due to 226Ra, 232Th and 40K in 5 cm soil layer were determined using conversion factors by UNSCEAR. The average external hazard indexes in samples from Fongo-Tongo were 0.78 and 1.06 while the internal hazard indexes in samples from Mini-Matap were 1.07 and 1.37. Comparing these values with the worldwide values set by UNSCEAR we realized that avoidance of high exposure from gamma radiation due to NORM to the populace should be of concern.