Calculated Rate Coefficients Research Articles

Partial and total dielectronic recombination rate coefficients forW73+toW56+

Dielectronic recombination (DR) is a key atomic process which affects the spectroscopic diagnostic modelling of tungsten, most of whose ionization stages will be found somewhere in the ITER fusion reactor: in the edge, divertor, or core plasma. Accurate DR data is sparse while complete DR coverage is unsophisticated (e.g. average-atom or Burgess General Formula) as illustrated by the large uncertainties which currently exist in the tungsten ionization balance. To this end, we present a series of partial final state-resolved and total DR rate coefficients for W73+ to W56+ Tungsten ions. This is part of a wider effort within The Tungsten Project to calculate accurate dielectronic recombination rate coefficients for the tungsten isonuclear sequence for use in collisional-radiative modelling of finite-density tokamak plasmas. The recombination rate coefficients have been calculated with autostructure using kappa-averaged relativistic wavefunctions in level resolution (intermediate coupling) and configuration resolution (configuration average). The results are available from OPEN-ADAS according to the adf09 and adf48 standard formats. Comparison with previous calculations of total DR rate coefficients for W63+ and W56+ yield agreement to within 20% and 10%, respectively, at peak temperature. It is also seen that the Juttner correction to the Maxwell distribution has a significant effect on the ionization balance of tungsten at the highest charge states, changing both the peak abundance temperatures and the ionization fractions of several ions.

Testing the Palma-Clary Reduced Dimensionality Model Using Classical Mechanics on the CH4 + H → CH3 + H2 Reaction.

Application of exact quantum scattering methods in theoretical reaction dynamics of bimolecular reactions is limited by the complexity of the equations of nuclear motion to be solved. Simplification is often achieved by reducing the number of degrees of freedom to be explicitly handled by freezing the less important spectator modes. The reaction cross sections obtained in reduced-dimensionality (RD) quantum scattering methods can be used in the calculation of rate coefficients, but their physical meaning is limited. The accurate test of the performance of a reduced-dimensionality method would be a comparison of the RD cross sections with those obtained in accurate full-dimensional (FD) calculations, which is not feasible because of the lack of complete full-dimensional results. However, classical mechanics allows one to perform reaction dynamics calculations using both the RD and the FD model. In this paper, an RD versus FD comparison is made for the 8-dimensional Palma-Clary model on the example of four isotopologs of the CH4 + H → CH3 + H2 reaction, which has 12 internal dimensions. In the Palma-Clary model, the only restriction is that the methyl group is confined to maintain C3v symmetry. Both RD and FD opacity and excitation functions as well as differential cross sections were calculated using the quasiclassical trajectory method. The initial reactant separation has been handled according to our one-period averaging method [ Nagy et al. J. Chem. Phys. 2016, 144, 014104 ]. The RD and FD excitation functions were found to be close to each other for some isotopologs, but in general, the RD reactivity parameters are lower than the FD reactivity parameters beyond statistical error, and for one of the isotopologs, the deviation is significant. This indicates that the goodness of RD cross sections cannot be taken for granted.

