Small-curvature Tunneling Research Articles

DFT Calculations for the HONO Elimination Process of CL-20 Conformers

The HONO elimination process is regarded to be an important initial decomposition process of energetic nitramines. Four CL-20 conformers based on the ε-CL-20 were obtained by the optimization at the m062x/cc-pvtz level in this study, and the Transition State (TS) and Intrinsic Reaction Coordinate (IRC) calculations were carried out at the same level. In addition, the rate coefficients and activation energy of the HONO elimination process were evaluated using conventional transition state theory (TST) and canonical variational transition state theory (CVT) with Eckart and small-curvature tunneling (SCT) methods to correct the transmission coefficients for the quantum tunneling effect. The calculation results have shown that the HONO elimination process concerning the nitro groups located on six numbered rings is the hardest to happen, and it seems that the longer distance between nitro groups and the adjacent hydrogen atom would result in the higher barrier energy; the HONO elimination process is most likely to happen for the axial positioning of nitro groups located on five numbered rings and most unlikely to happen for the ones located on six numbered rings; CL-20 II and CL-20 IV conformers are the most unstable one and most stable one concerning the reaction difficulty of the HONO elimination process.

Temperature dependent kinetics for the reaction between OH radicals and (E)- and (Z)- CHF = CHCl: A dual-level computational study

The temperature dependent kinetics for the reactions between (E)/(Z)- CHF = CHCl and OH radicals were studied using CCSD(T)/cc-pVTZ//M06-2X/6–31 + G(d,p) level of theory. The rate coefficients of the studied reaction paths were calculated by Canonical Variational Transition state theory followed by small curvature tunnelling corrections from 200 to 400 K. Various reaction pathways involved in both the reactions were exothermic and spontaneous. The Arrhenius expressions for the title reactions were obtained as k(E)-CHF=CHCl = 5.73 × 10-19 T2.44 exp (622.1/T) and k(Z)-CHF=CHCl = 2.22 × 10-18 T2.17 exp (803.6/T) cm3molecule-1s−1 respectively.

Theoretical study of mechanisms and kinetics of reactions of the O(3P) atom with alkyl hydroperoxides (ROOH) where (R = CH3 & C2H5)

An analysis of O(3P) reaction mechanisms and kinetics with CH3OOH and CH3CH2OOH has been examined. Potential energy diagrams for the named reactions have been evaluated at the W1 and CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ levels of theory. For the two reactions, a total of seven abstraction channels have been described. The reaction enthalpies and standard reaction Gibbs free energy are calculated at the W1 level. The kinetic study is conducted within the temperature range of 210–350 K using transition state theory (TST), TST with Wigner correction (TST/W), and TST with zero curvature tunneling (TST/ZCT). The rate coefficients were also calculated using canonical variational transition state theory (CVT), CVT with zero curvature tunneling (CVT/ZCT), and CVT with small curvature tunneling (CVT/SCT). Branching ratios have been computed using the CVT/SCT rate coefficients. Hydrogen abstraction from the hydroxyl group is identified as a significant pathway in both reactions.

Theoretical studies of the reactions of 1,2-Ethanediol with triplet oxygen and hydrogen atoms

An direct kinetics study of the (CH2OH)2 + O(3P) and (CH2OH)2 + H reactions was performed over the temperature range of 300–1500 K. The geometries of all stationary points were optimised at the M06-2X/cc-pVQZ level of theory. The electronic energies were refined by CCSD(T)/CBS and QCISD(T)/cc-pVTZ methods. The rate coefficients were calculated using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The results showed that the channels of hydrogen abstraction from the CH2 group were most kinetically favourable at the temperature less than 1300 K and have positive temperature dependence. The OH-transfer channels were ignorable.

