Calculated Rate Coefficients Research Articles

Reaction of OH with Aliphatic and Aromatic Isocyanates

Isocyanates are highly relevant industrial intermediates for polyurethane production. In this work, we used quantum chemistry and transition state theory (TST) to investigate the gas-phase reaction of isocyanates with the OH radical, which is likely one of the most significant chemical sinks for these compounds in the troposphere. para-Tolyl-isocyanate (p-tolyl-NCO) was chosen as a proxy substance for the large-volume aromatic diisocyanate species toluene diisocyanate and methylene diphenyl diisocyanate. Besides p-tolyl-NCO + OH, the model reactions CH3NCO + OH, H2C═CHNCO + OH, C6H5-NCO + OH, C6H5-CH3 + OH, and C6H6 + OH have been studied as well to analyze various substituent effects and to allow for comparison with literature. Quantum chemical computations at the CCSD(T)/cc-pV(T,Q → ∞)Z//M06-2X/def2-TZVP level were used as the basis for tunneling-corrected canonical TST calculations. For CH3NCO + OH, H abstraction by OH at the methyl group is the main reaction channel according to our calculations and predicted to be four orders of magnitude faster than OH addition at the NCO group. The calculated rate coefficient (8.8 × 10-14 cm3 molecule-1 s-1) at 298 K is in good agreement with experimental data from the literature. Likewise, for aromatic isocyanates, OH attack at the isocyanate group was found to be only a minor pathway compared to addition to the aromatic ring. In the OH + p-tolyl-NCO reaction, OH addition at the ortho-position relative to the NCO group has been identified as the main initial reaction channel (branching fraction: 53.2%), with smaller but significant branching fractions for the H abstraction at the methyl group (9.6%) as well as the other ring addition reactions (ipso: 2.3%, meta: 24.5%, para: 10.5%, all relative to the NCO group). By comparing OH addition to the aromatic ring in p-tolyl-NCO with the respective ring addition reactions of phenyl isocyanate and toluene, the site-selective reactivity trends observed for ring addition in the OH + p-tolyl-NCO could be rationalized by a dominating positive mesomeric effect of the NCO group and a positive electron-donating (inductive) effect of the CH3 group. Except for the OH ring adduct formed from OH addition in ipso-position to the NCO group, we estimate the first-generation radical intermediates in the OH + p-tolyl-NCO reaction to have sufficiently long lifetimes to react with O2 under atmospheric conditions and undergo typical oxidative reaction cascades like those known for benzene or toluene.

Precise quantum measurement of vacuum with cold atoms.

We describe the cold-atom vacuum standards (CAVS) under development at the National Institute of Standards and Technology (NIST). The CAVS measures pressure in the ultra-high and extreme-high vacuum regimes by measuring the loss rate of sub-millikelvin sensor atoms from a magnetic trap. Ab initio quantum scattering calculations of cross sections and rate coefficients relate the density of background gas molecules or atoms to the loss rate of ultra-cold sensor atoms. The resulting measurement of pressure through the ideal gas law is traceable to the second and the kelvin, making it a primary realization of the pascal. At NIST, two versions of the CAVS have been constructed: a laboratory standard used to achieve the lowest possible uncertainties and pressures, and a portable version that is a potential replacement for the Bayard-Alpert ionization gauge. Both types of CAVSs are connected to a combined extreme-high vacuum flowmeter and dynamic expansion system to enable sensing of a known pressure of gas. In the near future, we anticipate being able to compare the laboratory scale CAVS, the portable CAVS, and the flowmeter/dynamic expansion system to validate the operation of the CAVS as both a standard and vacuum gauge.

