Theoretical Rate Coefficients Research Articles

Breit interaction in dielectronic recombination of hydrogenlike xenon ions: storage-ring experiment and theory

Abstract Electron-ion collision spectroscopy of the KLL dielectronic recombination (DR) resonances of hydrogenlike xenon ions was performed at a heavy-ion storage ring with a resolving power that is competitive with x-ray spectroscopy of inner-shell transitions in highly charged ions. The $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 , $$KL_{1/2}L_{3/2}$$ K L 1 / 2 L 3 / 2 , and $$KL_{3/2}L_{3/2}$$ K L 3 / 2 L 3 / 2 resonance groups and even parts of their fine structure are individually resolved. The resonance strengths were measured on an absolute scale and compared with results from multi-configuration Dirac–Fock (MCDF) calculations. These are in excellent agreement with the experimental findings when QED effects on the resonance energies and the Breit interaction are considered. As already found for DR of hydrogenlike uranium (Bernhardt et al. in Phys Rev A 83:020701(R), 2011), this interaction is particularly strong for the $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 resonance group. For U$$^{91+}$$ 91 + , it increases the $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 DR resonance strength by 40%. For Xe$$^{53+}$$ 53 + , the increase is found to amount to 25%, confirming the prediction that the influence of the Breit interaction grows with increasing nuclear charge. A comprehensive appendix treats the derivation of experimental and theoretical merged-beams recombination rate coefficients for interacting beams of relativistic electrons and ions. Graphical abstract

Unexpected high yield of acrolein underlies the importance of the hydrogen-abstraction mechanism in photooxidation of allyl methyl sulfide (AMS).

This work explores theoretically the gas phase oxidation of allyl methyl sulfide (AMS, H2CCHCH2SCH3) initiated by •OH radicals, focusing on the H-abstraction pathway at the M06-2X-D3/aug-cc-pVTZ and MN15/aug-cc-pVTZ levels of theory (m06Tz and mn15Tz). The formation of a prereactive complex (PRC) is involved in H-abstraction processes with two potential directions of approach for the OH radical, denoted as “α” and “β”. The PRCs, demonstrate increased reactivity, primarily due to the interaction between the sulfur atoms and the hydroxyl hydrogen. A scheme for the H-abstraction mechanism that supports the experimentally identified products and predicts the formation of some S-containing low volatility products is proposed. The comparison of the potential energy surface (PES) between the double bond addition and H-abstraction paths in the AMS molecule shows that at the m06Tz level of theory, the H-abstraction on C3 and the addition to C1 have nearly the same profile of energy, while at the mn15Tz level, the minimum energy channel is the addition to C1. The theoretical rate coefficient for each reaction channel was calculated, considering the formation of a PRC prior to reaching the transition state of each channel and assuming thermal equilibrium between reactants and the PRC. The rate constants were calculated in a multi-TS/multi-conformer way at the SVECV-f12/m06Tz and SVECV-f12/mn15Tz levels of theory. The SVECV-f12 method is consistent in its predictions in both systems and exhibits only minor deviations from the experimental rate constants. Despite some specific differences due to the DFT method supporting the SVECV-f12 calculations, both methodologies predict a significant H-abstraction contribution in the AMS + OH gas phase reaction, which explains the high formation yield for acrolein determined experimentally.

The temperature variation of the CH+ + H reaction rate coefficients: a puzzle finally understood?

CH+ was the first molecular ion identified in the interstellar medium and is found to be ubiquitous in interstellar clouds. However, its formation and destruction paths are not well understood, especially at low temperatures. A new theoretical approach based on the canonical variational transition state theory was used to study the H + CH+ reactive collisions. Rate coefficients for formation of C+ ions are calculated as a function of temperature. We considered the participation of a direct path and an indirect path in which the reactants should overcome an entropic barrier to form a van der Waals complex or pass through a CH2+ intermediate complex, respectively. We show that the contribution of both pathways to the formation of C+ has to be taken into account. The new reactive rate coefficients for the title reaction, complemented by reactive data for CH+/CH2+ in the H/H2/He mixture, have been used to simulate the corresponding kinetics experimentally measured using an Atomic Beam 22 Pole Trap apparatus at low temperature. A good agreement with the experimental findings was found at 50 K. At a lower temperature, the model overestimates the formation of C+. This shows that secondary reactions are not responsible for the weak C+ production in the experiments at such temperature. Then, we discuss the possible impact of non-adiabatic effects in the study of the H + CH+ reactive collisions and we found that such effects can be responsible for the decrease of the H + CH+ rate coefficients at low temperature. This study offers an explanation for the disagreement between H + CH+ theoretical and experimental rate coefficients which has been going on for 20 years and highlights the need for performing non-adiabatic studies for this simple chemical reaction.

Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO2.

Methyl-ethyl-substituted Criegee intermediate (MECI) is a four-carbon carbonyl oxide that is formed in the ozonolysis of some asymmetric alkenes. MECI is structurally similar to the isoprene-derived methyl vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization, making it a promising candidate to help us unravel the effects of size, structure, and resonance stabilization that influence the reactivity of atmospherically important, highly functionalized Criegee intermediates. We present experimental and theoretical results from the first bimolecular study of MECI in its reaction with SO2, a reaction that shows significant sensitivity to the Criegee intermediate structure. Using multiplexed photoionization mass spectrometry, we obtain a rate coefficient of (1.3 ± 0.3) × 10-10 cm3 s-1 (95% confidence limits, 298 K, 10 Torr) and demonstrate the formation of SO3 under our experimental conditions. Through high-level theory, we explore the effect of Criegee intermediate structure on the minimum energy pathways for their reactions with SO2 and obtain modified Arrhenius fits to our predictions for the reaction of both syn and anti conformers of MECI with SO2 (ksyn = 4.42 × 1011 T-7.80exp(-1401/T) cm3 s-1 and kanti = 1.26 × 1011 T-7.55exp(-1397/T) cm3 s-1). Our experimental and theoretical rate coefficients (which are in reasonable agreement at 298 K) show that the reaction of MECI with SO2 is significantly faster than MVK-oxide + SO2, demonstrating the substantial effect of resonance stabilization on Criegee intermediate reactivity.

Machine learning rate constants of hydrogen abstraction reactions between ester and H atom

The rate constants of hydrogen abstraction reactions of alkyl ester by H atom are crucial for optimizing combustion reaction network and improving combustion efficiency of biodiesel. Due to the lack of experimental and theoretical rate coefficients data for some reactions - such as hydrogen abstraction of large biodiesel molecules by free radicals - machine learning provides a viable alternative to predict rate constants. In this study, three different machine learning (ML) methods - feedforward neural network (FNN), extreme gradient boosting (XGB) and XGB-FNN hybrid model - were used to predict rate constants of the reactions between alkyl ester and H atom. The rate constants of 41 reactions between H + CnH2n+1COOCmH2m+1 (n = 0–5, m = 1, 2) were calculated by the Master Equation System Solver (MESS) program over a temperature range of 300–2000 K for model training. The results showed that the XGB-FNN model with 8 descriptors has better overall performance than the other two ML methods. The average deviation of XGB-FNN model on the test set is 33.56% by performing leave one out (LOO) cross validations. The rate constants of the H + methyl decanoate (MD) reactions over a temperature range of 300∼2000 K were predicted by the XGB-FNN model, which follow well the modified three-parameter Arrhenius equation and agree well with theoretical values, indicating that the hybrid XGB-FNN model is robust in predicting the rate constants of alkyl ester and H atom in the temperature range of combustion. The present ML method in this study is supposed to be able to provide accurate and affordable rate constants prediction of larger systems, the molecular sizes of which are comparable to those of the dominant components of real biodiesel. That is important for the development of biodiesel kinetic models.

Mechanistic and kinetic study of the oxidation of trifluoroacetonitrile by hydroxyl and oxygen

AbstractTrifluoroacetonitrile (CF3CN) is one of the perfluoronitriles to be used as replacements for electrical insulation, working fluids, and biomedical applications. Potential energy surfaces for the reactions of CF3CN with OH radicals in the presence of molecular O2 have been calculated in details using the second‐order Møller−Plesset perturbation theory (MP2) and coupled‐cluster theory with single and double substitutions (CCSD) for geometrical optimization, and the restricted open‐shell complete basis set quadratic ROCBS‐QB3, multireference second‐order Rayleigh–Schrodinger perturbation theory extrapolated to complete basis set limit (RS2/CBS), and the explicitly‐correlated RS2 theory (RS2‐F12) for energetics. The CF3CN + OH reaction undergoes predominantly via the CO addition/elimination mechanism, leading to CF3C(OH)N radical adduct followed by the formation of CF3 and HOCN. The OH radicals could be recycled through the interception of CF3C(OH)N by O2 to form the orientation‐dependent CF3C(OH)NO2 radicals, leading to CF3C(O)NO + OH via the successive H‐migration and OO bond fission. Under the typical atmospheric conditions, the OH‐recycling mechanism is dominant for the oxidation of CF3CN. The production of CF3 and HOCN can be significant at high temperatures and low pressures. Theoretical rate coefficients are in excellent agreement with the experimental data. The effect of substitution on the reactivity of the perfluoronitriles toward OH was revealed. The present work provides a fundamental understanding on the degradation of CF3CN and serves as a theoretical basis for its secondary chemistry.

