Transition State Theory Results Research Articles

Mechanism analysis of fuel-N oxidation during ammonia-coal co-combustion: Influence of H2O

To reduce CO2 emissions from thermal power, gradual substitution of coal by ammonia has been widely considered. However, co-firing of ammonia and coal has the risk of increasing NOx emissions. Moreover, the combustion of hydrogen-rich carrier ammonia can generate H2O, which affects the conversion paths of N in ammonia-N/coal-N. Therefore, it is necessary to study the oxidation mechanism of N during ammonia-coal co-combustion under an H2O atmosphere. In this study, the NH/char/O2 co-combustion system, in the presence of OH, was constructed by density functional theory. The molecular mechanism of the oxidation of amino groups into NO, HNO, and N2O, with the participation of OH, was explored. Combined with wave function analysis, the promoting effect of OH on the oxidation of ammonia-N and coal char-N into N oxidation products was clarified. The energy barrier values of ammonia oxidation to HNO and char oxidation to NO could be reduced by 52.32 and 75.72 kJ/mol, respectively, compared with those without OH participation. By coupling with the kinetic results of transition state theory (TST), it was concluded that the participation of OH was more conducive to the oxidation of ammonia to NO, and its formation rate is higher than that of HNO or N2O in the study temperature range. Furthermore, this work revealed the molecular path of HO2 formation in the NH/char/O2 co-combustion system with the participation of OH, and obtained the necessary factors and temperature conditions for its formation (higher than 1673 K). The above results are conducive to understanding the characteristics of NOx generation during ammonia-coal co-combustion, which can provide theoretical guidance and reference for the development of N conversion mechanism of ammonia-coal co-firing.

Water Exchange Rates and Molecular Mechanism around Aqueous Halide Ions

Molecular dynamics simulations were performed to systematically study the water-exchange mechanism around aqueous chloride, bromide, and iodide ions. Transition state theory, Grote-Hynes theory, and the reactive flux method were employed to compute water exchange rates. We computed the pressure dependence of rate constants and the corresponding activation volumes to investigate the mechanism of the solvent exchange event. The activation volumes obtained using the transition state theory rate constants are negative for all the three anions, thus indicating an associative mechanism. Contrary to the transition state theory results, activation volumes obtained using rate constants from Grote-Hynes theory and the reactive flux method are positive, thus indicating a dissociative mechanism.

Rate coefficients and kinetic isotope effects of the X + CH4 → CH3 + HX (X = H, D, Mu) reactions from ring polymer molecular dynamics

The thermal rate coefficients and kinetic isotope effects have been calculated using ring polymer molecular dynamics (RPMD) for the prototypical reactions between methane and several hydrogen isotopes (H, D, and Mu). The excellent agreement with the theoretical rate coefficients of the H + CH4 reaction obtained previously from a multi-configuration time-dependent Hartree calculation on the same potential energy surface provides strong evidence for the accuracy of the RPMD approach. These quantum mechanical rate coefficients are also in good agreement with the results obtained previously using the transition-state theory with semi-classical tunneling corrections for the H∕D + CH4 reactions. However, it is shown that the RPMD rate coefficients for the ultralight Mu reaction with CH4 are significantly smaller than the experimental data, presumably suggesting inaccuracies in the potential energy surface and∕or experimental errors. Significant discrepancies between the RPMD and transition-state theory results have also been found for this challenging system.

An extensive investigation of reactions involved in the nitrogen trifluoride dissociation

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF3 + F and the other is composed of the unimolecular and the abstraction production of N2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF3.

