Calculated Rate Coefficients Research Articles

Formation of Criegee intermediates and peroxy acids: a computational study of gas-phase 1,3-cycloaddition of ozone with catechol.

A detailed theoretical investigation of gas-phase 1,3-cycloaddition of ozone with catechol is presented to explore the discrepancies in previous theoretical and experimental rate constants. DFT based PBE, TPSS, B3LYP, B3PW91, M06-2X, wB97XD, MN15 and high-level CCSD(T) methods are used for the calculation. Canonical transition state theory has been used to calculate the rate coefficients of individual steps. The calculated rate coefficients are compared with the experimental and previously calculated rate constant. The possible pathways for primary ozonide (POZ) formation and subsequent reactions to yield the Criegee Intermediates (CI) and peroxy acids (POA) are investigated. The endo-POZ may undergo conversion to exo-POZ or form the Creigee Intermediates. This work shows a novel pathway by which the exo-POZ can form more stable and chemically different species, peroxy acids, by abstracting an H atom from the OH group.

Dissection of the multichannel reaction of acetylene with atomic oxygen: from the global potential energy surface to rate coefficients and branching dynamics.

The O(3P) + C2H2 reaction is the first step in acetylene oxidation. The accurate kinetic data and the understanding of the reaction dynamics is of great importance. To this end, a full-dimensional global potential energy surface (PES) for the ground triplet state of the O(3P) + C2H2 reaction is constructed based on approximately 85 000 ab initio points calculated at the level of explicitly correlated unrestricted coupled cluster single, double, and perturbative triple excitations with the explicitly correlated polarized valence triple zeta basis set (UCCSD(T)-F12b/VTZ-F12). The PES is fit using the permutation invariant polynomial-neural network (PIP-NN) approach with a total root mean square error of 0.21 kcal mol-1. The key topographic features of the PES, including multiple potential wells and saddle points along different reaction pathways, are well represented by this fit PES. The kinetics and dynamics of the O(3P) + C2H2 reaction are investigated using the quasi-classical trajectory (QCT) method. The calculated rate coefficients are in good agreement with experimental data over a wide temperature range, especially when the temperature is lower than 1500 K. The product branch ratio has also been determined, which indicates the H + HCCO channel as the dominant reaction pathway at 298-3000 K, accounting for 80-90% of the overall rate coefficient, in agreement with experimental observations. The dynamics of the reaction is analyzed in detail.

Учет тонкой структуры атомов щелочных металлов при неупругих столкновениях с водородом

The method for calculations of rate coefficients for inelastic processes in collisions of alkali metal atoms and their positive ions with hydrogen atoms and its negative ions with accounting fine structure levels of alkali metals is proposed. The results of applications of the proposed method are reported in the present work on the examples of collisional systems KH and RbH.

Computational Study of the Reaction of Dimethyl Ether with Nitric Oxide. Mechanism and Kinetic Modeling.

To probe into the autoignition effect of nitric oxide (NO) on the combustion of dimethyl ether (DME), a detailed mechanism study and kinetic modeling for the reaction of DME with NO, which was considered to be very sensitive to the ignition delay time of DME, have been conducted using computational chemical methods. The CCSD(T)/6-311+G(2df,2p)//B2PLYP/TZVP compound method was employed to obtain the potential energy surface along the reaction coordinate, with the geometries, gradients, and force constants of nonstationary points calculated at the B2PLYP/TZVP theoretical level. The temperature-dependent rate coefficients from 200 to 3000 K were calculated using multistructural canonical variational transition-state theory (MS-CVT) with torsional motions and multidimensional tunneling effects included. The CCSD(T) calculations with both 6-311+G(2df,2pd) and cc-pVTZ basis sets give a zero-point inclusive barrier of 197-201 kJ mol-1 using the BMK/MG3S, B2PLYP/TZVP, and mPW2PLYP/TZVP based geometries. A van der Waals postreaction complex appears on the products HNO + CH3OCH2 side of the transition state. Two highly coupled torsions lead to four conformers for the transition state, and contributions from multiple structures and torsional anharmonicities substantially affect the rate coefficient evaluations. Variational effects can be argued to play an important role, especially at high temperatures, and tunneling probabilities increase with decreasing temperature. Because of large temperature-dependent feature of activation energy, the four-parameter formula 1.912 × 1011( T/300)3.191 exp[-178.417( T - 2.997)/( T2 + 2.9972)] cm3 mol-1 s-1 is recommended for the MS-CVT calculated rate coefficients including small-curvature tunneling. The kinetic model is shown to give a satisfactory interpretation of the inhibited and accelerated effect of NO on the oxidation of DME.