An Ab Initio RRKM-Based Master Equation Study for Kinetics of OH-Initiated Oxidation of 2-Methyltetrahydrofuran and Its Implications in Kinetic Modeling

Cyclic ethers (CEs) can be promising future biofuel candidates. Most CEs possess physico-chemical and combustion indicators comparable to conventional fuels, making them suitable for internal combustion engines. This work computationally investigates the kinetic behaviors of hydrogen abstraction from 2-methyl tetrahydrofuran (2MTHF), one of the promising CEs, by hydroxyl radicals under combustion and atmospheric relevant conditions. The various reaction pathways were explored using the CCSD(T)/cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. The Rice–Ramsperger–Kassel–Marcus-based master equation (RRKM-ME) rate model, including treatments for hindered internal rotation and tunneling, was employed to describe time-dependent species profiles and pressure and temperature-dependent rate coefficients. Our kinetic model revealed that the H-abstraction proceeds via an addition-elimination mechanism forming reaction complexes at both the entrance and exit channels. Eight different reaction channels yielding five radical products were located. The reaction exhibited complex kinetics yielding a U-shaped Arrhenius behavior. An unusual occurrence of negative temperature dependence was observed at low temperatures, owing to the negative barrier height for the hydrogen abstraction reaction from the C-H bond at the vicinity of the O-atom. A shift in the reaction mechanism was observed with the dominance of the abstraction at Cα-H of 2MTHF ring (causing negative-T dependence) and at CH3 (positive-T dependence) at low and high temperatures, respectively. Interestingly, the pressure effect was observed at low temperatures, revealing the kinetic significance of the pre-reaction complex. Under atmospheric pressure, our theoretical rate coefficients showed excellent agreement with the available literature data. Our model nicely captured the negative temperature-dependent behaviors at low temperatures. Our predicted global rate coefficients can be expressed as k (T, 760 Torr) = 3.55 × 101 × T−4.72 × exp [−340.0 K/T] + 8.21 × 10−23 × T3.49 × exp [918.8 K/T] (cm3/molecule/s). Our work provides a detailed kinetic picture of the OH-initiated oxidation kinetics of 2MTHF. Hence, this information is useful for building a kinetic me chanism for methylated cyclic ethers.

Thermochemistry and Kinetics of the OH- and Cl-Initiated Degradation Pathways of the HCFC-132b Atmospheric Pollutant

The gas-phase hydrogen abstraction reaction of 1,2-dichloro-1,1-difluoroethane (CH2ClCClF2) with OH and Cl radicals is theoretically investigated by employing density functional theory methods combined with coupled-cluster-based composite schemes. The mechanism and kinetics of the degradation entrance channels of CH2ClCClF2, recently detected with increasing concentration in the atmosphere, is elucidated by using cost-effective ab initio triple-slash dual-level direct dynamics, jChS//B2PLYP-D3(BJ)/jun-cc-pV(T+d)Z///M06-2X-D3/jun-cc-pV(T+d)Z. Thermal rate constants are calculated over the temperature range of 200–1000 K by adopting both canonical- (CVT) and microcanonical (μVT)-variational transition state theory also including the microcanonically optimized multidimensional tunneling transmission coefficient (μOMT). The theoretical rate coefficient for the H-abstraction reaction initiated by the OH radical is computed to be kOHCVT/μOMT = 1.72 × 10–14 cm3 molecule–1 s–1 and kOHμVT/μOMT = 1.57 × 10–14 cm3 molecule–1 s–1 at 298 K, in excellent agreement with the available experimental data. The rate constant for the H-abstraction reaction by the Cl atom, here obtained for the first time, is predicted to be kClCVT/μOMT = 5.96 × 10–14 cm3 molecule–1 s–1 and kClμVT/μOMT = 5.65 × 10–14 cm3 molecule–1 s–1 at 298.15 K, thus showing increased efficiency with respect to the OH-prompted reaction.

