Partial Rate Coefficients Research Articles

Inelastic Processes in Strontium-Hydrogen Collisions and Their Impact on Non-LTE Calculations

Inelastic processes rate coefficients for low-energy Sr + H, Sr+ + H−, Sr+ + H, and Sr2+ + H− collisions are calculated using the multichannel quantum model approach. A total of 31 scattering channels of SrH+ and 17 scattering channels of SrH are considered. The partial cross sections and the partial rate coefficients are hence calculated for 1202 partial processes in total. Using new quantum data for Sr ii + H i collisions, we updated the model atom of Sr ii and performed the non-local thermodynamic equilibrium (non-LTE) calculations. We provide the non-LTE abundance corrections for the Sr ii resonance lines in two grids of model atmospheres, which are applicable to very metal-poor ([Fe/H] ≤−2) dwarfs and giants.

Estimation of rate coefficients and branching ratios for gas-phase reactions of OH with aromatic organic compounds for use in automated mechanism construction

Abstract. Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for the reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving estimation of atmospheric lifetimes or oxidation rates for VOCs. A structure–activity relationship (SAR) method is presented for the reactions of OH with aromatic organic compounds, with the reactions of aliphatic organic compounds considered in the preceding companion paper. The SAR is optimized using a preferred set of data including reactions of OH with 67 monocyclic aromatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The SAR can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the reactions of the product radicals under tropospheric conditions are also summarized, specifically the rapid reaction sequences initiated by their reactions with O2.

Estimation of rate coefficients and branching ratios for gas-phase reactions of OH with aliphatic organic compounds for use in automated mechanism construction

Abstract. Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving the estimation of atmospheric lifetimes or oxidation rates for VOCs. Updated and extended structure–activity relationship (SAR) methods are presented for the reactions of OH with aliphatic organic compounds, with the reactions of aromatic organic compounds considered in a companion paper. The methods are optimized using a preferred set of data including reactions of OH with 489 aliphatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The information can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the subsequent reactions of the product radicals under tropospheric conditions are also summarized, specifically their reactions with O2 and competing processes.

Total dielectronic recombination rate coefficient for Co-like tungsten

Ab initio calculation of the total dielectronic recombination (DR) rate coefficient from the ground state 3 s 23 p 63 d 9( J=5/2) of Co-like tungsten is performed employing the relativistic distorted-wave approximation with configuration-interaction. The DR contributions mainly come from complex series 3 d 84 ln′ l′. The complex series 3 p 53 d 10 n′ l′, 3 p 53 d 94 ln′ l′ and 3 d 85 ln′ l′ also contribute significantly to the total DR rates at relatively high electron temperatures. The l′ and n′ dependences of the partial rate coefficient are investigated. The inclusion of decays into autoionizing levels followed by radiative cascades (DAC) enlarges the total DR rate coefficients by a factor of about 10%. The level-by-level extrapolation method is developed to include DAC effects. The total DR rate coefficients are fitted to an empirical formula. It is shown that at temperatures above 2.5 keV the Burgess–Merts (BM) semiempirical formula can provide DR results with an accuracy of about 15%, whereas at electron temperatures below 100 eV it underestimates the DR rate coefficients by up to a few orders of magnitude and its temperature dependence is completely inadequate. The comparison of the results for Ni-like and Co-like tungsten shows that these two sets of DR rate coefficients are very close in magnitude at relatively high electron temperatures.

Dielectronic recombination of Co-like gold ions

Ab initio calculation of the total dielectronic recombination (DR) rate coefficient from the ground and the first excited states of Co-like gold is performed employing the relativistic distorted-wave approximation with configuration interaction. The DR contributions are explicitly taken into account from the relevant complexes of a Ni-like ion: 3d33/2 3d65/2n′l′, 3p5 3d10n′l′, 3s 3p63d10n′l′, 3d8 4ln′l′, 3p5 3d9 4ln′l′ and 3s 3p6 3d9 4ln′l′ with n′ ⩽ 25, and 3d8 5ln′l′ with n′ ⩽ 9. The contributions from a higher n′ complex are evaluated by an extrapolation procedure. The DR contributions mainly come from complex series 3d8 4ln′l′ and 3p5 3d9 4ln′l′. The complex series 3p5 3d10n′l′ and 3d8 5ln′l′ also contribute significantly to the total DR rates at low and high electron temperatures, respectively. The l′ and n′ dependences of the partial rate coefficient are investigated. The possible important decays into autoionizing levels followed by radiative cascade (DAC) from the resonant levels are taken into account, as well as the resonant stabilizing and non-resonant stabilizing transitions. The inclusion of DAC transitions enlarges the total DR rate coefficients by a factor of about 10% and may break down the usual n′−3 scaling law of the partial DR rates along some complex series. To evaluate the high n′ contributions from these complex series, the level-by-level extrapolation method is developed to include DAC effects. The total DR rate coefficients are fitted to an empirical formula. The present results are compared with those from the semiempirical Burgess–Merts approximation. The DR rate coefficients of Ni-like gold are also presented and compared to those of Co-like gold. In addition, some comments on the published DR data for the NiI isoelectronic sequence are drawn from the present calculation.