Mechanism and kinetics of the oxidation of dimethyl carbonate by hydroxyl radical in the atmosphere.

The mechanism and kinetics for the reaction of dimethyl carbonate (DMC) with OH radical have been studied by using quantum chemical methods. Four reaction pathways were identified for the initial reaction. In the first two pathways, hydrogen atom abstraction is taking place and alkyl radical intermediate is formed with the energy barrier of 6.4 and 7.9kcal/mol. In the third pathway, OH addition reaction to the carbonyl carbon (C2) atom of DMC and intermediate, I2, is formed with an energy barrier of 11.9kcal/mol. In the fourth pathway, along with CH3O●, methyl hydrogen carbonate is formed. For this C-O bond breaking and O-H addition reaction, the energy barrier is 27kcal/mol. The calculated enthalpy and Gibbs energy values show that the studied initial reactions are exothermic and exoergic except the OH addition reaction. For the initial reactions, the rate constants were calculated by using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) correction over the temperature range of 278-1200K. At 298K, the calculated rate coefficient for the in-plane and out-of-plane hydrogen atom abstraction reaction pathway is 2.30 × 10-13 and 0.02 × 10-13cm3molecule-1s-1. Further, the reaction between alkyl radical intermediate formed from the first pathway and O2 is studied. The reaction of alkyl peroxy radical intermediate with atmospheric oxidants, HO2, NO, and NO2 is also studied. It was found that the formic (methyl carbonic) anhydride is the end product formed from the atmospheric oxidation and secondary reactions of DMC.

Intermolecular electron energy transfer processes in the quenching of N2(C3Πu,v = 0–4) by collisions with N2 molecules

The removal rates of electronically excited molecular nitrogen N2(C3Πu,v = 0–4) in the collisions with the ground state N2(X1Σg+,v* = 0–4) 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.

The effect of fuel oxidation on electron swarm properties and nanosecond discharge characteristics in combustible mixtures

The results of the calculations of electron transport and rate coefficients in oxidized methane:O2, propane:O2 and H2:O2 mixtures were presented for different temperatures and reduced electric fields E/N from 1 to 3 × 103 Td. The calculations were made for various oxidized fuel fractions. It was shown that fuel oxidation led to a drastic decrease in the average electron energy, electron drift velocity and attachment coefficient for E/N < 60 Td. At higher reduced electric fields, the effect of fuel oxidation on the average electron energy and electron drift velocity was small, whereas the attachment coefficient increased with increasing oxidation degree. In addition, in the hydrocarbon-containing mixtures for E/N ∼ 30 Td, the electron drift velocity varied nonmonotonously with the oxidation degree increase. The critical reduced electric field at which the average rate of electron production in the mixture was equal to the average attachment rate increased with fuel oxidation due to the increased attachment rate and decreased detachment rate. The effect of intermediate species on the electron transport and rate properties in partially oxidized mixtures was small. The calculated results were used to self-consistently simulate fuel oxidation and plasma characteristics for high-voltage nanosecond repetitive discharges in combustible mixtures. Zero-dimensional simulation in a H2:O2 mixture showed that the reduced electric field at the instant when the deposited energy peaked was close to the critical values of E/N and increased with increasing oxidized fuel fraction.

Rotational Excitation of HD by Hydrogen Revisited.

The HD molecules are key species for the cooling of pristine gas at temperatures below 100 K. They are also known to be key tracers of H2 in protoplanetary disks and thus, they can be used as a measure of protoplanetary disks mass. Accurate modeling of the cooling mechanism and of HD abundance in astrophysical media requires a proper modeling for its excitation by both radiative and collisional processes. Here, we report quantum time-independent calculations of collisional rate coefficients for the rotational excitation of HD by H for temperatures ranging from 10 to 1000 K. The reactive and hydrogen exchange channels are taken into account in the scattering calculations. New exact quantum results are compared to previous calculations performed neglecting reactive and exchange channels. We found that for temperatures higher than ∼300 K, the impact of these channels on the rate coefficients cannot be neglected. Such results suggest that the new HD-H collisional data have to be used for properly modeling HD cooling function and HD abundance in all the astrophysical environments where HD plays a role, e.g. in photon-dominated regions, protoplanetary disks, early Universe chemistry, and primordial star forming regions.