Unimolecular Reactions of 2,4-Dimethyloxetanyl Radicals.

Alkyl-substituted oxetanes are cyclic ethers formed via unimolecular reactions of QOOH radicals produced via a six-membered transition state in the preceding isomerization step of organic peroxy radicals, ROO. Owing to radical isomer-specific formation pathways, cyclic ethers are unambiguous proxies for inferring QOOH reaction rates. Therefore, accounting for subsequent oxidation of cyclic ethers is important in order to accurately determine rates for QOOH → products. Cyclic ethers can react via unimolecular reaction (ring-opening) or via bimolecular reaction with O2 to form cyclic ether-peroxy adducts. The computations herein provide reaction mechanisms and theoretical rate coefficients for the former type in order to determine competing pathways for the cyclic ether radicals. Rate coefficients of unimolecular reactions of 2,4-dimethyloxetanyl radicals were computed using master equation modeling from 0.01 to 100 atm and from 300 to 1000 K. Coupled-cluster methods were utilized for stationary-point energy calculations, and uncertainties in the computed rate coefficients were accounted for using variation in barrier heights and in well depths. The potential energy surfaces reveal accessible channels to several species via crossover reactions, such as 2-methyltetrahydrofuran-5-yl and pentanonyl isomers. For the range of temperature over which 2,4-dimethyloxetane forms during n-pentane oxidation, the following are the major channels: 2,4-dimethyloxetan-1-yl → acetaldehyde + allyl, 2,4-dimethyloxetan-2-yl → propene + acetyl, and 2,4-dimethyloxetan-3-yl → 3-butenal + methyl, or, 1-penten-3-yl-4-ol. Well-skipping reactions were significant in a number of channels and also exhibited a markedly different pressure dependence. The calculations show that rate coefficients for ring-opening are approximately an order of magnitude lower for the tertiary 2,4-dimethyloxetanyl radicals than for the primary and secondary 2,4-dimethyloxetanyl radicals. Unlike for reactions of the corresponding ROO radicals, however, unimolecular rate coefficients are independent of the stereochemistry. Moreover, rate coefficients of cyclic ether radical ring-opening are of the same order of magnitude as O2 addition, underscoring the point that a competing network of reactions is necessary to include for accurate chemical kinetics modeling of species profiles for cyclic ethers.

Theoretical study on the gas phase hydroxyl radical reaction with tetrahydrothiophene, tetrahydrofuran, thiophene and furan

Kinetic studies for the gas phase reaction of hydroxyl radical with tetrahydrothiophene, tetrahydrofuran, thiophene and furan has been performed using computational methods. Rate coefficients were evaluated using the energy grained master equation solver utilizing energies of transition states and complexes calculated at G4MP2 level of theory. Structures were optimized employing different methods to study their effectiveness and suitability. Theoretical rate coefficients were then compared with the experimental one and the suitability of the method is discussed. It has been observed that while tetrahydrothiophene and tetrahydrofuran results were reproduced with the single reference methods, thiophene and furan shows multi reference character.

Two-phase mass flow rate through restrictions in liquid-flooded twin-screw compressors or expanders

Liquid flooded twin-screw machines are commonly used as compressors for different applications and offer excellent prospects for energy conversion in lower and medium power ranges as expansion engines in Rankine cycles for example with regard to either waste heat recovery or electrical power generation from regenerative heat sources. With the aim of minimising internal leakages, lubricating moving machine parts, or reducing thermal stress and providing the benefits of wet expansion, a twin-screw machine can be operated with an auxiliary liquid or a two-phase working fluid. In this context, the present work provides a basic approach addressing the two-phase mass flow rate calculation in water-flooded twin-screw machines. Based on a generic inlet-geometry model, two-phase mass flow rates are recorded for different operating conditions and mass dryness fractions in particular. Appropriate approaches for the theoretical two-phase mass flow rate and discharge coefficient calculation considering restrictions in the flow section—such as orifices and nozzles—are applied. Finally, the two-phase discharge coefficients are compared with flow coefficients determined by means of experimental and theoretical two-phase mass flow rates revealing the limits and uncertainties of the models applied in this study.