Electron attachment to the interhalogen compounds ClF, ICl, and IBr

Thermal electron attachment rate coefficients for three interhalogen compounds (ClF, ICl, IBr) have been measured from 300 to 900 K at pressures of 1--2 Torr using a flowing afterglow--Langmuir probe apparatus. ClF attaches somewhat inefficiently ($k=7.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) at 300 K, with the rate coefficient rising to $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 700 K. At higher temperatures the apparent rate coefficient falls steeply; however, this is interpreted as an artifact due to decomposition on the walls of the inlet line. ICl attaches with even lower efficiency ($k=9.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 300 K) and a less steep increase with temperature. Attachment to IBr is too slow to confidently measure with the present experiment, with an upper limit on the rate coefficient of ${10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ from 300 to 600 K. Both ClF and ICl attach dissociatively to yield $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, likely exclusively, though ${\mathrm{F}}^{\ensuremath{-}}$ or ${\mathrm{I}}^{\ensuremath{-}}$ may be produced with limits of $l2%$ and $l5%$, respectively. The ClF attachment was further explored through ab initio calculation of the ClF and $\mathrm{Cl}{\mathrm{F}}^{\ensuremath{-}}$ potential energy curves and $R$-matrix calculations of the resonance parameters which were used then for calculations of the dissociative attachment cross sections and rate coefficients. While the magnitude of the attachment rate coefficient for ClF is similar to those for both $\mathrm{C}{\mathrm{l}}_{2}$ and ${\mathrm{F}}_{2}$, the calculated cross sections show qualitatively different threshold behavior due to the $s$-wave contribution allowed by the lack of inversion symmetry. The $v=1$ and 2 vibrational modes of ClF attach about three to four times faster than $v=0$ and 3 at energies lower than $\ensuremath{\sim}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The calculated rate coefficients are in good agreement with the experiment at 300 K and increase moderately less steeply with temperature.

Experimental and Theoretical Study of the OH-Initiated Photo-oxidation of Formamide.

The kinetics of OH radical reaction with formamide was studied by the relative rate method employing proton transfer reaction-mass spectrometry detection at the European Photochemical Reactor in Valencia, Spain. The rate coefficient was determined to be (4.5 ± 0.4) × 10(-12) cm(3) molecule(-1) s(-1) at 309 ± 3 K and 1013 ± 1 hPa. Isocyanic acid was observed as the sole product. The experimental results are supported by quantum chemical calculations and kinetic simulations using a master equation model. The calculated rate coefficient is independent of pressure at tropospheric conditions and can be accurately described by an Arrhenius expression having negative activation energy. The reaction is predicted to proceed exclusively via C-H abstraction.

Intermolecular electron energy transfer processes in the collisions of N2(A3Σu+, v = 0–10) with CO and N2 molecules

The removal rates of metastable molecular nitrogen N2(A3Σu+, v=0–10) in the collisions with the ground state CO(X1Σ+, v=0–3) and N2(X1Σg+, v=0–3) molecules are calculated according to analytical expressions. The study includes a consideration of intermolecular electron energy transfers with the electronic excitation of target molecules. Reasonable agreement of the calculated rate coefficients with data of experimental measurements is obtained.

Collisional excitation of NH(X(3)Σ(-)) by Ne: Potential energy surface, scattering calculations, and comparison with experiments.

We present a new three-dimensional potential energy surface (PES) for the NH(X(3)Σ(-))-Ne van der Waals system, which explicitly takes into account the NH vibrational motion. Ab initio calculations of the NH-Ne PES were carried out using the open-shell single- and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations [RCCSD(T)]. The augmented correlation-consistent quadruple zeta (aug-cc-pVQZ) basis set was employed. Mid-bond functions were also included in order to improve the accuracy in the van der Waals well. Using this new PES, we have studied the collisional excitation of NH(X(3)Σ(-)) by Ne. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of NH by Ne are performed for energies up to 3000 cm(-1), which yield, after thermal average, rate coefficients up to 350 K. The propensity rules between fine-structure levels are reported, and it is found that F-conserving cross sections are larger than F-changing cross sections even if the propensity rules are not as strong as for the NH-He system. The calculated rate coefficients are compared with available experimental measurements at room temperature and a fairly good agreement is found between experimental and theoretical data, confirming the good quality of the scattering calculations and also the accuracy of the potential energy surface used in this work.