TUMME: Tsinghua University Minnesota Master Equation program

TUMME is a program for assembling and solving master equations for gas-phase chemical kinetics based on chemically significant eigenmodes. TUMME has interfaces to the Gaussian, Polyrate, and/or MSTor output files that allow the master equation code to obtain the microcanonical flux coefficients needed for the coefficient matrix of the master equation. The flux coefficients for reactions with barriers can be calculated by multi-structural variational transition state theory with small-curvature tunneling (MS-VTST/SCT) or by simpler approximations to this such as conventional transition state theory without tunneling (also called RRKM theory). The flux coefficients for barrierless reactions are provided by a hard-sphere model. TUMME is written in double precision with Python 3; quadruple and octuple precision are also available for some subtasks in C++. The Python code can run in serial or parallel (MP or MPI), and the C++ code can run on a single processor or on multiple processors with OpenMP. Program summaryProgram Title: TUMME 2.2CPC Library link to program files:https://doi.org/10.17632/whcnvm2mc9.1Developer's repository link:https://comp.chem.umn.edu/tummeLicensing provisions: Apache-2.0Programming languages: Python 3 and C++External libraries: Numpy, Scipy, Numba, mpi4py (optional), modified mpack (optional), qd (optional), omp (optional)Nature of problem: Characterize a temperature-dependent and pressure-dependent complex reaction system.Solution method: Solve the energy master equation based on chemically significant eigenmodes to get phenomenological rate constants and time evolution of the populations.Additional comments including restrictions and unusual features: Interfaces to programs Gaussian, Polyrate, and/or MSTor; calculates rate constants and chemically significant eigenmodes; performs calculations by multi-structural variational transition state theory with small-curvature tunneling (MS-VTST/SCT) and/or by simpler theories; can use quadruple or octuple precision; can do calculations in parallelized modes.

The influence of the environment in chemical reactivity: the HCOOH formation from the H2O + CO reaction.

The reaction between carbon monoxide and water was studied occurring in an aerosol medium rich in methanol. This environment is plausible for the primitive and prebiotic Earth atmosphere. The chemical environment is expressed in terms of dielectric constant (ε) and the chemical system was modeled employing the polarizable continuum model (PCM). The main results were acquired from calculations employing the M06-2X density functional for the electronic structure calculations and the canonical variational theory with small curvature tunneling for the chemical kinetic calculations. The rise of ε affects both the thermochemistry and the kinetics of the reaction, increasing the barrier height and decreasing the rate constant for the reaction occurring at room temperature. For example, the rate constant at 300 K is 5-10× 10- 53 cm3 ⋅molecule- 1 ⋅s- 1 for low dielectric constant (ε < 3) and around 2-4× 10- 53 cm3 ⋅molecule- 1 ⋅s- 1 for ε between 7 and 40. Our results indicate that the ε variation allows a fine tuning to the rate of the reaction.

Theoretical investigations on the OH radical mediated kinetics of cis- and trans-CH3CF=CHF and CH3CH=CF2 over temperature range of 200-400K

Global reactivity for the OH-radical mediated kinetics of three Hydrofluoroolefins (HFOs), cis-CH3CF=CHF, trans-CH3CF=CHF and CH3CH=CF2 was evaluated at the CCSD(T)/cc-pVTZ//M062x/6-31G+(d,p) level of theory. The overall kinetics was computed using the Canonical Variational Transition State Theory (CVT) in combination with Small Curvature Tunneling (SCT) and Interpolated Single Point Energy (ISPE) corrections over the temperature range of 200 and 400 K. The total rate coefficients (k, in units of cm3 molecule−1 sec−1) for the reaction of OH radical with these HFOs at 298 K were computed to be, k298Kcis−CHF=CFCH3+OH=1.69×10−12, k298Ktrans−CHF=CFCH3+OH=3.12×10−12, and k298KCF2=CHCH3+OH=5.40×10−12. To further corroborate the computed kinetics, thermodynamic parameters and branching ratios were also computed for all the reaction pathways. Atmospheric lifetimes, Radiative forcing (RF), Global Warming Potentials (GWP) and Photochemical Ozone Creation Potentials (POCP) of these HFOs were also calculated and are presented in this manuscript.

