Accurate Rate Coefficients Research Articles

Rate coefficients for C and O2 reactive collisions relevant to interstellar clouds from QCT and machine learning.

The chemical reactions between certain interstellar molecules are exothermic in nature and barrierless in the entrance channel, allowing these reactions to occur rapidly even at low astronomical temperatures, e.g., C and O2 interaction. Obtaining detailed rovibrational transition parameters for the reaction between C and O2, such as state-selected rate coefficients, is crucial for studying the associated atmospheric and astronomical environments. Hence, this work presents an approach that combines quasi-classical trajectory calculations with machine learning techniques based on Neural Network (NN) and Gaussian Process Regression (GPR) to determine state-selected rate coefficients. Employing this approach, we significantly reduced the computational requirements while simultaneously obtaining the accurate state-selected reaction cross sections and rate coefficients for the collision of C and O2. Both the NN-based and GPR-based models established in this work accurately predict the results calculated from explicit numerical calculations in the explored temperature range of 50-1500K, achieving a coefficient of determination R2 > 0.96. Most importantly, the current work provides the most comprehensive dataset of rovibrational rate coefficients of v = 0-4, j = 0-70 → v' = 0-15 for the astrophysical modeling of the C-O2 collision system.

MASER and dasar lines of disilicon carbide (SiCSi) using accurate collisional rate coefficients

MASER and dasar lines of disilicon carbide (SiCSi) using accurate collisional rate coefficients

Methanimine in Cool Cosmic Objects Using Accurate Collisional Rate Coefficients

Methanimine in Cool Cosmic Objects Using Accurate Collisional Rate Coefficients

Methanimine in cool cosmic objects using accurate collisional rate coefficients

Methanimine in cool cosmic objects using accurate collisional rate coefficients

Revisiting the rovibrational (de-)excitation of molecular hydrogen by helium

Context. The collisional (de-)excitation of H2 by He plays an important role in the thermal balance and chemistry of various astro-physical environments, making accurate rate coefficients essential for interpreting observations of the interstellar medium. Aims. Our goal is to utilize a state-of-the-art potential energy surface (PES) to provide comprehensive state-to-state rate coefficients for He-induced transitions among rovibrational levels of H2. Methods. We performed quantum scattering calculations for the H2-He system. Thus, we were able to provide state-to-state rate coefficients for 1059 transitions between rovibrational levels of H2, with internal energies up to ≃15 000 cm−1, for temperatures ranging from 20 to 8000 K. Results. Our results demonstrate a good agreement with previous calculations for pure rotational transitions between low-lying rotational levels. However, we do find significant discrepancies for rovibrational processes involving highly-excited rotational and vibrational states. We attribute these differences to two key factors: (1) the broader range of intramolecular distances covered by ab initio points and (2) the superior accuracy of the PES, resulting from the utilization of the state-of-the-art quantum chemistry methods, compared to previous lower-level calculations. Conclusions. Radiative transfer calculations performed with the new collisional data indicate that the population of rotational levels in excited vibrational states experiences significant modifications, highlighting the critical need for this updated dataset in models of high-temperature astrophysical environments.

Hyperfine excitation of NH and ND by molecular hydrogen

The NH and ND radicals are of key importance in the comprehension of nitrogen chemistry and the enhancement of deuterated molecules in the interstellar medium. Observations by space telescopes yield spectra that can resolve the fine and hyperfine structure of these radicals, a consequence of the electronic and magnetic interactions of nitrogen, hydrogen, and deuterium nuclei. Accurate rate coefficients, induced by collisions with H2, are required to interpret spectra of these radicals. We report the first rate coefficients for fine and hyperfine transitions of NH and ND in collision with both ortho- and para-H2. Based on a recent four-dimensional potential energy surface, fine-structure resolved cross sections and rate coefficients are computed with the time-independent close-coupling method over a temperature range of 5–300 K. Our calculations include the first 25 energy levels of NH and ND. Hyperfine resolved cross sections and rate coefficients are determined using the infinite-order sudden (IOS) approximation between 5 and 200 K for NH and 100 K for ND. We consider the first 71 and 105 energy levels of NH and ND, respectively. General propensity rules are discussed. We found a significant isotopic substitution effect in the rate coefficients. In addition, the rate coefficients for collisions with H2 are larger than those with He by a factor of up to 5, leading to lower critical densities for collisional excitation with H2 than He. The impact of the new set of collisional data has been investigated in simple radiative transfer models of the NH emission seen toward the Orion Bar and the ejecta of the η Carinae binary star. We observed significant differences by a factor of 5 between the presently determined column densities for NH compared to those from the literature using He as a collider.