Homogeneous, unimolecular gas‐phase elimination kinetics of ethyl esters of glyoxylic, 2‐oxo‐propanoic, and 3‐methyl‐2‐oxo‐butanoic acids

AbstractThe rates of elimination of several ethyl esters of 2‐oxo‐carboxylic acid were determined in a seasoned static reaction vessel over the temperature range 350–430°C and pressure range 33–240 Torr. The reactions, in the presence of a free‐radical inhibitor, are homogeneous, unimolecular, and follow a first‐order rate law. The overall and partial rate coefficients are expressed by the Arrhenius equation.Ethyl glyoxalate equation image Ethyl 2‐oxo‐propionate equation image Ethyl 3‐methyl‐2‐oxo‐butyrate equation image The mechanisms of these elimination reactions are described in terms of concerted cyclic transition state structures. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 268–275, 2007

Vibrational dependence in the dissociative recombination of O2+

We report data on a study into the vibrational dependence in the dissociative recombination of O+2 (X2Πg). The experiment has been performed with a merged-beam apparatus in the heavy-ion storage ring, CRYRING. We present the total rate coefficients as function of collision energy for five different vibrational populations of the ion beam and the partial rate coefficients and branching fractions near 0 eV collision energy for the vibrational levels v = 0, 1 and 2. Furthermore, we report on the control over and characterization of vibrational populations. The total rate coefficient is found to be weakly dependent on the vibrational excitation. The rate decreases upon increasing vibrational excitation, though only within an order of magnitude. The partial rate coefficients are strongly dependent on the vibrational level; with the v = 0 oxygen ions recombining the fastest. The partial branching fractions are also found to be strongly dependent on the vibrational level, specifically the O(1S) quantum yield which increases strongly upon increasing vibrational level.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

On the role of s-cis conformers in the reaction of dienes with OH radicals

The reactions of OH radicals with s-cis and s-trans-butadiene and s-cis-isoprene have been modeled by ab initio Molecular Orbital Theory. Density Functional Theory (BHandHLYP) calculations have been performed for both butadiene and isoprene, and Moller–Plesset Perturbation Theory to the second-order (MP2) has also been used for s-cis-isoprene in order to compare with previous work. Pre-reactive complexes are identified in all cases, with the OH radical placed over either one of the double bonds at a distance of about 2 A and the H atom pointing towards the C–C bond. The geometries of the transition states corresponding to OH addition at all positions have been fully optimized. The calculated apparent activation energies are negative for addition at the terminal carbon atoms and in excellent agreement with the experimental measurements. The possible role of direct additions at the internal carbon atoms in the formation of furan-like compounds is discussed. Energy barriers for the s-cis conformers are found to be smaller than those for the s-trans conformers, especially for addition at the internal carbons, suggesting that the s-cis conformers could play a role in the tropospheric oxidation of dienes. Calculated overall rate constants are in good agreement with experimental values. Partial rate coefficients corresponding to the different channels are reported. The temperature dependence is studied in the 290–500 K range and two-parameter equations are reported for each rate coefficient. The calculated partial rate coefficients of addition to internal carbon atoms are not large enough to account for the observed yield of 3-methylfuran.

Specific and non‐specific solvent effects on aromatic nucleophilic substitution. Kinetics of the reaction of 1‐fluoro‐2,6‐dinitrobenzene and homopiperidine in binary solvent mixtures

AbstractThis paper presents a comparative study of the solvent effects on the kinetics of the reactions between 1‐fluoro‐2,6‐dinitrobenzene and hexahydro‐1H‐azepine in ethyl acetate + chloroform or acetonitrile and acetonitrile + chloroform binary solvent mixtures. The purpose was to asses the contribution of each molecular‐microscopic solvent property to the overall solvation effect of the reaction critical state. The influence of the dipolarity/polarizability, hydrogen‐bond donor acidity and hydrogen‐bond acceptor basicity of the solvent mixtures on the reaction rate was analyzed by the application of the multiparametric approach of Kamlet, Abboud and Taft. The evaluation of the correlation coefficients shows that the solvation effects are dominated by the non‐specific interactions. Moreover, the incidence of the hydrogen‐bond acceptor solvent ability is more important than that corresponding to the hydrogen‐bond donor character, except for the reactions that manifest kinetic retarding effects. The influence of the solvation effects on the base catalysis was analyzed, describing the response patterns of the partial rate coefficients to the mixture composition and also to each solvent property. The application of a preferential solvation model to the kinetic results yields information on the preferential solvation of the reaction critical state by any of the pure solvents, or by the hydrogen‐bonded complexed media. Copyright © 2002 John Wiley & Sons, Ltd.