Theoretical study of the reactions of triplet oxygen atom with 1-propanol and 2-propanol

The reactions of triplet oxygen atom (O(3P)) with 1-Propanol and 2--Propanol were characterized at CCSD(T)/CBS//M06-2X/cc-pVTZ level of theory. The rate coefficients were calculated by the canonical variational transition state theory with the small curvature tunneling correction. The calculated total rate coefficients agreed with the available reference values. The dominant channel in the 1-C3H7OH + O(3P) reaction is hydrogen-abstraction from the α-CH2 group at temperature less 450 K or hydrogen-abstraction from β-CH2 groups at temperatures higher than 500 K; the dominant channel in the 2-C3H7OH + O(3P) reaction is hydrogen-abstraction from the CH group at temperature less than 1200 K.

Kinetics of the Reactions of Ozone with Halogen Atoms in the Stratosphere

It is well established that reaction cycles involving inorganic halogens contribute to the depletion of ozone in the atmosphere. Here, the kinetics of O3 with halogen atoms (Cl, Br, and I) were investigated between 180 and 400 K, expanding the temperature range relative to prior studies. Canonical variational transition state theory including small curvature tunneling correction (CVT/SCT) were considered, following the construction of the potential energy surfaces. MRCI + Q/aug-ano-pVTZ//MP2/aug-cc-pV(T + d)Z and MRCI + Q/aug-ano-RCC-VTZP//MP2/aug-cc-pV(T + d)Z levels of theory were used to calculate the kinetic parameters. Calculated rate coefficients were used to fit the Arrhenius equations, which are obtained to be k1 = (3.48 ± 0.4) × 10−11 exp[(−301 ± 64)/T] cm3 molecule−1 s−1, k2 = (3.54 ± 0.2) × 10−11 exp[(−990 ± 35)/T] cm3 molecule−1 s−1 and k3 = (1.47 ± 0.1) × 10−11 exp[(−720 ± 42)/T] cm3 molecule−1 s−1 for the reactions of O3 with Cl, Br, and I atoms, respectively. The obtained rate coefficients for the reactions of O3 with halogen atoms using CVT/SCT are compared to the latest recommended rate coefficients by the NASA/JPL and IUPAC evaluations. The reactivity trends and pathways of these reactions are discussed.

Investigation of kinetics of phenyl radicals with ethyl formate in the gas phase using cavity ring-down spectroscopy and theoretical methodologies.

The gas-phase kinetics of phenyl radical (·C6H5) with ethyl formate (HCO2Et, EF) was investigated experimentally using ultrasensitive laser-based cavity ring-down spectroscopy (CRDS). Phenyl radicals were generated by photolyzing nitrosobenzene (C6H5NO) at 248nm and thereby probed at 504.8nm. The rate coefficients for the (phenyl radical + EF) reaction were investigated between the temperatures of 260 and 361K and at a pressure of 61Torr with nitrogen (N2) as diluent. The temperature-dependent Arrhenius expression for the test reaction was obtained as: [Formula: see text]=(1.20 ± 0.16) × 10-13 exp[-(435.6 ± 50.0)/T] cm3 molecule-1s-1 and the rate coefficient at room temperature was measured out to be: [Formula: see text]=(4.54 ± 0.42) × 10-14 cm3 molecule-1s-1. The effects of pressure and laser fluence on the kinetics of the test reaction were found to be negligible within the experimental uncertainties. To complement the experimental findings, kinetics for the reaction of phenyl radicals with EF was investigated theoretically using Canonical Variational Transition State Theory (CVT) with Small Curvature Tunnelling (SCT) at CCSD(T)/cc-pVDZ//B3LYP/6-31 + G(d,p) level of theory in the temperatures between 200 and 400K. The theoretically calculated rate coefficients for the title reaction were expressed in the Arrhenius form as: [Formula: see text]= (1.48 ± 0.56) × 10-38 × T8.47 × exp[(2431.3 ± 322.0)/T] cm3 molecule-1s-1 and the corresponding rate coefficient at room temperature was calculated to be: [Formula: see text]= 4.91 × 10-14 cm3 molecule-1s-1. A very good agreement was observed between the experimentally measured and theoretically calculated rate coefficients at 298K. Thermochemical parameters as well as branching ratios for the reaction of (phenyl radical + EF) are also discussed in this manuscript.