Time-resolved quantification of key species and mechanistic insights in low-temperature tetrahydrofuran oxidation.

We investigate the kinetics and report the time-resolved concentrations of key chemical species in the oxidation of tetrahydrofuran (THF) at 7500 torr and 450-675 K. Experiments are carried out using high-pressure multiplexed photoionization mass spectrometry (MPIMS) combined with tunable vacuum ultraviolet radiation from the Berkely Lab Advanced Light Source. Intermediates and products are quantified using reference photoionization (PI) cross sections, when available, and constrained by a global carbon balance tracking approach at all experimental temperatures simultaneously for the species without reference cross sections. From carbon balancing, we determine time-resolved concentrations for the ROO˙ and ˙OOQOOH radical intermediates, butanedial, and the combined concentration of ketohydroperoxide (KHP) and unsaturated hydroperoxide (UHP) products stemming from the ˙QOOH + O2 reaction. Furthermore, we quantify a product that we tentatively assign as fumaraldehyde, which arises from UHP decomposition via H2O or ˙OH + H loss. The experimentally derived species concentrations are compared with model predictions using the most recent literature THF oxidation mechanism of Fenard et al., (Combust. Flame, 2018, 191, 252-269). Our results indicate that the literature mechanism significantly overestimates THF consumption and the UHP + KHP concentration at our conditions. The model predictions are sensitive to the rate coefficient for the ROO˙ isomerization to ˙QOOH, which is the gateway for radical chain propagating and branching pathways. Comparisons with our recent results for cyclopentane (Demireva et al., Combust. Flame, 2023, 257, 112506) provide insights into the effect of the ether group on reactivity and highlight the need to determine accurate rate coefficients of ROO˙ isomerization and subsequent reactions.

Theoretical Study of Intramolecular H-Migration Reactions of Peroxyl Radicals of JP-10 (Exo-Tetrahydrodicyclopentadiene)

ABSTRACT Intramolecular H-migration reactions of peroxyl radicals of JP-10 (C10H15OO•) play an important role in developing the oxidation mechanism of JP-10 at low temperature. However, accurate rate coefficients for these reactions are lacking in the literature. Therefore, 33 H-migration reactions of C10H15OO•, which belong to six series of reactions, were investigated. Quantum chemical computations at the CBS-QB3 level reveal that C10H15OO• do not obey the rule that the energy barrier of 1,4-H migration reaction is higher than that of 1,5-H migration reaction because of the special steric cyclic structure of C10H15OO• and the ring strain in the transition state. The transition state theory and Rice-Ramsberger-Kassel-Marcus/Master-Equation theory were used to obtain high-pressure limit rate coefficients and pressure-dependent rate coefficients, respectively. The results show that H-migration reactions of C10H15OO• have obvious pressure-dependent effect. The most competitive reaction was obtained among 33 reactions. Moreover, branching ratios of each series of reactions were used as weight ratios to lump this series of reactions. Six series of reactions were lumped as six lumped reactions by the isomer lumping method. This will provide accurate rate coefficients for constructing a mechanism with the low number of species and suitability for computational fluid dynamics simulation.

An accurate set of H3O+ − H2 collisional rate coefficients for non-LTE modelling of warm interstellar clouds

ABSTRACT Hydronium (H3O+) was first detected in 1986 in interstellar molecular clouds. It was reported in many Galactic diffuse and dense regions, as well as in extragalactic sources. H3O+ plays a major role both in interstellar oxygen and water chemistry. However, despite the large number of H3O+ observations, its collisional excitation was investigated only partially. In this work, we study the state-to-state rotational (de-)excitation of ortho- and para-H3O+ in collisions both with ortho- and para-H2. The cross sections are calculated within the close-coupling formalism using a highly accurate potential energy surface developed for this system. The rate coefficients are computed up to a kinetic temperature of 300 K. Transitions between the lowest 21 rotation-inversion states were studied for para-H3O+, and the lowest 11 states for ortho-H3O+, i.e. all levels with rotational energies below 430 K (∼300 cm−1) are considered. In order to estimate the impact of the new rate coefficients on the astrophysical models for H3O+, radiative transfer calculations were also carried out. We have examined how the new collisional data affect the line intensities with respect to older data previously used for the interpretation of observations. By analysing all detected transitions we find that our new, accurate rate coefficients have a significant impact (typically within a factor of 2) on radiation temperatures, allowing more accurate estimation of column densities and relative abundances of hydronium, especially in warm molecular clouds, paving the path towards better interpretation of interstellar water and oxygen chemistry.