Mechanistic study of the reaction of methyl peroxy radical (CH3O2) with formaldehyde (CH2O)

ABSTRACTA direct dynamic study on the reactions of CH3O2 + CH2O was carried out over the temperature range of 300–1500 K. All stationary points were calculated with the M06-2X/6-311++G(d,p) level of theory and identified for local minimum. The energetic parameters were refined at QCISD (T)/cc-pVTZ and CCSD (T)/cc-pVTZ levels of theory. Three channels were explored and a reaction of hydrogen abstraction from CH2O by CH3O2 was identified as dominant channel which involves the formation of a prereactive complex in the entrance channel. The rate coefficient of the dominant channel was calculated with TST and TST/Eck and the Eckart tunnelling effect is only important over the lower temperature region. The calculated rate coefficient of the dominant channel has positive temperature dependence and agrees reasonably with the available literature data.

Ro-vibrational excitation of H2 by H extended to high temperatures

Collisions between H2 and H are crucial to the thermal balance of astrophysical media, especially that of the early Universe. We report Quasi-Classical Trajectory (QCT) calculations of rate coefficients for the collisional (de-)excitation of H2 by H – including the three-body collisional dissociation – on a highly accurate H3 potential energy surface (PES). Translational temperatures ranging from 100 to 15 000 K, and ro-vibrational energy levels of H2 up to ≈50 000 K are considered. Our QCT rate coefficients agree well with those obtained from previous quantum calculations on the same PES. This good agreement supports our claim that, in order to study the 5000 to 15 000 K temperature range, typical of the early Universe, inexpensive QCT calculations should be preferred to extrapolation of costly quantum calculations run at lower temperatures. The new collisional rate coefficients should enhance our ability to model the cooling and heating budget of the primordial gas.

Influence of space charge density on electron energy distribution function and on composition of atmospheric pressure He/O2/air plasmas

Atmospheric pressure non-equilibrium plasma represents an efficient source of reactive species for different kinds of applications. Rich chemistry of such plasmas develops on longer time scales and is difficult to handle by kinetic models so global models are often applied to study such processes. The Maxwell-Boltzmann (MB) distribution is often used in global modelling of these non-equilibrium systems, even for the calculation of electron rate coefficients. In order to test the sensitivity of plasma composition on the space charge density and consequently on the assumed electron energy distribution function (EEDF), the zero-dimensional global model is applied to the helium/oxygen mixture (0.5% of O2 with humid air impurities. To test the effect of the distribution function on global models we have included the data calculated based on the non-equilibrium EEDF for the processes where often exponential Arrhenius-like formulae as a function of the effective temperature are used. The initial calculation showed that the change in the form of distribution function mainly affects the processes with thresholds considerably higher than the mean electron energy while it does not change much the rates for the processes with the thresholds and peaks of the cross sections in the region of the mean energy. We have calculated variations of the EEDF with the charge density and the resulting changes in chemical kinetics.

Electron–ion Recombination Rate Coefficients of Be-like 40Ca16+

Abstract Electron–ion recombination rate coefficients for beryllium-like calcium ions in the center of mass energy from 0 to 51.88 eV have been measured by means of the electron–ion merged-beam technique at the main cooler storage ring at the Institute of Modern Physics in Lanzhou, China. The measurement energy range covers the dielectronic recombination (DR) resonances associated with the core excitations and the trielectronic recombination (TR) resonances associated with the core excitations. In addition, the AUTOSTRUCTURE code was used to calculate the recombination rate coefficients for comparison with the experimental results. Resonant recombination originating from parent ions in the long-lived metastable state ions has been identified in the recombination spectrum below 1.25 eV. A good agreement is achieved between the experimental recombination spectrum and the result of the AUTOSTRUCTURE calculations when fractions of 95% ground-state ions and 5% metastable ions are assumed in the calculation. It is found that the calculated TR resonance positions agree with the experimental peaks, while the resonance strengths are underestimated by the theoretical calculation. Temperature dependent plasma rate coefficients for DR and TR in the temperature range of 103–108 K were derived from the measured electron–ion recombination rate coefficients and compared with the available theoretical results from the literature. In the temperature range of photoionized plasmas, the presently calculated rate coefficients and the recent results of Gu &amp; Colgan et al. are up to 30% lower than the experimentally derived ones, and the older atomic data are even up to 50% lower than the present experimental result. This is because strong resonances situated below electron–ion collision energies of 50 meV were underestimated by the theoretical calculation, which also has a severe influence on the rate coefficients in low-temperature plasmas. In the temperature range of collisionally ionized plasmas, agreement within 25% was found between the experimental result and the present calculation as well as the calculation by Colgan et al. The present result constitutes a set of benchmark data for use in astrophysical modeling.