Investigation of the Gas-Phase Reaction of Nopinone with OH Radicals: Experimental and Theoretical Study

Monoterpenes are the most essential reactive biogenic volatile organic compounds. Their removal from the atmosphere leads to the formation of oxygenated compounds, such as nopinone (C9H14O), one of the most important first-generation β-pinene oxidation products that play a pivotal role in environmental and biological applications. In this study, experimental and theoretical rate coefficients were determined for the gas-phase reaction of nopinone with hydroxyl radicals (OH). The absolute rate coefficient was measured for the first time using a cryogenically cooled cell along with the pulsed laser photolysis–laser-induced fluorescence technique at 298 K and 7 Torr. The hydrogen abstraction pathways were found by using electronic structure calculations to determine the most favourable H-abstraction position. Pathway 5 (bridgehead position) was more favourable, with a small barrier height of −1.23 kcal/mol. The rate coefficients were calculated based on the canonical variational transition state theory with the small-curvature tunnelling method (CVT/SCT) as a function of temperature. The average experimental rate coefficient (1.74 × 10−11 cm3 molecule−1 s−1) was in good agreement with the theoretical value (2.2 × 10−11 cm3 molecule−1 s−1). Conclusively, the results of this study pave the way to understand the atmospheric chemistry of nopinone with OH radicals.

Kinetic insights into the reaction of hydroxyl radicals with 1,4-pentadiene: A combined experimental and theoretical study

Olefinic hydrocarbons are one of the main intermediates of the oxidation of large hydrocarbons, and they are also found in distillate transportation fuels. Combustion characteristics of olefinic species, particularly non-conjugated ones, are not well understood. This work reports chemical insights into the reaction of OH radicals with 1,4-pentadiene (14PTDN), an important non-conjugated diolefin. High-temperature rate coefficients of OH + 14PTDN were measured in a shock tube over T = 881 – 1314 K and p ∼ 1150 Torr. The progress of the reaction was monitored by detecting OH radicals near 307 nm using a UV laser-absorption technique. OH radicals were generated by the fast thermal decomposition of tert‑butyl hydroperoxide (TBHP). Stochastic RRKM-based Master Equation (RRKM-ME) simulations were carried out on the potential energy surface computed at M06–2X/aug-cc-pVTZ level of theory to gain mechanistic insights. The theoretical model captured high-temperature experimental data from this work and previously reported data at low temperatures (294 - 468 K). This combined experimental and theoretical study reveals: (i) no discernible pressure dependence of the total rate constants, (ii) mechanism shift occurs with temperature, (iii) similar to conjugated dienes + OH reactions, the addition of OH radicals to 14PTDN dominates compared to abstraction pathways for T < 500 K, (iv) among the bimolecular product channels originating from the OH-adducts, only vinyl alcohol + allyl radical is important, contributing ∼28% of the reactant consumption at 0.1 Torr and 400 K, (iv) beyond 1000 K, the addition channel contributes negligibly small for all pressures. To our knowledge, this is the first detailed experimental and theoretical kinetic study of the reaction of 1,4-pentadiene with OH radicals. The expression for the theoretical total rate coefficients (in units of cm3 molecule−1s−1) is provided as k(T)=0.172×T−3.82exp(−125.6KT)+1.49×10−20T3.09exp(−53.7KT) over T = 200 – 2000 K and P = 760 Torr.

Shock tube study of the pyrolysis kinetics of Di- and trimethoxy methane

The high potential of oxymethylene ethers (OMEs) and related compounds as fuels and fuel-additives motivated a multitude of experimental and theoretical investigations on, e.g., dimethoxy methane (DMM), the smallest member of the OME family. The present work adds to this research by providing combined experimental and theoretical rate coefficients for di- and trimethoxy methane (TMM) pyrolysis. For DMM pyrolysis, the branching ratios between the major dissociation pathways remained elusive in recent studies and is elucidated in the present work using four independent sets of shock tube experiments and master equation modeling. For TMM pyrolysis, the present work provides the very first detailed chemical kinetics model. A key consumption reaction of both compounds, DMM and TMM, is the methoxy-induced H-atom migration, which yields methanol and a singlet diradical. While for DMM this reaction is in direct competition to the C–O bond fission reactions, TMM is found to be a prime example for methoxy-induced H-atom migration, as its pyrolysis chemistry is exclusively governed by this reaction. With the present work, the details of DMM pyrolysis are elucidated and the foundation is laid for detailed chemical kinetics modeling of TMM.