Testing The Applicability Of Scaling Law For Collisional Rate Coefficients

Calculation of collisional rate coefficients is a challenging and difficult task. When these data are not available, their values may be estimated using a scaling law. In some investigations, we have utilized a scaling law. Accurate collisional rate coefficients for cyclopropenylidene (c-C3H2) as a complete set are available, and thus it may be appropriate occasion to test the applicability of that scaling law. We have considered 50 rotational levels of each of the ortho and para species of c-C3H2 having energies up to 91.53 cm-1 and 87.21 cm-1, respectively. We have compared the results of radiative transfer using accurate collisional rate coefficients with those obtained using the scaling law, and have found that the results of the scaling law are qualitative. Hence, for getting the qualitative behaviour of an analysis, the use of the scaling law is quite reasonable.

Cost-Effective Implementation of Multiconformer Transition State Theory for Alkoxy Radical Unimolecular Reactions.

Alkoxy radicals are important intermediates in the gas-phase oxidation of volatile organic compounds (VOCs) determining the nature of the first-generation products. An accurate description of their chemistry under atmospheric conditions is essential for understanding the atmospheric oxidation of VOCs. Unfortunately, experimental measurements of the rate coefficients of unimolecular alkoxy radical reactions are scarce, especially for larger systems. As has previously been done for peroxy radical hydrogen shift reactions, we present a cost-effective approach to the practical implementation of multiconformer transition state theory (MC-TST) for alkoxy radical unimolecular (H-shift and decomposition) reactions. Specifically, we test the optimal approach for the conformational sampling as well as the best value for a cutoff of high-energy conformers. In order to obtain accurate rate coefficients at a reduced computational cost, an energy cutoff is employed to reduce the required number of high-level calculations. The rate coefficients obtained with the developed theoretical approach are compared to available experimental rate coefficients for both 1,5 H-shifts and decomposition reactions. For all but one of the reactions tested, the calculated MC-TST rate coefficients agree with experimental results to within a factor of 7. The discrepancy for the final reaction is about a factor of 15, but part of the discrepancy is caused by pressure effects, which are not included in MC-TST. Thus, for the fastest alkoxy reactions, deviation from the high-pressure limit even at 1 bar should be considered.

Hyperfine resolved rate coefficients of HC17O+ with H2 (j = 0)

ABSTRACT The formyl cation (HCO+) is one of the most abundant ions in molecular clouds and plays a major role in the interstellar chemistry. For this reason, accurate collisional rate coefficients for the rotational excitation of HCO+ and its isotopes due to the most abundant perturbing species in interstellar environments are crucial for non-local thermal equilibrium models and deserve special attention. In this work, we determined the first hyperfine resolved rate coefficients of HC17O+ in collision with H2 (j = 0). Indeed, despite no scattering calculations on its collisional parameters have been performed so far, the HC17O+ isotope assumes a prominent role for astrophysical modelling applications. Computations are based on a new four dimensional (4D) potential energy surface obtained at the CCSD(T)-F12a/aug-cc-pVQZ level of theory. A test on the corresponding cross-section values pointed out that, to a good approximation, the influence of the coupling between rotational levels of H2 can be ignored. For this reason, the H2 collider has been treated as a spherical body and an average of the potential based on five orientations of H2 has been employed for scattering calculations. State-to-state rate coefficients resolved for the HC17O+ hyperfine structure for temperature ranging from 5 to 100K have been computed using recoupling techniques. This study provides the first determination of HC17O+–H2 inelastic rate coefficients directly computed from full quantum close-coupling equations, thus supporting the reliability of future radiative transfer modellings of HC17O+ in interstellar environments.

High accuracy barrier heights, enthalpies, and rate coefficients for chemical reactions

Quantitative chemical reaction data, including activation energies and reaction rates, are crucial for developing detailed kinetic mechanisms and accurately predicting reaction outcomes. However, such data are often difficult to find, and high-quality datasets are especially rare. Here, we use CCSD(T)-F12a/cc-pVDZ-F12//ωB97X-D3/def2-TZVP to obtain high-quality single point calculations for nearly 22,000 unique stable species and transition states. We report the results from these quantum chemistry calculations and extract the barrier heights and reaction enthalpies to create a kinetics dataset of nearly 12,000 gas-phase reactions. These reactions involve H, C, N, and O, contain up to seven heavy atoms, and have cleaned atom-mapped SMILES. Our higher-accuracy coupled-cluster barrier heights differ significantly (RMSE of ∼5 kcal mol−1) relative to those calculated at ωB97X-D3/def2-TZVP. We also report accurate transition state theory rate coefficients {k}_{infty }(T) between 300 K and 2000 K and the corresponding Arrhenius parameters for a subset of rigid reactions. We believe this data will accelerate development of automated and reliable methods for quantitative reaction prediction.