Multiconformer transition state theory rate constant and branching ratios for the OH‐initiated reaction of CH3OCF2CHF2 and its primary product, HC(O)OCF2CHF2

AbstractObtaining rate constants and branching ratios for the atmospheric degradation of organic compounds is crucial for their environmental assessment and for a deeper understanding of their atmospheric chemistry and reactivity. The theoretical prediction of such quantities is a challenging endeavor that grows with conformational complexity. In this work, we have theoretically calculated the rate constants and branching ratios for the tropospheric OH‐initiated oxidation reactions of the hydrofluoroether and its primary product, the fluorinated ester. The calculations were performed with a cost‐effective multiconformer transition state theory protocol employing the recently developed constrained transition state randomization method. The calculated rate constant lies within the error bars of the recommended experimental value, while the calculated rate coefficient of represents, to our knowledge, the first determination of this reaction's rate constant. The results provide new insight and elucidate the main mechanistic issues discussed in recent experimental work while they also encourage the use of this protocol in investigations of the tropospheric reactivity of other fluorinated organic compounds.

Collisional excitation of hyperfine levels of OH by hydrogen atoms

ABSTRACT Observations of transitions between the hyperfine levels of the hydroxyl radical (OH) can provide crucial information on the physical conditions in interstellar clouds. Accurate modelling of the spectra requires calculated rate coefficients for the excitation of OH by H atoms, which is often present in molecular clouds in addition to the dominant H2 molecule. In this work, rate coefficients for the (de-)excitation of hyperfine levels of OH through collisions with hydrogen atoms are presented. In previous work, nuclear-spin-free scattering calculations were carried out; these took account of the fact that four electronic states (1A′, 1A″, 3A′, and 3A″) arise from the interaction of OH(X2Π) with H(2S). Because of the deep H2O($\tilde{X}^1A^{\prime }$) well, inelastic transitions can occur through direct collisions or by formation and decay of a collision complex. The rates of collision-induced hyperfine transitions were computed by the recoupling method and the MJ randomization approximations, respectively. These data will be useful in astrophysical models of OH excitation.

Computational investigation of thermochemistry and kinetics of the reaction between n-butyl peroxy and hydroperoxyl radicals

The energetics, thermochemistry and kinetics for the reaction between n-butyl peroxy radical and HO2 radical was computed using CCSD(T)/aug-cc-pVDZ// B3LYP/6-311G(d,p) level of theory coupled with Canonical Variational Transition state theory incorporating small curvature tunnelling corrections. The H abstraction pathway involving addition–elimination mechanism on the triplet energy surface was found to be most feasible reaction pathway. The calculated rate coefficients showed a negative temperature dependency in the studied range of 200 to 400 K. The Arrhenius expression for the title reaction was derived as, ktotal = 1.35 × 10−16 exp(3703/T) cm3molecule−1 s−1.

The role of radical-radical chain-propagating pathways in the phenyl + propargyl reaction

Well-skipping radical-radical reactions can provide a chain-propagating pathway for formation of polycyclic radicals implicated in soot inception. Here we use controlled pyrolysis in a microreactor to isolate and examine the role of well-skipping channels in the phenyl (C6H5) + propargyl (C3H3) radical-radical reaction at temperatures of 800–1600 K and pressures near 25 Torr. The temperature and concentration dependence of the closed-shell (C9H8) and radical (C9H7) products are observed using electron-ionization mass spectrometry. The flow in the reactor is simulated using a boundary layer model employing a chemical mechanism based on recent rate coefficient calculations. Comparison between simulation and experiment shows reasonable agreement, within a factor of 3, while suggesting possible improvements to the model. In contrast, eliminating the well-skipping reactions from the chemistry mechanism causes a much larger discrepancy between simulation and experiment in the temperature dependence of the radical concentration, revealing that the well-skipping pathways, especially to form indenyl radical, are significant at temperatures of 1200 K and higher. While most C9H7 forms by well-skipping at 25 Torr, an additional simulation indicates that the well-skipping channels only contribute around 3% of the C9Hx yield at atmospheric pressure, thus indicating a negligible role of the well-skipping pathways at atmospheric and higher pressures.

Kinetics of IO radicals with C1, C2 aliphatic alcohols in tropospherically relevant conditions.

