Multi-structural Canonical Variational Transition State Research Articles

Strong dependence on multistructural anharmonicity of the relative rates of intramolecular H-migration in alkylperoxyl and methylcyclohexylperoxyl radicals

Alkylperoxyl (RO2) and hydroperoxyalkyl (QOOH) species are crucial intermediates produced during the autoignition of transport fuels, and consequently their thermodynamic and kinetic data are very important. Since these data are not available experimentally, we take up the challenge of computing them from first principles. We compare the 1,5 shifts in 2-methyl-4-heptylperoxyl radical and 3-methylcyclohexylperoxy radical to clarify the similarities and differences of the kinetics of chain alkanes and cycloalkanes in the fuel oxidation mechanism. In order to take account of the multistructural and coupled torsional anharmonicity, we employ multistructural canonical variational transition state theory (MS-VTST) with multidimensional tunneling to calculate rate constants. The calculations explicitly include all conformers: 6 and 154 for the two reactants, 7, 6, 1, and 1 for the four transition states, and 289, 282, 8, and 5 for the four products (which are the reactants for the reverse reactions). The effects of various factors on the thermodynamic data and calculated rate constants are evaluated in detail. We found that multistructural torsional anharmonicity has a very large effect on the rate constants. This work illustrates how modern density functional and dynamics methods make it possible to calculate accurate rate constants for both branched-chain and cycloalkane peroxy radicals and hydroperoxy species.

Theoretical study on hydrogen abstraction reactions from cyclopentanol by hydroxyl radical

Cyclopentanol is a novel biofuel with high resistance to auto-ignition, which can be used as a promising additive to gasoline fuel. It is of great importance to gain a comprehensive understanding on the oxidation chemistry of cyclopentanol. The reaction of cyclopentanol with hydroxyl radical plays an important role in both combustion and atmospheric chemistry. In this study, we calculate the rate constants of cyclopentanol reaction with hydroxyl radical over a wide temperature range of 200–2500 K, using multistructural canonical variational transition state theory. The effects of variational, quantum tunneling and multistructural torsional anharmonicity were analyzed. The results show that the variational and multistructural torsional anharmonicity effects play a key role, while the quantum tunneling effect is nearly negligible for the studied reaction system. Moreover, the rate constants of reactions with submerged forward barrier heights show a non-monotonic behavior, while a positive temperature dependence is found for the rate constants of reactions with positive forward barrier heights. Our calculated total rate constants are in good agreement with those estimated in the literature (Cai et al., CNF, 2019), however, significant discrepancies are observed in the branching ratios of reaction channels. The rate constants reported in this study are expected to be more accurate and will benefit the understanding on the oxidation chemistry of cyclopentanol.

Kinetic investigation of the reaction of ethylperoxy radicals with ethanol

AbstractThe thermodynamic and kinetic investigation for the reaction of ethylperoxy radical (CH3CH2OO•) with ethanol (CH3CH2OH) was studied computationally with variational transition state theory. The geometry optimization calculations showed that both the reactants have two conformers. The energetics and thermodynamic properties were calculated using the G4 composite method. The results showed that the abstraction of H atom from the ─CH2─ site is most feasible, when compared to the other pathways, since it has the lowest energy barrier. The rate coefficient calculations for the title reaction were carried out using the multistructural canonical variational transition state theory with small curvature tunneling corrections in the temperature range of 400‐1500 K. The results showed a similar trend to that of the energetics in which the abstraction of the H atom from the ─CH2─ site has a larger rate coefficient, when compared to other reaction pathways. The reactivity trend towards the H atom abstraction by ethylperoxy radicals varies as ─CH2 > ─CH3 > ─OH. The contributions from the ground‐state conformers of both the reactants were included into the total kinetics using the Boltzmann probability distribution. The obtained temperature‐dependent expression for the studied reaction is ktotal = 1.51 × 10−32 T6.3 exp(−3691/T) cm3 molecule−1 s−1.