Selected Ion Flow Tube – Mass Spectrometry (SIFT-MS) study of the reactions of H3O+, NO+ and O2+ with a range of oxygenated volatile organic carbons (OVOCs)

Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS) uses soft chemical ionisation, typically by H3O+, NO+ and O2+, to analyse trace gases in real time at mixing ratios down to low parts per trillion by volume (pptv) and has recently been demonstrated as an effective method for monitoring volatile organic compounds (VOCs) in the atmosphere. SIFT-MS analysis requires accurate rate coefficient and product ion branching ratio data to allow for accurate mixing ratio calculations without the need for calibration standards. Most rate constant data have been measured using helium as the carrier gas. Here we report rate constants of a number of oxygenated volatile organic compounds (OVOCs) alongside benzene for H3O+, NO+ and O2+ reagent ions in nitrogen carrier gas, using a permeation tube calibration source and a certified gas standard to introduce known concentrations into the SIFT-MS. This includes the first reported literature rate constant for nonanal using SIFT-MS. We found that the rate constants for most compounds measured in nitrogen carrier gas were similar to those measured in helium. However, the NO+ rate constants for ketones measured in nitrogen were 1.2–5 times lower than those measured in helium. We observe higher fragmentation due to changes made to the lens voltages of the instrument in order to tune for higher sensitivity, altering the reagent ion energies. We also show the impact of increasing the flow tube temperature and voltage on the sensitivity and ionisation of benzene, butanone and butanal under both dry and humid conditions. Some of the SIFT-MS compound library entries are data from historic SIFT-MS instruments that lacked a heated flow tube, and therefore suffered from higher water clustering than current commercial SIFT-MS models. We show that a higher flow tube temperature reduces the amount of water adduct formation thereby simplifying compound concentration determination. These results demonstrate the need to ensure that the library data used are acquired under the same conditions as the experiment being performed.

Anomalous phenomena in cyclopropenylidene (c-C3H2) with accurate collisional rate coefficients

ABSTRACT Collisional rate coefficients are important parameters, but their calculation is a tedious task. Accurate collisional rate coefficients for collisional transitions between pure rotational levels of cyclopropenylidene (c-C3H2), colliding with He atom, are now available. Strong anomalous absorption line 220–211 at 21.587 GHz of cyclopropenylidene is found ubiquitous in interstellar medium. It is therefore worth to analyse the line spectrum of c-C3H2 using accurate collisional rate coefficients. After performing the Sobolev analysis of c-C3H2, we have found four anomalous absorption lines namely 22,0–21,1, 44,0–43,1, 33,0–32,1, and 43,2–50,5. We have also found two weak MASER lines, 40,4–33,1 and 51,4–44,1, which may make the detection of c-C3H2 in a cosmic object more convenient. Even for low kinetic temperature and low column density of cyclopropenylidene, the detection of 22,0–21,1 transition is possible, and it may be the reason for its detection in a large number of cosmic objects. The stability of results is tested.

Kinetics of OH + SO<sub>2</sub> + M: temperature-dependent rate coefficients in the fall-off regime and the influence of water vapour