Kinetics of the reaction of IO radicals with methanol (MeOH) and ethanol (EtOH) were experimentally studied in the gas phase using pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS). IO radicals were produced in situ at the reaction zone by photolysing a mixture of precursors (CH3I + O3 + N2) at 248nm and thereby electronically excited at 445.04nm. The rate coefficients for the reactions of (IO + MeOH) and (IO + EtOH) were measured at a total pressure of 60Torr/N2 in the range of 258-360K. At room temperature, the experimental rate coefficients of the title reactions were measured to be [Formula: see text] and [Formula: see text]. Dependencies of the kinetics with photolysis laser fluence and experimental pressures were verified. Effects of pressure over the kinetic behaviour of the studied systems were observed to be insignificant within the statistical uncertainties when studied in the range of ~ 30-150Torr/N2, whereas a minor and linear fluence dependency was observed within the studied limit. From the measured kinetic parameters, the atmospheric lifetimes of MeOH and EtOH were calculated in the tropospherically relevant conditions regarding their reactions with important atmospheric oxidants like Cl atom, OH and IO radicals. To complement experimental results, kinetics and thermochemistry for the title reactions were investigated theoretically via canonical variational transition state (CVT) theory in combination with small curvature tunnelling (SCT) corrections with a dual-level Interpolated Single Point Energy (ISPE) approach at the CCSD(T)/def2-QZVPP//M06-2X/def2-TZVPP level of theory/basis set in the temperatures between 200 and 400K. Good degree of agreement was encountered between experimentally measured and theoretically calculated rate coefficients. This article also discusses the thermochemical parameters and kinetic branching ratios (BRs) of all the pathways involved in the title reactions.

A Theoretical Study of H-Abstractions of Benzaldehyde by H, O3(P), 3O2, OH, HO2, and CH3 Radicals: Ab Initio Rate Coefficients and Their Uncertainty Quantification.

Benzaldehyde is a vital intermediate during the oxidation of toluene and oxygenated aromatics, but benzaldehyde's combustion chemistry currently gains little attention. The H-atom abstraction of benzaldehyde is one critical reaction class, yet its rate coefficients rely on the analogy of molecular similarity. For this reason, we have employed the ab initio calculations at the CCSD(T)/cc-pVTZ//M06-2X/6-311+g(d,p) level of theory, along with the RRKM master equation to predict the rate coefficients of H-abstraction reactions of benzaldehyde by H, O3(P), 3O2, OH, HO2, and CH3 radicals. The calculated rate coefficients of benzaldehyde + OH generally agree with literature measurements. From the branching ratio analysis, all the H-abstractions from ring sites can be neglected in kinetic model construction except by OH radical at temperature above 1700 K, where the ring sites contribute a relatively large branching ratio to consume benzaldehyde. A random sampling method has been used to estimate the global uncertainty in the calculated rate coefficients. The logarithm of uncertainty is proportional to the reciprocal temperature, and the global uncertainty is primarily derived from the energy errors and is almost independent of the attacking species. Comparison between benzaldehyde and acetaldehyde reveals that their rate coefficients are consistent only in H-abstractions by H and HO2 but are conflictive in other reactions, especially in the case of OH. By incorporating the new calculations, existing models show a faster prediction in autoignition delay times of benzaldehyde. This study reports the first high-level ab initio calculations for the H-atom abstraction reaction class of benzaldehyde. It is necessary for the future comprehensive kinetic modeling of aromatic aldehyde.

Electron attachment to interstellar grains

ABSTRACT We have performed quantum mechanical calculations of cross-sections and rate coefficients for electron attachment to small, neutral, and positively charged grains. In addition to the polarization potential and – for charged grains – the Coulomb potential, we considered the influence of the interaction between the charge of the electron and a possible permanent electric dipole on the grain. We found large discrepancies between the predictions of classical and quantal models of electron scattering on neutral grains at low energies (temperatures) in the absence of a permanent dipole moment, but much closer agreement when a dipole moment was present. The presence of the long-range Coulomb potential also leads to better agreement between classical and quantal models.

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.