QUANTUM CALCULATION OF INELASTIC CO COLLISIONS WITH H. III. RATE COEFFICIENTS FOR RO-VIBRATIONAL TRANSITIONS

We present calculated rate coefficients for ro-vibrational transitions of CO in collisions with H atoms for a gas temperature range of 10 K $\leq T \leq$ 3000 K, based on the recent three-dimensional ab initio H-CO interaction potential of Song et al(2013). Rate coefficients for ro-vibrational $v=1,j=0-30 \rightarrow v'=0, j'$ transitions were obtained from scattering cross sections previously computed with the close-coupling method by Song et al(2015). Combining these with the rate coefficients for vibrational $v=1-5 \rightarrow v' < v$ quenching obtained with the infinite-order sudden approximation, we propose a new extrapolation scheme that yields the rate coefficients for ro-vibrational $v=2-5,j=0-30 \rightarrow v',j'$ de-excitation. Cross sections and rate coefficients for ro-vibrational $v=2, j=0-30 \rightarrow v'=1,j'$ transitions calculated with the close-coupling method confirm the effectiveness of this extrapolation scheme. Our calculated and extrapolated rates are very different from those that have been adopted in the modeling of many astrophysical environments. The current work provides the most comprehensive and accurate set of ro-vibrational de-excitation rate coefficients for the astrophysical modeling of the H-CO collision system. Application of the previously available and new data sets in astrophysical slab models shows that the line fluxes typically change by 20-70% in high temperature environments (800 K) with an H/H$_2$ ratio of 1; larger changes occur for lower temperatures.

Dynamics and Kinetics Study of "In-Water" Chemical Reactions by Enhanced Sampling of Reactive Trajectories.

High potential energy barriers and engagement of solvent coordinates set challenges for in silico studies of chemical reactions, and one is quite commonly limited to study reactions along predefined reaction coordinate(s). A systematic protocol, QM/MM MD simulations using enhanced sampling of reactive trajectories (ESoRT), is established to quantitatively study chemical transitions in complex systems. A number of trajectories for Claisen rearrangement in water and toluene were collected and analyzed, respectively. Evidence was found that the bond making and breaking during this reaction are concerted processes in solutions, preferentially through a chairlike configuration. Water plays an important dynamic role that helps stabilize the transition sate, and the dipole-dipole interaction between water and the solute also lowers the transition barrier. The calculated rate coefficient is consistent with the experimental measurement. Compared with water, the reaction pathway in toluene is "narrower" and the reaction rate is slower by almost three orders of magnitude due to the absence of proper interactions to stabilize the transition state. This study suggests that the "in-water" nature of the Claisen rearrangement in aqueous solution influences its thermodynamics, kinetics, as well as dynamics.

Reactions between neutral molecules and cation-radicals in the gas-phase: Can protonation occur without proton transfer?

Gas-phase collisions of Gly and Ala with free electrons were employed to form positive ions at well determined energies. At elevated molecular vapor pressure, protonated [Gly+H]+ and [Ala+H]+ ions were also observed. The appearance energies of [Gly+H]+ and [Ala+H]+ were both determined as 0.30±0.14eV higher than the ionization energies of their M+ congeners. Quantum chemical calculations predict proton transfer reactions rather than hydrogen atom abstraction reactions as the likely [M+H]+ forming method in our experiment. Microcanonical rate coefficient calculations using B3LYP/6-311++G(2d,p) frequencies and rotational constants, support abstraction of a Calpha proton for glycine, whereas the C-terminal proton abstraction mechanism was found as being more favorable for alanine. Additionally, substantial immonium ion formation was observed at experimental energies identical to either the protonated product or cation radical for glycine and alanine, respectively. Theoretical barriers to M+ cation radical fragmentation to HRCNH2+ indicate that they are product-limited and that the glycine barriers are generally larger than those for alanine.