A quantum theory investigation on atmospheric oxidation mechanisms of acrylic acid by OH radical and its implication for atmospheric chemistry.

The hydroxyl radical, as the most important oxidant, controls the removal of some volatile organic compounds (VOCs) in the atmosphere. In this work, the atmospheric oxidation processes of acrylic acid by OH radical have been investigated by density functional theory (DFT). The energetic routes of the reaction of CH2CHCOOH with OH radical have been calculated accurately at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level. It is implicated that the oxidation has five elementary reaction pathways mostly hinging on how hydroxyl radical approaches to the carbon skeleton of acrylic acid. The atmospheric degradation mechanisms of the CH2CHCOOH by OH radical are the formation of reactive intermediates IM1 and IM2. Meanwhile, the further oxidation mechanisms of IM1 and IM2 by O3 and NO are also investigated. The rate coefficients have been computed using tight transition state theory of the variflex code. The calculated rate coefficient is 2.3×10-11cm3molecule-1s-1 at standard pressure and 298K, which is very close to the laboratory data (1.75 ± 0.47×10-11cm3molecule-1s-1). Moreover, the atmospheric lifetime of acrylic acid is about 6h at 298K and 1atm, implying that the fast sinks of acrylic acid by hydroxyl radical.

Theoretical Insight into the Kinetics of H‐Abstraction Reaction of CHF2CH2OH with OH Radical, Atmospheric Lifetime and Global Warming Potential

AbstractThe mechanistic, kinetic and thermochemical analysis of the reaction of CHF2CH2OH with OH radical have been carried out at the CCSD(T)/aug‐cc‐pVTZ//M06‐2X/6‐311++G(d,p) level. The rate coefficients for all the reaction channels are evaluated using conventional transition state theory with Eckart's tunneling correction over a temperature range of 250–500 K. Our calculated rate coefficients agree reasonably well with those reported from the experiments. Heats of formation of CHF2CH2OH molecule and C•F2CH2OH, CHF2C•HOH and CHF2CH2O• radicals have been reported for the first time. Atmospheric lifetime, Global Warming Potential (GWP) and atmospheric degradation pathway of CHF2CH2OH have also been determined in the present study.

Investigations of the dielectronic recombination of phosphorus-like tin at CSRm**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300) and the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant Nos. U1732133, 11320101003, 11611530684, and 11604003).

The electron–ion recombination for phosphorus-like 112Sn35+ has been measured at the main cooler storage ring of the Heavy Ion Research Facility in Lanzhou, China, employing an electron–ion merged-beams technique. The absolute total recombination rate coefficients for electron–ion collision energies from 0 eV–14 eV are presented. Theoretical calculations of recombination rate coefficients were performed using the Flexible Atomic Code to compare with the experimental results. The contributions of dielectronic recombination and trielectronic recombination on the experimental rate coefficients have been identified with the help of the theoretical calculation. The present results show that the trielectronic recombination has a substantial contribution to the measured electron–ion recombination spectrum of 112Sn35+. Although a reasonable agreement is found between the experimental and theoretical results the precise calculation of the electron–ion recombination rate coefficients for M-shell ions is still challengeable for the current theory.

Atmospheric degradation of HCFO-1233zd(E) initiated by OH radical, Cl atom and O3 molecule: Kinetics, reaction mechanisms and implications

A degradation of trans-CF3CH=CHCl (HCFO-1233zd(E)) by reaction with OH radical, Cl atom and O3 molecule over the temperature range 200–1000 K has been studied employing the density functional theory. The potential energy surfaces accompnying the degradation have further been characterized in terms of single point energies derived from the coupled cluster CCSD(T) theory. Different possible pathways including H-abstraction, addition, isomerisation, dissociation and secondary reactions in absence and presence of O2/NOx are considered to elucidate the underlying mechanisms of the titled reactions. It has been shown that the initiation of degradation proceeds predominantly via addition to olefinic carbon with contributions from the direct H-abstraction channels being negligible as a result of the higher energy barrier. Calculated rate coefficients for the reaction of HCFO-1233zd(E) with OH was estimated to be 3.2 × 10−13 cm3 molecule‒1 sec‒1 whereas those for the reactions with Cl and O3 are predicted to be 3.7 × 10‒11 cm3 molecule‒1 sec‒1 and 50.0 × 10‒21 cm3 molecule‒1 sec‒1, respectively, which compare well with those from the earlier experiments. The present endeavor describes the atmospheric fate and oxidation processes of titled molecule and provides comprehensive data on the product branching ratios, atmospheric lifetimes and global warming potentials.