Computed Pre-reactive Complex Association Lifetimes Explain Trends in Experimental Reaction Rates for Peroxy Radical Recombinations

The lifetimes of pre-reactive complexes, although implicitly part of the equations used to model many gas-phase bimolecular reactions, have seldom been included in quantitative calculations of rate coefficients. Here, we demonstrate the application of empirical molecular dynamics simulations of collisions between peroxy radicals to model association lifetimes. With the exception of the methyl peroxy–acetyl peroxy system, measurements of the lifetimes based on a phenomenological model are shown to correlate well with available experimental data for recombination reactions of peroxy radicals in cases where the rate-limiting transition state lies below the reactants in energy. Further, we predict reaction rates for larger α-pinene-derived peroxy radicals, and we interpret our results in tandem with available experimental data on these systems, which are of great relevance to improve our understanding of atmospheric aerosol formation.

The Gas-phase Reaction of NH2 with Formaldehyde (CH2O) is not a Source of Formamide (NH2CHO) in Interstellar Environments

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH2 with formaldehyde (CH2O) has been performed. This reaction has previously been suggested as a source of formamide (NH2CHO) in interstellar environments. A pulsed Laval nozzle equipped with laser-flash photolysis and laser-induced fluorescence spectroscopy was used to create and monitor the temporal decay of NH2 in the presence of CH2O. No loss of NH2 could be observed via reaction with CH2O, and we place an upper limit on the rate coefficient of <6 × 10−12 cm3 molecule−1 s−1 at 34 K. Ab initio calculations of the potential energy surface were combined with Rice–Rampsberger–Kassel–Marcus (RRKM) calculations to predict a rate coefficient of 6.2 × 10−14 cm3 molecule−1 s−1 at 35 K, consistent with the experimental results. The presence of a significant barrier, 18 kJ mol−1, for the formation of formamide as a product, means that only the H-abstraction channel producing NH3 + CHO, in which the transfer of an H atom can occur by quantum mechanical tunneling through a 23 kJ mol−1 barrier, is open at low temperatures. These results are in contrast with a recent theoretical study, which suggested that the reaction could proceed without a barrier and was therefore a viable route to gas-phase formamide formation. The calculated rate coefficients were used in an astrochemical model, which demonstrated that this reaction produces only negligible amounts of gas-phase formamide under interstellar and circumstellar conditions. The reaction of NH2 with CH2O is therefore not an important source of formamide at low temperatures in interstellar environments.

Formation of NaCl by radiative association in interstellar environments

Context. Radiative association is a possible way of sodium chloride (NaCl) formation in interstellar and related environments. Theoretical studies are essential since laboratory experiments are unavailable and difficult to perform. Aims. The total rate coefficient was calculated for the formation of NaCl by radiative association at 30–750 K. Methods. We included two contributing processes for the total rate-coefficient computation. One of them takes the nonadiabatic coupling between the two lowest 1Σ+ states, Χ1Σ+ and Β1Σ+, into account. The other one was calculated conventionally as a single channel and started in the continuum of the A1Π state. The individual rate coefficients were calculated from cross sections obtained up to 0.8 eV, which enabled us to calculate the rate coefficients up to 750 K. The cross section was also calculated for a one-state process within the Χ1 Σ+ state. Results. The nonadiabatic coupling enhances the formation of NaCl by radiative association by two orders of magnitude at about 30 K and by around one order of magnitude at about 750 K. The single-channel process starting in the continuum of the A1 Π state starts to contribute above around 200 K. The one-state transition model, within the Χ1Σ+ state, is not an adequate approximation for collisions in 1Σ+ symmetry. Instead, these collisions are treated in the diabatic representation in the total rate-coefficient calculation. Conclusions. The calculated total rate-coefficient function at 30–750 K can improve the astrochemical reaction networks for the CRL 2688, IRC+10216, and Orion SrcI environments, where NaCl was detected before.