Direct dynamics of a large complex hydrocarbon reaction system: The reaction of OH with exo-tricyclodecane (the main component of Jet Propellant-10)

It remains a long-standing challenge to predict the reaction kinetics of large complex hydrocarbon system from first principles. Exo-tricyclo[5.2.1.02,6]decane, which is further called tricyclodecane and which is the major component in JP-10 aviation fuel, and therefore its reaction with hydroxyl radical is very significant. We report high-level theoretical calculations of the rate constants of tricyclodecane + OH for the first time. We show that the exchange-correlation hybrid functional M06-2X with MG3S basis set provides an accurate and efficient calculation of the barrier heights and reaction energies. A total of 20 reaction pathways for hydrogen abstractions from chair and boat tricyclodecane conformations are considered for direct dynamics. Multi-structural canonical variational transition state theory with small-curvature tunneling (MS-CVT/SCT) is adopted to calculate the rate constants of the title reaction at 200–2000 K. The theoretical results obtained using MS-CVT/SCT with curvilinear coordinates are in fairly good agreement with the experimental measurements. We also determine the branching fractions as a function of temperature and find the tertiary abstraction from tricyclodecane is dominant. This work highlights the significance of modern DFT methods for studying combustion kinetics of large fuel molecules.

Kinetic modeling for unimolecular β-scission of the methoxymethyl radical from quantum chemical and RRKM analyses

Unimolecular β-scission of the methoxymethyl (CH3OCH2) radical has been considered to be the crucial chain-propagating step in both oxidation and pyrolysis of dimethyl ether. The present work employs hybrid density functionals M06-2X, BB1K, B3LYP, and MPW1K with the MG3S basis set as well as double-hybrid density functional B2PLYP and Moller-Plesset perturbation theory MP2 with the TZVP basis set to study the detailed mechanism of unimolecular decomposition of CH3OCH2. Energies of all stationary points are refined with the CCSD(T), QCISD(T), CBS-QB3, and G4 calculations. The minimum energy path was computed at the CCSD(T)/aug-cc-PVTZ//M062X/MG3S level. Kinetic calculations are performed by means of high-pressure multi-structural canonical variational transition state (MS-CVT) theory and pressure-dependent Rice–Ramsperger–Kassel–Marcus (RRKM) theory to clarify the available experimental observations and previous theoretical results. A kinetic model for the low and the high-pressure limiting, and falloff region was extracted. For high pressure limit, k∞ = 2.08 × 1012 (T/300)1.002 exp(–11097.64/T) s−1 at temperatures of 200–2600 K based on the MS-CVT/SCT method. Furthermore, the intermediate falloff curve was found to be best represented by k/k∞=[x/(1+x)]Fcent1/[1+(a+logx)2/(N±ΔN)2] with x = k0/k∞, a = 0.263, N = 1.208, ΔN = 0.096, (+ΔN for (a + logx) < 0 and –ΔN for (a + logx) > 0), and Fcent(DME) = 0.348 independent of temperature. The low and high pressure limiting rate constants have been extracted by extrapolation of the fall-off curves: k0 = [DME] 2.49 × 1016 (T/300)0.053 exp(–9067.58/T) cm3 mol–1 s–1 and k∞ = 1.88 × 1012 (T/300)1.05 exp(–11061.79/T) s−1 at temperatures of 450–800 K, which agree well with the reported experimental low and high pressure limit results.

Kinetics of Hydrogen Radical Reactions with Toluene Including Chemical Activation Theory Employing System-Specific Quantum RRK Theory Calibrated by Variational Transition State Theory.

Pressure-dependent reactions are ubiquitous in combustion and atmospheric chemistry. We employ a new calibration procedure for quantum Rice-Ramsperger-Kassel (QRRK) unimolecular rate theory within a chemical activation mechanism to calculate the pressure-falloff effect of a radical association with an aromatic ring. The new theoretical framework is applied to the reaction of H with toluene, which is a prototypical reaction in the combustion chemistry of aromatic hydrocarbons present in most fuels. Both the hydrogen abstraction reactions and the hydrogen addition reactions are calculated. Our system-specific (SS) QRRK approach is adjusted with SS parameters to agree with multistructural canonical variational transition state theory with multidimensional tunneling (MS-CVT/SCT) at the high-pressure limit. The new method avoids the need for the usual empirical estimations of the QRRK parameters, and it eliminates the need for variational transition state theory calculations as a function of energy, although in this first application we do validate the falloff curves by comparing SS-QRRK results without tunneling to multistructural microcanonical variational transition state theory (MS-μVT) rate constants without tunneling. At low temperatures, the two approaches agree well with each other, but at high temperatures, SS-QRRK tends to overestimate falloff slightly. We also show that the variational effect is important in computing the energy-resolved rate constants. Multiple-structure anharmonicity, torsional-potential anharmonicity, and high-frequency-mode vibrational anharmonicity are all included in the rate computations, and torsional anharmonicity effects on the density of states are investigated. Branching fractions, which are both temperature- and pressure-dependent (and for which only limited data is available from experiment), are predicted as a function of pressure.