Theoretical Investigations on the Hydroxyl-Initialized Oxidation of Hexafluoro-2-butyne in the Presence of Oxygen.

Hexafluoro-2-butyne (C4F6) is a potential eco-friendly alternative gas in plasma, refrigerants, and electrical insulation applications. Mechanisms for the reactions of C4F6 with OH/O2 have been revealed in detail using various theoretical methods including ROCBS-Q, RCCSD(T), multireference RS2C, and extrapolations to the complete basis-set limit with Aug-cc-pVnZ (n = T, Q, 5) basis sets. Rate coefficients and product branching ratios were predicted for a wide range of temperatures and pressures using the solution of master equations. The vibrationally adiabatic ground-state barrier for the initial C4F6 + OH association was best estimated to be 1.53-2.26 kcal/mol. Energetically preferable decomposition paths for the conformation-dependent C4F6OH adducts include six-center HF elimination, four-center proton migration, and C-C bond cleavage, but the collisional deactivation is dominant under ambient conditions. The subsequent oxidation of C4F6OH by O2 bifurcates in two orientations and proceeds without any well-defined barrier followed by the successive isomerization/elimination steps, forming perfluorobiacetyl to regenerate OH radicals or trifluoroacetic acid with trifluoroacetyl radicals. The OH-recycling path accounts for a branching ratio of 70% under ambient conditions. Theoretical rate coefficients are in good agreement with the available experimental results. The effect of fluorination on the reactivity of alkynes toward OH/O2 is discussed.

Experimental dielectronic recombination rate coefficients for lithium-like 40Ca17+

The rate coefficients for dielectronic recombination (DR) of lithium-like 40Ca17+ ions with ∆n = 0 core excitations are derived from electron–ion recombination spectra measured with merged-beams method at the heavy-ion storage ring CSRm. The experimental DR spectrum, in the electron–ion collision energy range of 0 to 42 eV in the center-of-mass frame, comprises of all DR resonance peaks belong to the 2s 2 S 1/2 → 2p 2 P 1/2, 3/2 core excitations. The resonant energies and strengths for the resolved resonances in 2pjnl series are determined by fitting of the measured DR peaks. The further interpretation of the measured DR rate coefficients has been performed by calculating the DR rate coefficients with relativistic configuration-interaction method implemented in flexible atomic code (FAC) and compared with the experimental results. The experimental results and FAC calculations are found to be in a good agreement within the experimental uncertainties. Moreover, temperature dependent plasma rate coefficients were constructed from 4 × 103 to 1 × 107 K energy region by convoluting experimental and theoretical DR rate coefficients with the Maxwellian energy distribution function and then compared with previously available data. The plasma DR rate coefficient is found to be significantly underestimated by the early theoretical data calculated by Jacobs et al, and Mazotta et al in the low temperature. In contrast, a very good agreement has been found between the theoretical DR data of Gu and Colgan et al and the presently measured results at the low temperature region. Therefore, the results in this work composed of a bench-mark data set for plasma modeling at the photoionized temperature range. We have also provided a fit to our measured and theoretical plasma rate coefficients for low temperature plasma modeling.

Calculations of electron-impact excitation and dielectronic recombination rate coefficients of highly charged silicon ions

The rate coefficients for dielectronic recombination and electron-impact excitation processes with H-like to Be-like silicon ions (${\mathrm{Si}}^{13+}\text{\ensuremath{-}}{\mathrm{Si}}^{10+}$) are calculated using relativistic distorted-wave approach. The prominent $▵n=1$ and the weaker $▵n=2$ and 3 dielectronic recombination (DR) resonances with $K$-shell excitations are presented, and compared with the existing DR experimental rate coefficients of ${\mathrm{Si}}^{13+}, {\mathrm{Si}}^{12+}$, and ${\mathrm{Si}}^{11+}$ ions, it is found that the theoretical DR rate coefficients are in very good agreement with the experimental results. With the same approach, the direct and resonant electron-impact excitation (EIE) cross sections associated with $1s\ensuremath{-}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ core excitations are calculated for the ground states of ${\mathrm{Si}}^{13+}\text{\ensuremath{-}}{\mathrm{Si}}^{10+}$ ions and found in excellent agreement with the available experiment. Finally, we present the synthesized DR and EIE rate coefficients for the sum of all detected $(13+\ensuremath{\sim}10+)$ charge states of Si and these agree well with the experimental results.