Fine-structure Excitation of O(3P) Induced by Collisions with Atomic Hydrogen

The utility of the far-infrared lines of oxygen as diagnostics of gas outflows and for other applications depends on accurate descriptions of the rate coefficients for excitation (and relaxation) through collisions with electrons and with hydrogen atoms. For O and H collisions, earlier calculations of rate coefficients show discrepancies leading to ambiguity in astrophysical applications. In this note we introduce a methodology that yields consistent sets of rate coefficients for a number of cases. We then apply our method to the O–H system in order to investigate the discrepancies. The present rate coefficients will be of particular interest for modeling observations of astrophysical environments in the far-infrared.

Toward Efficient Direct Dynamics Studies of Chemical Reactions: A Novel Matrix Completion Algorithm.

This paper describes the development and testing of a polynomial variety-based matrix completion (PVMC) algorithm. Our goal is to reduce computational effort associated with reaction rate coefficient calculations using variational transition state theory with multidimensional tunneling (VTST-MT). The algorithm recovers eigenvalues of quantum mechanical Hessians constituting the minimum energy path (MEP) of a reaction using only a small sample of the information, by leveraging underlying properties of these eigenvalues. In addition to the low-rank property that constitutes the basis for most matrix completion (MC) algorithms, this work introduces a polynomial constraint in the objective function. This enables us to sample matrix columns unlike most conventional MC methods that can only sample elements, which makes PVMC readily compatible with quantum chemistry calculations as sampling a single column requires one Hessian calculation. For various types of reactions─SN2, hydrogen atom transfer, metal-ligand cooperative catalysis, and enzyme chemistry─we demonstrate that PVMC on average requires only six to seven Hessian calculations to accurately predict both quantum and variational effects.

Calculated rate coefficients between CI-MS reagent ions and organosulfur compounds causing food taints and off-flavours

Volatile sulfur compounds play a crucial role in the aroma profile of food and fermented beverages. We explore chemical-ionization mass spectrometry (CI-MS) ion-molecule reaction kinetics of commonly used reagent ions to a list of volatile organic sulfur compounds (VOSCs). We compute the rate coefficients of ion-molecule reactions, useful for the accurate identification and quantification of trace gases, using capture collision models based on the electric dipole moment and polarizability of the neutral VOSCs. To this aim, we evaluate molecular properties, such as the electric dipole moment, polarizability, proton affinity (PA), and ionization energy (IE) for each VOSC, by means of hybrid density functional theory (DFT) simulations. The PA and IE values are useful in the selection of appropriate reagent ions to be used in CI-MS. We thoroughly investigate collision rate coefficients at effective temperatures and internal energies, as relevant for highly energetic proton transfer reaction mass spectrometry (PTR-MS) drift tube conditions. The data provided will be valuable for the rapid quantification of VOSCs in food and fermented beverages.

A matrix completion algorithm for efficient calculation of quantum and variational effects in chemical reactions.

This work examines the viability of matrix completion methods as cost-effective alternatives to full nuclear Hessians for calculating quantum and variational effects in chemical reactions. The harmonic variety-based matrix completion (HVMC) algorithm, developed in a previous study [S. J. Quiton et al., J. Chem. Phys. 153, 054122 (2020)], exploits the low-rank character of the polynomial expansion of potential energy to recover vibrational frequencies (square roots of eigenvalues of nuclear Hessians) constituting the reaction path using a small sample of its entities. These frequencies are essential for calculating rate coefficients using variational transition state theory with multidimensional tunneling (VTST-MT). HVMC performance is examined for four SN2 reactions and five hydrogen transfer reactions, with each H-transfer reaction consisting of at least one vibrational mode strongly coupled to the reaction coordinate. HVMC is robust and captures zero-point energies, vibrational free energies, zero-curvature tunneling, and adiabatic ground state and free energy barriers as well as their positions on the reaction coordinate. For medium to large reactions involving H-transfer, with the sole exception of the most complex Ir catalysis system, less than 35% of total eigenvalue information is necessary for accurate recovery of key VTST-MT observables.