Kinetics study on the reactions of dimethyl ether with triplet oxygen and hydrogen atoms

The reaction kinetics of dimethyl ether (CH3OCH3) with O(3P) and hydrogen atom have been studied. Electronic energies were calculated by single-point energy calculations at the CCSD(T)/cc-PVnZ (n = D, T, Q) levels. Rate coefficients of the reactions were calculated by the canonical variational transition state theory (CVT) with the small curvature tunneling (SCT) correction. The hydrogen-abstraction channels involve the formation of reactant complexes in the entrance channels and show positive temperature dependence. Their branching rates are almost 1.0 at the temperature less than 1000 K.

Multistructural Variational Reaction Kinetics of the Simplest Unsaturated Methyl Ester: H-Abstraction from Methyl Acrylate by H, OH, CH3, and HO2 Radicals.

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH3/HO2 radicals have been investigated theoretically in the present work. For these reactions, the reaction energies and barrier heights are first computed using several density functionals and compared to the coupled cluster CCSD(T)-F12/jun-cc-pVTZ benchmark calculations. The M062X/maug-cc-pVTZ method shows the best performance with the smallest mean unsigned deviation (MUD) of 0.42 kcal mol-1. Combined with the electronic structure calculations using the M062X/maug-cc-pVTZ method, the multistructural canonical variational transition-state theory (MS-CVT) with small-curvature tunneling (SCT) is employed to calculate the reaction rate constants at 500-2000 K. The variational effect is between 0.56 and 1.0, the multistructural torsional anharmonicity factor ranges from 0.004 to 4.57, and the tunneling coefficient is in the range of 0.5-4.70. Notably, given the existence of reactant complexes (RCs) between reactants and transition states for the reaction systems MA + OH/HO2, we further compare the rate constants under the low-pressure limit (LPL) kinetic model, which treats the reaction as a single-step process and neglects RCs, and the pre-equilibrium model, which takes RCs into account in the reaction and treats the reaction as a two-step process. The rate constants calculated by these two models are similar within the combustion temperature range, and apparent differences occur at lower temperatures. In addition, we determine the branching ratios as a function of temperature and find that the methyl site (S3) abstractions by OH and H radicals are dominant in the low- and high-temperature ranges, respectively. Moreover, we update the kinetic model with the calculated H-abstraction rate constants to simulate the ignition delay times of MA. The simulations of the updated model are in good agreement with experimental results. The accurate reaction kinetics determined in this work are useful for the understanding and prediction of consumption branching fractions and ignition properties of the unsaturated methyl esters.

Initial Thermal Decomposition Mechanism of (NH2)2C=C(NO2)(ONO) Revealed by Double-Hybrid Density Functional Calculations.

This work employs double-hybrid density functionals to re-examine the CO–NO bond dissociation mechanism of nitrite isomer of 1,1-diamino-2,2-dinitro-ethylene (DADNE) into (NH2)2C=C(NO2)O and nitric monoxide (NO). The calculated results confirm that an activated barrier is present in the CO–NO bond dissociation process of (NH2)2C=C(NO2)(ONO). Furthermore, it is proposed that a radical–radical adduct is involved in the exit dissociation path with subsequent dissociation to separate (NH2)2C=C(NO2)O and NO radicals. The activation and reaction enthalpies at 298.15 K for the nitrite isomer dissociation are predicted to be 43.6 and 5.4 kJ mol–1 at the B2PLYP/6-31G(d,p) level, respectively. Employing the B2PLYP/6-31G(d,p) reaction energetics, gradient, Hessian, and geometries, the kinetic model for the CO–NO bond dissociation of (NH2)2C=C(NO2)(ONO) is obtained by a fitting to the modified Arrhenius form 1.05 × 1013(T/300)0.39 exp[−27.80(T + 205.32)/R(T2 + 205.322)] in units of per second over the temperature range 200–3000 K based on the canonical variational transition-state theory with multidimensional small-curvature tunneling.