Phosphorus Chemistry in the Earth's Upper Atmosphere

AbstractThe ablation of phosphorus from interplanetary dust particles entering the Earth's atmosphere is a potentially significant source of this key bioelement. In this study, the atmospheric chemistry of phosphorus is explored by developing a reaction network of possible routes from PO, the major ablation product in the upper mesosphere/lower thermosphere region, to the stable reservoirs H3PO3 and H3PO4 that become incorporated into meteoric smoke particles (MSPs) as metal phosphites and phosphates, respectively. The network is constructed with reactions whose kinetics have been measured experimentally, together with reactions where theoretical rate coefficients are estimated using a combination of electronic structure theory calculations and a Rice‐Ramsperger‐Kassel‐Markus master equation treatment. The network is then incorporated into a global chemistry‐climate model, together with a phosphorus meteoric input function. The estimated global mean P deposition flux, in the form of submicron‐sized MSPs, is 1 × 10−8 g m−2 yr−1, with a maximum of ∼5 × 10−8 g m−2 yr−1 over the northern Rockies, Himalayas, and southern Andes. The estimated fraction of ablated phosphorus forming bioavailable metal phosphites is 11%, which results from the very large concentrations of O and H compared to OH in the upper mesosphere. A layer of OPO is predicted to occur at 90 km with a peak of concentration of ∼50 cm−3; this is the counterpart of the well‐known layers of meteoric metals such as Na and Fe, and may be observable spectroscopically.

Gas phase hydroxyl radical reaction with 3,4-Dichloro-1,2,5-thiadiazole in the temperature range of 265–353 K: A kinetic and theoretical study

Kinetic studies for the gas phase reaction of OH radical with 3,4-Dichloro-1,2,5-thiadiazole has been performed using the laser photolysis – laser induced fluorescence technique. The rate coefficients were measured over the temperature range of 265–353 K and at a total pressure of~ 40 Torr of N2. The bimolecular rate coefficients obtained was fitted to the expression k(T)expt=(1.38 ± 0.91) × 10−13 exp[(903 ± 128)/T] cm3 molecule−1 s−1. Theoretical rate coefficients were also evaluated using the energy grained master equation solver which uses energies and structures of transition states and complexes calculated by ab initio method at G4MP2//M06-2X/6-311++G(d,p) level of theory. The reaction shows weak inverse temperature dependence.

Theoretical studies of carbon isotopic fractionation in reactions of C with C2: dynamics, kinetics, and isotopologue equilibria

Context. Our current understanding of interstellar carbon fractionation hinges on the interpretation of astrochemical kinetic models. Yet, the various reactions included carry large uncertainties in their (estimated) rate coefficients, notably those involving C with C2. Aims. We aim to supply theoretical thermal rate coefficients as a function of the temperature for the gas-phase isotope-exchange reactions 13C+12C2(X1Σg+,a3Πu)⇌13C12C(X1Σg+,a3Πu)+12C and 13C+13C12C(X1Σg+,a3Πu)⇌13C2(X1Σg+,a3Πu)+12C. Methods. By relying on the large masses of the atoms involved, we employ a variation of the quasi-classical trajectory method, with the previously obtained (mass-independent) potential energy surfaces of C3 dictating the forces between the colliding partners. Results. The calculated rate coefficients within the range of 25 ≤ T∕K ≤ 500 show a positive temperature dependence and are markedly different from previous theoretical estimates. While the forward reactions are fast and inherently exothermic owing to the lower zero-point energy content of the products, the reverse processes have temperature thresholds. For each reaction considered, analytic three-parameter Arrhenius-Kooij formulas are provided that readily interpolate and extrapolate the associated forward and backward rates. These forms can further be introduced in astrochemical networks. Apart from the proper kinetic attributes, we also provide equilibrium constants for these processes, confirming their prominence in the overall C fractionation chemistry. In this respect, the 13C+12C2(X1Σg+) and 13C+12C2(a3Πu) reactions are found to be particularly conspicuous, notably at the typical temperatures of dense molecular clouds. For these reactions and considering both equilibrium and time-dependent chemistry, theoretical 12C/13C ratios as a function of the gas kinetic temperature are also derived and shown to be consistent with available model chemistry and observational data on C2.