Kinetics and dynamics of the NH3 + H → NH2 + H2 reaction using transition state methods, quasi-classical trajectories, and quantum-mechanical scattering

On a recent analytical potential energy surface developed by two of the authors, an exhaustive kinetics study, using variational transition state theory with multidimensional tunneling effect, and dynamics study, using both quasi-classical trajectory and full-dimensional quantum scattering methods, was carried out to understand the reactivity of the NH(3) + H → NH(2) + H(2) gas-phase reaction. Initial state-selected time-dependent wave packet calculations using a full-dimensional model were performed, where the total reaction probabilities were calculated for the initial ground vibrational state and for four excited vibrational states of ammonia. Thermal rate constants were calculated for the temperature range 200-2000 K using the three methods and compared with available experimental data. We found that (a) the total reaction probabilities are very small, (b) the symmetric and asymmetric N-H stretch excitations enhance the reactivity, (c) the quantum-mechanical calculated thermal rate constants are about one order of magnitude smaller than the transition state theory results, which reproduce the experimental evidence, and (d) quasi-classical trajectory calculations, which were performed with the main goal of analyzing the influence of the zero-point energy problem on the final dynamics results, reproduce the quantum scattering calculations on the same surface.

Global dynamics and transition state theories: Comparative study of reaction rate constants for gas‐phase chemical reactions

In this review article, we present a systematic comparison of the theoretical rate constants for a range of bimolecular reactions that are calculated by using three different classes of theoretical methods: quantum dynamics (QD), quasi-classical trajectory (QCT), and transition state theory (TST) approaches. The study shows that the difference of rate constants between TST results and those of the global dynamics methods (QD and QCT) are seen to be related to a number of factors including the number of degrees-of-freedom (DOF), the density of states at transition state (TS), etc. For reactions with more DOF and higher density of states at the TS, it is found that the rate constants from TST calculations are systematically higher than those obtained from global dynamics calculations, indicating large recrossing effect for these systems. The physical insight of this phenomenon is elucidated in the present review.

Seven-dimensional quantum dynamics study of the H+NH3→H2+NH2 reaction

Initial state-selected time-dependent wave packet dynamics calculations have been performed for the H+NH3-->H2+NH2 reaction using a seven-dimensional model and an analytical potential energy surface based on the one developed by Corchado and Espinosa-Garcia [J. Chem. Phys. 106, 4013 (1997)]. The model assumes that the two spectator NH bonds are fixed at their equilibrium values. The total reaction probabilities are calculated for the initial ground and seven excited states of NH3 with total angular momentum J=0. The converged cross sections for the reaction are also reported for these initial states. Thermal rate constants are calculated for the temperature range 200-2000 K and compared with transition state theory results and the available experimental data. The study shows that (a) the total reaction probabilities are overall very small, (b) the symmetric and asymmetric NH stretch excitations enhance the reaction significantly and almost all of the excited energy deposited was used to reduce the reaction threshold, (c) the excitation of the umbrella and bending motion have a smaller contribution to the enhancement of reactivity, (d) the main contribution to the thermal rate constants is thought to come from the ground state at low temperatures and from the stretch excited states at high temperatures, and (e) the calculated thermal rate constants are three to ten times smaller than the experimental data and transition state theory results.

Transition state theory and dynamical corrections in ergodic systems

The results of transition state theory are derived rigorously in the general context of ergodic dynamical systems defined by a vector field on a Riemannian manifold. A new perspective on how to compute the dynamical corrections to the transition state theory transition frequency is given. Hamiltonian dynamical systems are considered a special case and the so-called Marcus formula for the rate constant is re-derived.

Ab initio rate constants from hyperspherical quantum scattering: application to H+C2H6 and H+CH3OH.