Theoretical kinetics of hydrogen abstraction reactions from propanol isomers by hydroperoxyl radical: Implication for combustion modeling

Propanol isomers are potential alternatives to conventional fossil fuels and have attracted considerable attentions from combustion community. Hydrogen atom abstraction reactions from alcohols by HO2 radical plays an important role in alcohol oxidation chemistry. In this work, the temperature-dependent rate constants and branching ratios of the hydrogen atom abstraction reactions from n-propanol and iso-propanol by HO2 were calculated using the multistructural variational transition state theory with small-curvature tunneling approach. Results show that the multistructural torsional anharmonicity effects significantly influence the calculation of site-specific rate constants and branching ratios, which need to be considered for reaction systems having multiple distinguishable structures. Significant discrepancy observed in the theoretically calculated rate constants from different sources highlight the necessity and importance of accurate calculation efforts, as it can provide the experimentally unavailable site-dependent rate constants and branching ratios. The effects of our newly calculated rate constants were examined by incorporating them into a recent chemical kinetic model developed by Saggese et al. The chemical kinetic model implemented with our newly calculated rate constants exhibits decreased fuel reactivity, especially at low temperatures and high pressures. Also, the chemical kinetic model using our theoretical calculations shows better prediction in the mole fractions of aldehydes, which are key oxidation intermediates in propanol oxidation.

Atmospheric fate of formyl chloride and mechanisms of the gas-phase reactions with OH radicals and Cl atoms

Formyl chloride (HCOCl) is a major product in the atmospheric degradation of several chlorinated hydrocarbons, but its atmospheric fate is not well known. Here, the kinetics of HCOCl with OH radicals and Cl atoms were studied over the temperature range of 180 to 400 K using Canonical Variational Transition state theory including Small Curvature Tunneling correction (CVT/SCT). The obtained temperature-dependent rate coefficients for the reactions of HCOCl with OH and Cl are (1.86 ± 0.7) × 10−11 exp[(−1217 ± 19)/T] cm3molecule−1s−1 and (9.87 ± 0.6) × 10−12 exp[(−747 ± 33)/T] cm3molecule−1s−1, respectively. Reaction mechanisms and thermodynamic parameters for the title reactions were studied, and the atmospheric lifetime of HCOCl was determined.

The Origin of the E/Z Isomer Ratio of Imines in the Interstellar Medium

Abstract Recent astronomical observations of both isomers E and Z of imines such as cyanomethanimine, ethanimine, and 2-propyn-1-imine have revealed that the abundances in the interstellar medium (ISM) of these isomers differ by factors of ∼3–10. Several theories have been proposed to explain the observed behavior, but none of them successfully explains the [E]/[Z] ratios. In this work we present a detailed study of the kinetics of the one-step E-Z isomerization reactions of cyanomethanimine, ethanimine, and 2-propyn-1-imine under interstellar conditions (in the 10–400 K temperature range). This reaction was previously thought to be nonviable in the ISM due to its associated high-energy barrier (about 13,000 K). In this Letter, we show that considering the multidimensional small curvature tunneling approximation, the tunneling effect enables the isomerization even at low temperatures. This is due to the fact that the representative tunneling energy lies in the vibrational ground state of the least stable isomer up to approximately 150 K, making the reaction constants of the isomerization from the least stable to the most stable isomer basically constant. The predicted [E]/[Z] ratios are almost the same as those reported from the astronomical observations for all imines observed. This study demonstrates that the [E]/[Z] ratio of imines in the ISM strongly depends on their relative stability.