Abstract. The OH-initiated oxidation of SO2 is the dominant, first step in the transformation of this atmospherically important trace gas to particulate sulfate, and accurate rate coefficients for the title reaction under all atmospheric conditions (pressures, temperatures, and humidity) are required to assess its role in, for example, new particle formation. Prior to this study, no temperature-dependent data were available in the fall-off regime for atmospherically relevant bath gases. We thus address an important omission in the kinetic database for this reaction and highlight significant discrepancies in recommended parameterizations. In this work, generation of OH via pulsed laser photolysis at 248 and 351 nm was coupled to its detection by laser-induced fluorescence to obtain rate coefficients (k1) for the title reaction at pressures of 14–742 Torr (1 Torr = 1.333 hPa) and temperatures of 220–333 K in N2 bath gas. In situ SO2 concentrations, central to accurate kinetic measurements under pseudo-first-order conditions, were measured by optical absorption. Under the conditions of the present study, the termolecular reaction between OH and SO2 is in the fall-off regime, and we parameterized the rate coefficients in N2 in terms of low- (k1,0) and high-pressure (k1,∞) limiting rate coefficients and a broadening factor (FC) to obtain k1,0N2=3.03×10-31 (T/300 K)−4.10 cm6 molecule−2 s−1, k1,∞=2.00×10-12 cm3 molecule−1 s−1, and FC=0.58. The effects of water vapour on the title reaction were explored through measurements in N2–H2O mixtures at 273, 298, and 333 K using the same experimental methods. The rate coefficients are significantly enhanced by the presence of water vapour with k1,0H2O=1.65×10-30 (T/300 K)−4.90 cm6 molecule−2 s−1, which indicates that H2O is a factor >5 more efficient in quenching the HOSO2* association complex than N2. A model-based comparison of our rate coefficients and parameterization with previous literature measurements and recommendations of evaluation panels are presented and discussed. The use of the new parameterization instead of the IUPAC or NASA evaluations, particularly after including H2O as a third-body quencher, leads to a significant (10 %–30 %) reduction in the lifetime of SO2 in some parts of the atmosphere and increases the H2SO4/SO2 ratio concomitantly.

Benchmarking an improved statistical adiabatic channel model for competing inelastic and reactive processes.

Inelastic collisions and elementary chemical reactions proceeding through the formation and subsequent decay of an intermediate collision complex, with an associated deep well on the potential energy surface, pose a challenge for accurate fully quantum mechanical approaches, such as the close-coupling method. In this study, we report on the theoretical prediction of temperature-dependent state-to-state rate coefficients for these complex-mode processes, using a statistical quantum method. This statistical adiabatic channel model is benchmarked by a direct comparison using accurate rate coefficients from the literature for a number of systems (H2 + H+, HD + H+, SH+ + H, and CH+ + H) of interest in astrochemistry and astrophysics. For all of the systems considered, an error of less than factor 2 was found, at least for the dominant transitions and at low temperatures, which is sufficiently accurate for applications in the above mentioned disciplines.

Transition 110−111 of methanimine in interstellar medium

Being precursor of glycine (NH2CH2COOH)—the simplest amino acid, the methanimine (CH2NH) in gas phase is of great importance for spectroscopists and astronomers both. Using accurate rate coefficients for collisional transitions between 15 lower rotational levels of methanimine, we have performed Sobolev large velocity gradient analysis and have discussed about weak MASER action of 110−111 transition of methanimine. We have done analytical analysis also for understanding the MASER action of 110−111 transition of methanimine. It is found that contribution of collisional transitions is less significant as compared to that of radiative ones.

Experimental and kinetic study on the low temperature oxidation and pyrolysis of formic acid in a jet-stirred reactor

A comprehensive experimental and kinetic study on the low temperature oxidation and pyrolysis of formic acid was conducted. Species profiles measurements were performed in a jet-stirred reactor (JSR) at the temperature range of 600–1100 K under atmospheric pressure, with a fixed residence time of 2.0 s and for the equivalence ratios ranging from 0.5 to ∞ (pyrolysis). High-level quantum calculation was used to obtain the accurate rate coefficients of missing reactions in Glarborg model. A detailed kinetic mechanism, derived from Glarborg model, has been developed based on high-level quantum calculation and validated with the species profiles obtained in this work and laminar flame speeds presented in our group. Reaction pathway and sensitivity analysis were investigated to get the deep insight of the oxidation of formic acid via the modified model. The results showed that unimolecular fuel decomposition reactions dominated the pyrolysis process while H abstraction reactions with the further consumption of the radicals are more important in the oxidation of formic acid. In addition, H abstraction reactions via HO2 become important at relatively low temperature.

Collisional excitation of the formyl radical (HCO) by molecular hydrogen

ABSTRACT This paper addresses the need for accurate rate coefficients for transitions between fine- and hyperfine-structure resolved rotational transitions in the formyl (HCO) radical induced by collisions with the two nuclear spin modifications of H2, the dominant molecule in the interstellar medium (ISM). These rate coefficients, as well as radiative transition rate coefficients, are required for accurate determination of the abundance of HCO in the ISM. Time-independent close-coupling quantum scattering calculations have been used to compute rate coefficients for (de-)excitation of HCO in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of HCO with H2 recently computed by the explicitly correlated RCCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 5 to 400 K were calculated for all transitions among the fine and hyperfine levels associated with the first 22 rotational levels of HCO, whose energies are less than or equal to 144 K.