A new ab initio potential energy surface for the NH-He complex.

We present a new three-dimensional potential energy surface (PES) for the NH(X3Σ-)-He van der Waals system, which explicitly takes into account the NH vibrational motion. The NH-He PES was obtained using the open-shell single- and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations. The augmented correlation-consistent aug-cc-pVXZ (X = Q, 5, 6) basis sets were employed, and the energies obtained were then extrapolated to the complete basis set limit. Using this new PES, we have studied the spectroscopy of the NH-He complex and we have determined a new rotational constant that agrees well with the available experimental data. Collisional excitation of NH(X3Σ-) by He was also studied at the close-coupling level. Calculations of the collisional excitation cross sections of the fine-structure levels of NH by He were performed for energies up to 3500 cm-1, which yield, after thermal average, rate coefficients up to 350 K. The calculated rate coefficients are compared with available experimental measurements at room temperature, and a reasonably good agreement is found between experimental and theoretical data.

Thermal Rate Coefficients and Kinetic Isotope Effects for the Reaction OH + CH4 → H2O + CH3 on an ab Initio-Based Potential Energy Surface.

Thermal rate coefficients for the title reaction and its various isotopologues are computed using a tunneling-corrected transition-state theory on a global potential energy surface recently developed by fitting a large number of high-level ab initio points. The calculated rate coefficients are found to agree well with the measured ones in a wide temperature range, validating the accuracy of the potential energy surface. Strong non-Arrhenius effects are found at low temperatures. In addition, the calculations reproduced the primary and secondary kinetic isotope effects. These results confirm the strong influence of tunneling to this heavy-light-heavy hydrogen abstraction reaction.

Dielectronic and Trielectronic Recombination Rate Coefficients of Be-like Ar 14+

Electron–ion recombination of Be-like 40Ar14+ has been measured by employing the electron–ion merged-beams method at the cooler storage ring CSRm. The measured absolute recombination rate coefficients for collision energies from 0 to 60 eV are presented, covering all dielectronic recombination (DR) resonances associated with 2s 2 → 2s2p core transitions. In addition, strong trielectronic recombination (TR) resonances associated with 2s 2 → 2p 2 core transitions were observed. Both DR and TR processes lead to series of peaks in the measured recombination spectrum, which have been identified by the Rydberg formula. Theoretical calculations of recombination rate coefficients were performed using the state-of-the-art multi-configuration Breit–Pauli atomic structure code AUTOSTRUCTURE to compare with the experimental results. The plasma rate coefficients for DR+TR of Ar14+ were deduced from the measured electron–ion recombination rate coefficients in the temperature range from 103 to 107 K, and compared with calculated data from the literature. The experimentally derived plasma rate coefficients are 60% larger and 30% lower than the previously recommended atomic data for the temperature ranges of photoionized plasmas and collisionally ionized plasmas, respectively. However, good agreement was found between experimental results and the calculations by Gu and Colgan et al. The plasma rate coefficients deduced from experiment and calculated by the current AUTOSTRUCTURE code show agreement that is better than 30% from 104 to 107 K. The present results constitute a set of benchmark data for use in astrophysical modeling.

Cross-sections and rate coefficients calculations for rotational excitation of cyanoethynylide ions () induced by collision with He atoms at low temperature

The focal points of the present paper are the calculations of cross-sections and rate coefficients for rotational (de-)excitation of C3N− induced by collision with He and H2 atoms for energies of up to 2000 cm−1 and temperatures ranging from 1 K to 300 K. For this study, the new two-dimensional potential energy surface (PES) of the C3N−−He complex has been calculated at the ab initio restricted coupled cluster level of theory, including single, double and perturbative triple excitation [RCCSD(T)-F12a] with the cc-pVTZ-F12 basis set augmented by mid-bond functions. Basis set superposition errors were taken into account. To allow for the dynamical calculations, the PES was then fitted to a global analytical form, the main features of which are discussed. Collisional cross-sections among the first 17 rotational levels were calculated and employed to compute downward and upward rate coefficients. A propensity towards was observed.