Low temperature rate coefficients of the H + CH+ → C+ + H2 reaction: New potential energy surface and time-independent quantum scattering

The observed abundances of the methylidyne cation, CH(+), in diffuse molecular clouds can be two orders of magnitude higher than the prediction of the standard gas-phase models which, in turn, predict rather well the abundances of neutral CH. It is therefore necessary to investigate all the possible formation and destruction processes of CH(+) in the interstellar medium with the most abundant species H, H2, and e(-). In this work, we address the destruction process of CH(+) by hydrogen abstraction. We report a new calculation of the low temperature rate coefficients for the abstraction reaction, using accurate time-independent quantum scattering and a new high-level ab initio global potential energy surface including a realistic model of the long-range interaction between the reactants H and CH(+). The calculated thermal rate coefficient is in good agreement with the experimental data in the range 50 K-800 K. However, at lower temperatures, the experimental rate coefficient takes exceedingly small values which are not reproduced by the calculated rate coefficient. Instead, the latter rate coefficient is close to the one given by the Langevin capture model, as expected for a reaction involving an ion and a neutral species. Several recent theoretical works have reported a seemingly good agreement with the experiment below 50 K, but an analysis of these works show that they are based on potential energy surfaces with incorrect long-range behavior. The experimental results were explained by a loss of reactivity of the lowest rotational states of the reactant; however, the quantum scattering calculations show the opposite, namely, a reactivity enhancement with rotational excitation.

Theoretical study on the nitrate radical oxidation of methyl vinyl ether

This study investigates the nitrate radical oxidation of methyl vinyl ether (CH2CHOCH3, MVE) by quantum chemical methods. The oxidation initiates with the formation of open-cyclic NO3-adducts, followed by a series of unimolecular decomposition or bimolecular reactions. Kinetic data suggest that pressure and temperature have strong influence on the environmental fate of MVE. Under atmospheric condition, methyl formate, formaldehyde and NO2 are the main products by association with O2/NO. At low pressure (<50Torr) 2-methoxyoxirane and NO2 are main products by decomposition. Total rate coefficient of the bimolecular reaction exhibits negative dependence on temperature (200–300K) and positive dependence on pressure (1–7600Torr). The calculated rate coefficient (7.09×10−13cm3molecule−1s−1) is in well agreement with experimental data at 293K and 760Torr.

Study of Ionization Rates of Helium Maxellian Plasma

We have contributed to the study and calculation of ionization rate coefficients from the cross sections of neutral helium.Numerical methods and FAC code are used in this work. Our calculations are comparable to published results.

Computational Study and Kinetic Analysis of the Aminolysis of Thiolactones.

The aminolysis of three differently α-substituted γ-thiolactones (C4H5OSX, X = H, NH2, and NH(CO)CH3) is modeled based on CBS-QB3 calculated free energies corrected for solvation using COSMO-RS. For the first time, quantitative kinetic and thermodynamic data are provided for the concerted path and the stepwise path over a neutral tetrahedral intermediate. These paths can take place via an unassisted, an amine-assisted, or a thiol-assisted mechanism. Amine assistance lowers the free energy barriers along both paths, while thiol assistance only lowers the formation of the neutral tetrahedral intermediate. Based on the ab initio calculated rate coefficients, a kinetic model is constructed that is able to reliably describe experimental observations for the aminolysis of N-acetyl-dl-homocysteine thiolactone with n-butylamine in THF and CHCl3. Reaction path analysis shows that for all conditions relevant for applications in polymer synthesis and postpolymer modification, an assisted stepwise mechanism is operative in which the formation of the neutral tetrahedral intermediate is rate-determining and which is mainly amine-assisted at low conversions and thiol-assisted at high conversions.

Hyperfine transitions of13CN from pre-protostellar sources

Recent quantum mechanical calculations of rate coefficients for collisional transfer of population between the hyperfine states of 13CN enable their population densities to be determined. We have computed the relative populations of the hyperfine states of the N = 0, 1, 2 rotational states for kinetic temperatures 5 $\le$ T $\le$ 20 K and molecular hydrogen densities 1 $\le$ n(H2) $\le$10 10 cm --3. Spontaneous and induced radiative transitions were taken into account. Our calculations show that, if the lines are optically thin, the populations of the hyperfine states, F, within a given rotational manifold are proportional to their statistical weights, (2F + 1) -- i.e. in local thermodynamic equilibrium -- over the entire range of densities. We have re-analysed IRAM 30 m telescope observations of 13CN hyperfine transitions (N = 1 $\rightarrow$ 0) in four starless cores. A comparison of these observations with our calculations confirms that the hyperfine states are statistically populated in these sources.