Experimental and model investigation of the low temperature oxidation and pyrolysis of 2-methyl-2-butene in a jet-stirred reactor

Species concentrations for 2-methyl-2-butene oxidation and pyrolysis were measured in a jet-stirred reactor at atmospheric pressure, in the temperature range of 650 to 1200 K and the residence time of 1.5 s. The model presented in literature was modified based on the high-level quantum calculated rate coefficients published, including 1,2-dimethyl-allyl (AC5H9C) + HO2, 1,2-dimethyl-allyl + O2, 1,2-dimethyl-allyl + 1,2-dimethyl-allyl, isoprene + H reaction classes. The modified model was validated against the measured species concentrations, ignition delay times and laminar flame speeds. The modified model can accurately predict experimental data under various conditions. Moreover, flux analysis showed that the major pathways were abstraction reactions that consumed more than 40% 2-methyl-2-butene. At lower temperatures, reactions of HO2 addition, O2 addition and radical self-combination were the dominant consumption path for allylic pentenyl radicals. Sensitivity analysis showed that HO2 additions to AC5H9C radicals, forming AC5H9COOH, BC5H9-AOOH, and C5H9O oxy-radicals + OH played important roles in promoting the overall reactivity.

Scattering calculation of the newly observed pentacarbon monoxide (C5O) with He atom: de-excitation rates and radiative transfer

ABSTRACT To determine the chemical composition of gases in molecular clouds, the oxygen-bearing systems CnO are needed as probe elements. The pentacarbon monoxide C5O was recently detected in TMC-1, and in order to derive accurate physical conditions from its rotational transitions, calculation of rate coefficients of C5O(1Σ+) induced by collision with He are performed for thermal temperature below 100 K. These calculations are based on a new 2D potential energy surface (2D-PES) obtained from the explicit correlated coupled cluster with single, double, and pertubative triple excitation (ccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C5O–He PES presents two minima below its dissociation limit with a well depths of −59.321 and −53.059 cm−1. By mean of this PES, the integral cross sections are calculated in the close-coupling quantum time independant formalism for $E\le 500 \, {\rm cm}^{-1}$ and J ≤ 20. The de-excitation rate coefficients are obtained after averaging these cross sections at low temperatures. We expect that the new collisional data will allow accurate determination of the C5O abundance in the interstellar medium, as well as the interpretation of its emission lines. These new data are crucial to understand the chemistry of carbon chain species in the interstellar gas.

The reactions of propanal/acetone with hydroperoxyl radicals: A theoretical and modeling study

Propanal (C2H5CHO) and acetone (CH3COCH3), the C3 carbonyl compound isomers, are the main byproducts of bio-alcohol combustion. In this work, the reaction rate coefficients between propanal/acetone and hydroperoxyl radicals (HO2) over 500–2000 K and 0.01–100 atm were obtained by the RRKM/master equation method based on high-level quantum chemical calculations. The competition relationship among the H-abstraction, HO2 addition, addition-dissociation reaction pathways and their temperature and pressure dependence were explored. The results show that for propanal + HO2, the aldehydic H-abstraction reaction is highly favored over the addition-dissociation and other abstraction channels, while for acetone + HO2, the H-abstraction reaction is comparable with those of addition-dissociation reactions at low temperature. At temperatures ≤ 700 K and pressures ≥ 1 atm, collisional stabilization of the peroxy-alcohol radical is the dominant channel. To reveal the impact of the studied reactions on model predictions, the rate coefficients of various reaction channels calculated in this work and those of peroxy-alcohol radicals + HO2 from the literature were incorporated into a recently proposed low-temperature mechanism of propanal. The results show that the aldehydic H-abstraction reaction of propanal by HO2 greatly contributes to the combustion behavior in the negative temperature coefficient regime. The peroxy-alcohol radical favors its decomposition channel with temperatures ranging from 500 to 900 K at 1 atm in a JSR. Modified Arrhenius representations of the calculated rate coefficients are given and should be valuable for the future development of low-temperature oxidation mechanisms for aldehydes, ketones, and alcohols.