Torsional anharmonicity in transition state theory calculations

Global Master Equation Analysis of Rate Data for the Reaction C2H4 + H ⇄ C2H5: ΔfH0⊖C2H5

While forward and reverse rate constants are frequently used to determine enthalpies of reaction and formation, this process is more difficult for pressure-dependent association/dissociation reactions, especially since the forward and reverse reactions are usually studied at very different temperatures. The problems can be overcome by using a data-fitting procedure based on a master equation model. This approach has been applied to existing experimental pressure-dependent forward and reverse rate coefficients for the reaction C₂H₄ + H ⇄ C₂H₅ (k₁, k_-1) using the MESMER code to determine Δ_fH₀^⊖C₂H₅ from the enthalpy of the reaction. New measurements of k₁, k_-1 were included in analysis. They are based on laser flash photolysis with direct observation of H atom time profiles by vacuum ultraviolet laser-induced fluorescence under conditions where the approach to equilibrium could be observed. Measurements were made over the temperature range 798-828 K and with [He] from 2.33 to 7.21 × 10¹⁸ molecule cm^-3. These data were then combined with a wide range of existing experimental data with helium as the bath gas (112 measurements of k₁ and k_-1, covering the temperature range 285-1094 K, and [He] = 7.1 × 10¹⁵-1.9 × 10¹⁹ molecule cm^-3) and fitted using the master equation solver MESMER. The required vibrational frequencies and rotational constants of the system were obtained from ab initio calculations, and the activation threshold for association (ΔE_thresh), enthalpy of reaction (Δ_rH₀^⊖), imaginary frequency (υ_imag), and helium energy-transfer parameters (⟨ΔE⟩_d,298(T/298)ⁿ) were optimized. The resulting parameters (errors are 2σ) are ΔE_thresh = 11.43 ± 0.34 kJ mol^-1, Δ_rH₀^⊖ = -145.34 ± 0.60 kJ mol^-1, υ_imag = 730 ± 130 cm^-1, ⟨ΔE⟩_d,298 = 54.2 ± 7.6 cm^-1, and n = 1.17 ± 0.12. A value of Δ_fH_298.15^⊖(C₂H₅) = 120.49 ± 0.57 kJ mol^-1 is obtained by combining Δ_rH₀^⊖ with standard enthalpies of formation for H and C₂H₄ and making the appropriate temperature corrections. The dependence of these parameters on how the internal rotor and CH₂ inversion modes are treated has been explored. The experimental data for other bath gases have been analyzed, and data sets compatible with the potential energy surface parameters determined above have been identified. The parameters are virtually identical but with slightly smaller error limits. Parameterization of k₁, k_-1 using the Troe formalization has been used to investigate competition between ethyl decomposition and reaction with oxygen under combustion conditions.

Accounting for conformational flexibility and torsional anharmonicity in the H + CH3CH2OH hydrogen abstraction reactions: a multi-path variational transition state theory study

This work reports a detailed theoretical study of the hydrogen abstraction reactions from ethanol by atomic hydrogen. The calculated thermal rate constants take into account torsional anharmonicity and conformational flexibility, in addition to the variational and tunneling effects. Specifically, the kinetics calculations were performed by using multi-path canonical variational transition state theory with least-action path tunneling corrections, to which we have added the two-dimensional non-separable method to take into account torsional anharmonicity. The multi-path thermal rate constant is expressed as a sum over conformational reaction channels. Each of these channels includes all the transition states that can be reached by internal rotations. The results show that, in the interval of temperatures between 250 and 2500 K, the account for multiple paths leads to higher thermal rate constants with respect to the single path approach, mainly at low and at high temperatures. In addition, torsional anharmonicity enhances the slope of the Arrhenius plot in this range of temperatures. Finally, we show that the incorporation of tunneling into the hydrogen abstraction reactions substantially changes the contribution of each of the transition states to the conformational reaction channel.

Quantum instanton calculation of rate constant for CH4 + OH → CH3 + H2O reaction: torsional anharmonicity and kinetic isotope effect

Thermal rate constants for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The results reveal that the quantum effect is remarkable for the reaction at both low and high temperatures, and the obtained rates are in good agreement with experimental measurements at high temperatures. Compared to the harmonic approximation, the torsional anharmonic effect of the internal rotation has a little influence on the rates at low temperatures, however, it enhances the rate by about 20% at 1000 K. In addition, the free energy barriers for the isotopic reactions and the temperature dependence of kinetic isotope effects are also investigated. Generally speaking, for the title reaction, the replacement of OH with OD will reduce the free energy barrier, while substituting D for H (connected to C) will increase the free energy barrier.

Pathways for conformational change in nitrogen regulatory protein C from discrete path sampling

Pathways corresponding to the conformational change in nitrogen regulatory protein C are calculated using the CHARMM19 force field with an implicit solvation model. Our analysis employs the discrete path sampling approach to grow a database of local minima and transition states from the potential energy surface that contains kinetically relevant pathways. The pathways with the largest contribution to the phenomenological two-state rate constants are found to exhibit a number of structural features that agree with experimental observations. Further details of the calculated pathways for conformational change may therefore provide useful predictions of how this large-scale motion is achieved.