Quantum chemistry and nuclear dynamics as diagnostic tools for stellar atmosphere modeling

We report full quantum scattering calculations of inelastic cross sections and rate coefficients for Mg in collision with H atoms. Transitions until the Mg(3s3d) 3D level were considered, as well as mutual neutralization and ion-pair production. These calculations are based on new ab initio potentials and nonadiabatic couplings for the nine 2Σ+ and five 2Π lowest electronic states of MgH. The collision energy range is from threshold up to 10eV. The main mechanisms underlying the scattering processes are discussed. An extension of these calculations is performed by means of a multichannel model approach. Rate coefficients are thus obtained for all processes between the 12 lowest atomic states of Mg in collisions with H, plus the ionic channel. Those data permit to model spectral lines in stellar atmospheres. Their impact on statistical equilibrium of Mg populations through the non-LTE departure coefficients is investigated for the Sun and the HD122563 metal-poor star.

Reanalysis of Rate Data for the Reaction CH3 + CH3 → C2H6 Using Revised Cross Sections and a Linearized Second-Order Master Equation.

Rate coefficients for the CH3 + CH3 reaction, over the temperature range 300-900 K, have been corrected for errors in the absorption coefficients used in the original publication ( Slagle et al., J. Phys. Chem. 1988 , 92 , 2455 - 2462 ). These corrections necessitated the development of a detailed model of the B̃(2)A1' (3s)-X̃(2)A2″ transition in CH3 and its validation against both low temperature and high temperature experimental absorption cross sections. A master equation (ME) model was developed, using a local linearization of the second-order decay, which allows the use of standard matrix diagonalization methods for the determination of the rate coefficients for CH3 + CH3. The ME model utilized inverse Laplace transformation to link the microcanonical rate constants for dissociation of C2H6 to the limiting high pressure rate coefficient for association, k∞(T); it was used to fit the experimental rate coefficients using the Levenberg-Marquardt algorithm to minimize χ(2) calculated from the differences between experimental and calculated rate coefficients. Parameters for both k∞(T) and for energy transfer ⟨ΔE⟩down(T) were varied and optimized in the fitting procedure. A wide range of experimental data were fitted, covering the temperature range 300-2000 K. A high pressure limit of k∞(T) = 5.76 × 10(-11)(T/298 K)(-0.34) cm(3) molecule(-1) s(-1) was obtained, which agrees well with the best available theoretical expression.

Cyanides/isocyanides abundances in the interstellar medium – IV. Temperature dependence of SiCN/SiNC rate coefficients and astrophysical applications

Accurate determination of collisional rate coefficients is an essential step in the estimation of the SiCN and SiNC abundances in the interstellar and circumstellar media. In this paper, we carry out calculations of rate coefficients for the rotational (de-)excitation of SiCN and SiNC molecules in collision with He. The calculations are based on new two-dimensional potential energy surfaces obtained from highly correlated ab initio calculations. Coupled-States quantum approximation was used in the scattering calculations to obtain collisional (de-)excitation cross-sections of SiCN and SiNC by He. The spin–orbit coupling and Λ-doublet splitting of SiCN and SiNC levels were taken into account explicitly. Rate coefficients for transitions among the first 92 rotational levels of SiCN and SiNC were calculated for temperatures ranging from 5 to 100 K. Moderate differences exist between the rate coefficients of both isomers. Subsequently, the new collisional data are used to simulate the excitation of SiCN and SiNC in the circumstellar gas. We obtain the brightness and excitation temperatures of selected lines frequently observed towards the circumstellar envelopes and we find that local thermodynamic equilibrium conditions are not fulfilled for these species. Radiative transfer calculations are then needed in order to accurately determine their abundances. Our results also show that previous estimations of the cyanides/isocyanides abundance ratios were incorrect and the present calculations show that SiCN, the most stable isomer, is more abundant than SiNC. This shows again the evidence of selective cyanide chemistry.