Product ion distributions and rate coefficients for the attachment reactions of electrons with CHCl 2Br, CCl 2Br 2, and CHClBr 2

Attachment rate coefficients, β, and product ion distributions, R, have been determined for the chlorobromomethanes CHCl 2Br, CCl 2Br 2, and CHClBr 2 using the flowing afterglow/Langmuir probe (FALP) method. Thus β and R have been determined at two attaching gas temperatures, T g , of 300 K and 540 K and at electron temperatures, T e , ranging from T g to about 5000 K. The results show that T g rather than T e has the stronger influence on both β and R and that Br − ions are the major products of these three reactions. Both β and Cl − increase markedly with T g , but the variations of β and R with T e , although not so straightforward, are nevertheless well defined. Thus, it is apparent that dissociation to reaction products of the nascent negative ions is efficiently promoted by preheating the attaching molecules (increasing T g ), and to a lesser extent by increasing T e , which is generally in accordance with a model we previously developed to describe other haloalkane attachment reactions. Electron capture by CHCl 2Br and CCl 2Br 2 vary with T e as predicted by capture rate theory, whereas autodetachment apparently seriously decreases the effective capture rate by CHClBr 2. By combining β and R values for these reactions under truly thermalised conditions, the partial rate coefficients for the Br − and Cl − reaction channels have been derived and from their variation with T g , Arrhenius activation energies for Br − and Cl − production have been estimated. Small fractions of dihalogen negative ions are produced in these attachment reactions with ClBr − ions representing a significant fraction of the product ion distribution for the CHClBr 2 reaction.

REACTIONS OF LITHIUM WITH BENZALDEHYDE IN SOLUTION

Benzaldehyde in THF reacts with lithium metal to give the expected reduction product, benzyl alcohol, plus benzoin and benzyl as minor products. The kinetics of the overall reaction as well as the partial rate coefficients of the several steps have been determined under various reaction conditions. It was found that adsorption on the surface and electron transfer from the lithium to benzaldehyde are the slow steps. The experimental results show interesting surface effects, and evidence for significant radical intermediates, which were characterized by their epr spectra and trapping experiments, results useful to gain insight into the mechanisms of these and other related reactions. © 1997 John Wiley & Sons, Ltd.

Conversion of flavanones into chalcones in alkaline medium. Kinetic and spectroscopic studies

AbstractThe conversion of flavanone and 7‐hydroxyflavanone in alkaline water and heavy water and of the same compounds and of 4′ ‐nitroflavanone in alkali‐methanolic media into the corresponding substituted chalcones was studied kinetically and spectroscopically. Treatment of kinetic data in this work and data for the reverse reaction determined previously allowed the estimation of the partial rate coefficients for each step and of the free‐energy changes for the three systems studied. To disentangle isotope effects, the conversion of [3‐D2]‐flavanone was also studied. The present results confirm a previously suggested mechanism for the spontaneous reaction and afford essential information that may contribute to a more detailed understanding of the mechanism of the enzyme‐catalysed reaction.

Theoretical partial dielectronic recombination rate coefficients for ground-state helium-like argon

Theoretical dielectronic recombination rate coefficients are calculated for ground-state helium-like argon. Partial dielectronic recombination rate coefficients are obtained by summing over the individual lines within the five transition arrays. 1s2 lnl′ → 1 s 2 nl′ for n = 2, 3 and 4 and 1s2 lnl′ → 1s 22 l for n = 3 and 4. The partial rate coefficients are presented for electron temperatures corresponding to values of kT e ranging from 0.4 to 10 keV.

The product channels in the quenching of O(1S) by O2(a 1Δg)

The discovery that quenching of O(1S) by O2(a 1Δg) is a very rapid process, and thus significant in both atmospheric and afterglow systems, makes it important to determine the products of this reaction. Using chemiluminescence and resonance fluorescence techniques, it has been found that 17% of the interaction proceeds through an O(1D)+O2(b 1Σ+g) product channel, whereas 19% results in fragmentation, yielding three ground state atoms. The remainder gives a ground state atom and presumably a triplet O2, most probably in either the A 3Σ+u or A′ 3Δu state. Through detailed balancing, the reverse of the fragmentation process leads to a partial rate coefficient of 1×10−37 cm6 molecule−2 s−1 for the Chapman mechanism for generating thermospheric O(1S).

Excitation of oxygen permitted line emissions in the tropical nightglow

The ultraviolet oxygen emissions at 1304 and 1356 A in the tropical nightglow seen from Ogo 4 by Hicks and Chubb (1970) and Barth and Schaffner (1970) are accompanied by emission at 7774 and 4368 A, which have been studied from the ground by looking through the beam of an ionosonde operated under the Appleton anomaly ionization peaks. Simultaneous forbidden O I 6300-A measurements were also made. A theoretical value for the partial rate coefficient for 7774 emission by radiative recombination has been obtained, and from the ionospheric data and a model atmosphere the expected rates of radiative recombination and ion-ion recombination were calculated. The time variations and absolute intensity of the calculated and observed intensities agree reasonably well, when the uncertainties involved are considered. It is concluded that radiative recombination is the major source of the tropical oxygen permitted line emissions, accompanied by a small contribution from ion-ion recombination.