Silane-initiated nucleation in chemically active plasmas: validation of density functionals, mechanisms, and pressure-dependent variational transition state calculations.

The growth of anionic silicon hydride clusters is a critically important process in nanodusty plasmas. In the current study, we focus on the formation of homologs of silylene (Sin+1H2n+2(-), n = 3, 4) and silyl (SinH2n+1(-), n = 4, 5) anions via anion-neutral reaction pathways. Species like silyl or silylene anions and their related elementary reactions, which are involved in the formation of silicon hydride clusters, were not used in developing exchange-correlation (xc) density functionals (i.e., they were not included in the training set of semiempirical density functionals); therefore, we explored the accuracy of various widely used xc density functionals based on reaction energies and barrier heights. Among the 21 density functionals we tested, M06-2X has the best performance for a hybrid functional, and MN15-L has the best performance for a local functional. Thermal rate constants of the elementary reactions involved in the reaction mechanism are calculated using M06-2X and multistructural canonical variational transition state theory with the small-curvature tunneling approximation (MS-CVT/SCT). The pressure dependence of unimolecular isomerization reactions is treated with system-specific quantum RRK theory (SS-QRRK) and the Lindemann-Hinshelwood mechanism.

Multistructural Variational Transition State Theory: Kinetics of the Hydrogen Abstraction from Carbon-2 of 2-Methyl-1-propanol by Hydroperoxyl Radical Including All Structures and Torsional Anharmonicity

We calculated the forward and reverse rate constants of the hydrogen abstraction reaction from carbon-2 of 2-methyl-1-propanol by hydroperoxyl radical over the temperature range 250-2400 K by using multistructural canonical variational transition state theory (MS-CVT) including both multiple-structure and torsional potential anharmonicity effects by the multistructural torsional anharmonicity (MS-T) method. In these calculations, multidimensional tunneling (MT) probabilities used to compute the tunneling transmission coefficients were evaluated by the small-curvature tunneling (SCT) approximation. Comparison with the rate constants obtained by the single-structural harmonic oscillator (SS-HO) approximation shows that multistructural anharmonicity increases the forward rate constants for all temperatures, but the reverse rate constants are reduced for temperatures lower than 430 K and increased for higher temperatures. The neglect of multistructural torsional anharmonicity would lead to errors of factors of 1.5, 8.8, and 13 at 300, 1000, and 2400 K, respectively, for the forward reaction, and would lead to errors of factors of 0.76, 3.0, and 6.0, respectively, at these temperatures for the reverse reaction.

Multi-structural variational transition state theory: kinetics of the 1,5-hydrogen shift isomerization of the 1-butoxyl radical including all structures and torsional anharmonicity

We investigate the statistical thermodynamics and kinetics of the 1,5-hydrogen shift isomerization reaction of the 1-butoxyl radical and its reverse isomerization. The partition functions and thermodynamic functions (entropy, enthalpy, heat capacity, and Gibbs free energy) are calculated using the multi-structural torsional (MS-T) anharmonicity method including all structures for three species (reactant, product, and transition state) involved in the reaction. The calculated thermodynamic quantities have been compared to those estimated by the empirical group additivity (GA) method. The kinetics of the unimolecular isomerization reaction was investigated using multi-structural canonical variational transition state theory (MS-CVT) including both multiple-structure and torsional (MS-T) anharmonicity effects. In these calculations, multidimensional tunneling (MT) probabilities were evaluated by the small-curvature tunneling (SCT) approximation and compared to results obtained with the zero-curvature tunneling (ZCT) approximation. The high-pressure-limit rate constants for both the forward and reverse reactions are reported as calculated by MS-CVT/MT, where MT can be ZCT or SCT. Comparison with the rate constants obtained by the single-structural harmonic oscillator (SS-HO) approximation shows the importance of anharmonicity in the rate constants of these reactions, and the effect of multi-structural anharmonicity is found to be very large. Whereas the tunneling effect increases the rate constants, the MS-T anharmonicity decreases them at all temperatures. The two effects counteract each other at temperatures 385 K and 264 K for forward and reverse reactions, respectively, and tunneling dominates at lower temperatures while MS-T anharmonicity has a larger effect at higher temperatures. The multi-structural torsional anharmonicity effect reduces the final reverse reaction rate constants by a much larger factor than it does to the forward ones as a result of the existence of more low-energy structures of the product 4-hydroxy-1-butyl radical than the reactant 1-butoxyl radical. As a consequence there is also a very large effect on the equilibrium constant. The neglect of multi-structural anharmonicity will lead to large errors in the estimation of reverse reaction rate constants.