The dynamics and kinetics of the abstraction reactions of H atoms with ethane and methanol have been studied using a quantum mechanical procedure. Bonds being broken and formed are treated with explicit hyperspherical quantum dynamics. The ab initio potential energy surfaces for these reactions have been developed from a minimal number of grid points (average of 48 points) and are given by analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimized using the second order perturbation theory method with a correlation consistent polarized valence triple zeta basis set. Single point energies are calculated on the optimized geometries with the coupled cluster theory and the same basis set. The reaction of H with C2H6 is endothermic by 1.5 kcal/mol and has a vibrationally adiabatic barrier of 12 kcal/mol. The reaction of H with CH3OH presents two reactive channels: the methoxy and the hydroxymethyl channels. The former is endothermic by 0.24 kcal/mol and has a vibrationally adiabatic barrier of 13.29 kcal/mol, the latter reaction is exothermic by 7.87 kcal/mol and has a vibrationally adiabatic barrier of 8.56 kcal/mol. We report state-to-state and state-selected cross sections together with state-to-state rate constants for the title reactions. Thermal rate constants for these reactions exhibit large quantum tunneling effects when compared to conventional transition state theory results. For H+CH3OH, it is found that the CH2OH product is the dominant channel, and that the CH3O channel contributes just 2% at 500 K. For both reactions, rate constants are in good agreement with some measurements.

A Quantum Dynamics Study of D2 + OH → DOH + D on the WSLFH Potential Energy Function

The quantum dynamics of the reaction D2 + OH → DOH + D on the Wu−Schatz−Lendvay−Fang−Harding ab initio-based potential energy function is investigated. A recently developed four-atom implementation of the real wave packet method is employed. Extensive six-dimensional calculations for a total angular momentum of J = 0 and, within the helicity-decoupled approximation, numerous J > 0 calculations are performed. Cross sections and rate constants for reaction are estimated using a J-shifting procedure and compared with quasi-classical trajectory, transition state theory, and experimental results. The results are also contrasted with comparable results for H2 + OH. A surprising feature is that our rate constants agree best with zero-curvature transition state theory results, indicating that tunneling may not be as important as expected.

Comparison of transition state theory with quantum scattering theory for the reaction Li+HF→LiF+H

The validity of transition state theory is examined for the bimolecular reaction of Li+HF→LiF+H. Accurate three-dimensional quantum scattering theory calculations of the cumulative reaction probability are reported for energies ranging from threshold (0.255 eV) up to 0.600 eV and a total angular momentum J of 0. Transition state theory estimates of the effect of both the entrance and exit channels on the cumulative reaction probability are reported for the same energy range and J value. The transition state theory results are found to provide an accurate description of the smoothed energy dependence of the cumulative reaction probabilities with a maximum disagreement between the two calculations of about 25% arising at the highest energy considered of 0.6 eV.

The diffusion coefficient beyond TST and jump theory

Jump theory (JT) and transition state theory (TST) are usually employed to study atom diffusion at crystal surfaces. In the Fokker-Planck (FP) approach the continuous motion of the diffusing particle and the non-equilibrium effects are incorporated in a natural way; JT and TST results are obtained as limiting cases in certain ranges of the parameters. High-temperature and low-friction effects are studied solving the FP equation with a periodic adiabatic potential and a position-dependent friction.

The diffusion coefficient beyond TST and jump theory

Abstract Jump theory (JT) and transition state theory (TST) are usually employed to study atom diffusion at crystal surfaces. In the Fokker-Planck (FP) approach the continuous motion of the diffusing particle and the non-equilibrium effects are incorporated in a natural way; JT and TST results are obtained as limiting cases in certain ranges of the parameters. High-temperature and low-friction effects are studied solving the FP equation with a periodic adiabatic potential and a position-dependent friction.

The collinear Cl + XY system (X,Y=H,D,T). A comparison between quantum mechanical, classical, and transition state theory results

In this work, exact quantum mechanical (QM) and classical (CL) transition probabilities for the Cl+XY (X,Y=H,D,T) collinear system are compared. The calculations were performed using a semiempirical LEPS surface. The main features discussed are tunneling and threshold behavior for both the ground state and the first excited state of the hydrogen molecule. In the second part, kinetic isotope effects are presented and discussed. Those were calculated in three different ways, using (i) QM transition probabilities, (ii) CL transition probabilities, and (iii) transition state theory (TST-1D). Tunneling coefficients kQM/kCL and kQM/kTST were calculated and discussed. Finally, a brief comparison with experimental data was performed.