Collisional quenching of highly rotationally excited HF

Collisional excitation rate coefficients play an important role in the dynamics of energy transfer in the interstellar medium. In particular, accurate rotational excitation rates are needed to interpret microwave and infrared observations of the interstellar gas for nonlocal thermodynamic equilibrium line formation. Theoretical cross sections and rate coefficients for collisional deexcitation of rotationally excited HF in the vibrational ground state are reported. The quantum-mechanical close-coupling approach implemented in the nonreactive scattering code MOLSCAT was applied in the cross section and rate coefficient calculations on an accurate 2D HF-He potential energy surface. Estimates of rate coefficients for H and H$_2$ colliders were obtained from the HF-He collisional data with a reduced-potential scaling approach. The calculation of state-to-state rotational quenching cross sections for HF due to He with initial rotational levels up to $j=20$ were performed for kinetic energies from 10$^{-5}$ to 15000 cm$^{-1}$. State-to-state rate coefficients for temperatures between 0.1 and 3000 K are also presented. The comparison of the present results with previous work for lowly-excited rotational levels reveals significant differences. In estimating HF-H$_2$ rate coefficients, the reduced-potential method is found to be more reliable than the standard reduced-mass approach. The current state-to-state rate coefficient calculations are the most comprehensive to date for HF-He collisions. We attribute the differences between previously reported and our results to differences in the adopted interaction potential energy surfaces. The new He rate coefficients can be used in a variety of applications. The estimated H$_2$ and H collision rates can also augment the smaller datasets previously developed for H$_2$ and electrons.

Rotational excitation induced by collision of AlOH with helium

The present paper deals with the calculation of rate coefficients for the 16 first rotational levels of the AlOH molecule in its ground vibrational state, induced by collision with helium. Such data are useful for studies of low-temperature interstellar environments. A new potential energy surface (PES) for the AlOH-He system, calculated at a AlO and OH r-distance frozen at their experimental equilibrium distances (r 1[Al-O]=3.1796a 0 and r 2[O-H]=1.6597a 0) was obtained by using the CCSD(T) ab initio method associated with the basis sets aVQZ, completed with bond functions placed at mid-distance between the center of mass of AlOH and its projectile. Dynamical calculations of pure rotational (de)excitation of AlOH-He were performed within the close-coupling method and the collisional cross-sections for the 16 first rotational levels of AlOH were calculated for the kinetic energies up to 1500 cm−1. State-to-state rate coefficients were computed for the temperatures ranging from 3 K up to 170 K. We found that the ΔJ=1 transitions are dominant and a propensity rule that favors odd ΔJ transitions is observed.

Oxidation of Graphene-Edge Six- and Five-Member Rings by Molecular Oxygen

To gain qualitative and quantitative understanding of oxidation processes of large polycyclic aromatics, soot particles, and graphene edges, a theoretical study is reported for the pyrenyl-O2 reaction system. First, possible reaction pathways and their energetics were investigated using high-level ab initio calculations. The results were utilized in RRKM-master equation calculations of rate coefficients and relative product yields at temperatures and pressures relevant to combustion. Finally, the deduced oxidation mechanisms of six- and five-member rings and the computed rate coefficients were employed in kinetic Monte Carlo simulations of oxidation of a graphene "molecule" evolving in flame-like environments. Among the major findings from the latter simulations are the following: The oxidation system exhibits two basic pathways, thermal decomposition and regeneration of oxyradicals. Their competition is temperature-dependent, with the former dominating at higher and the latter at lower temperatures. The overall oxidation of the graphene substrate is computed to be time-dependent, with the initial rates consistent with the known experimental data.