Theoretical study of the reactions of hydrogen atom with methyl and ethyl hydroperoxides

An ab initio dynamic study of the reactions of H atom with CH3OOH and CH3CH2OOH was performed over the temperature range of 250–1500 K. The geometries of all stationary points were optimised at the MP2, B3LYP and M06-2X theories. The final electronic energies for all species were refined by the CCSD(T) and QCISD(T) methods. The rate coefficients were calculated using canonical variational transition state theory (CVT) with small-curvature tunnelling (SCT) correction or the multichannel Rice−Ramsperger−Kassel−Marcus (RRKM) and CVT methods. The results showed that the channel of H-abstraction from OH group is dominant channel only at temperature less than 350 K. With the temperature higher than 350 K, the OH-transfer channel becomes dominant channel. For the CH3OOH + H reaction, the channel to produce H2 + CH2O + OH becomes the dominant channel at temperature higher than 800 K. For the CH3CH2OOH + H reaction, the channel to produce H2 + CH3CH2O + OH is important at temperature less than 600 K and has a maximum branching ratio of 0.3 around 350 K.

Theoretical study of the reactions of nitrogen dioxide with hydrogen and methyl peroxides

Four channels for HOOH + NO2 system and eight channels for CH3OOH + NO2 system were explored by the MP2 level of theory. Energies for all species were evaluated at the CCSD(T) theory. The rate coefficients calculated using CVT theory including the small curvature tunneling (SCT) correction agreed well with the available value. The results showed that the three channels to produce HNO2, trans-HONO and cis-HONO proceeded simultaneously for the same H-abstraction from HOOH or CH3OOH by NO2, and the H-abstraction from hydroperoxyl to produce cis-HONO was the dominant channel at the temperature less than 1000 K.

Quantum chemical insights into the atmospheric reactions of CH2FCH2OH with OH radical, fate of CH2FC•HOH radical and ozone formation potential

Quantum chemical calculations on the gas-phase H-abstraction reactions of CH2FCH2OH with OH radical were performed using the CCSD(T)//M06-2X/Aug-cc-pVTZ method. The rate coefficients were evaluated using the VTST−ISPE method incorporating small curvature tunnelling (SCT) correction. The orientation of F and –OH groups plays a major role in the reactivity of the CH2FCH2OH compound. The calculated rate coefficient k(CH2FCH2OH + OH) = 6.73 × 10−13 cm3 molecule−1 s−1 at 298K is in close agreement with the reported experimental result. Atmospheric lifetime and Global Warming Potential at a 100-year time horizon of CH2FCH2OH are reported. Atmospheric loss processes of CH2FC•HOH radical through several competing reaction channels are discussed. Rate coefficient values of the primary oxidation products: CH2FCOOH + OH = 5.63 × 10−14 and CH2FCHO + OH = 2.49 × 10−12 cm3 molecule−1 s−1 were reported for the first time. Regeneration of hydroxyl radical (OH) from the degradation reaction of CH2FC•HOH along with the ozone formation potential in the troposphere are deliberated.

Trifluoroacetaldehyde aminolysis catalyzed by a single water molecule: An important sink pathway for trifluoroacetaldehyde and a potential pathway for secondary organic aerosol growth

High-level ab initio calculations and variational transition state theory with small curvature tunneling have been used to study the aminolysis of trifluoroacetaldehyde catalyzed by a single water molecule. The results of energetic studies indicate that the energy barrier of the trifluoroacetaldehyde aminolysis reaction decreases along ammonia, methylamine, and dimethylamine. A single water molecule can significantly reduce the reaction energy barrier of trifluoroacetaldehyde aminolysis. In particular, the reaction involving dimethylamine has the lowest reaction energy barrier and the energy barrier is decreased to be −9.69 kcal/mol in the CF3CHO + (CH3)2NH + H2O reaction relative to CF3CHO, (CH3)2NH, and H2O separated reactants. Kinetic calculation shows that the rate coefficient of CF3CHO + (CH3)2NH⋯H2O ranges from 7.78 × 10−13 to 4.45 × 10−16 cm3⋅molecules−1⋅s−1 at 190–350 K. Here, we find an important CF3CHO elimination pathway, which can compete with the reaction of CF3CHO + OH when the OH concentration is 104 molecules⋅cm−3 and the dimethylamine concentration is higher than 109 molecules⋅cm−3 in the temperature range between 240 and 330 K. In addition, once (CH3)2NCH(OH)CF3 is formed by the reaction of CF3CHO + (CH3)2NH + H2O, it will further promote the growth of secondary organic aerosols.