Computational studies on the kinetics and mechanisms for NH3 reactions with ClOx (x=0-4) radicals
ABSTRACT Kinetics and mechanisms for NH3 reactions with ClOx (x = 0-4) radicals have been investigated at the G2M level of theory in conjunction with statistical theory calculations. The geometric parameters of the species and stationary points involved in the reactions have been optimized at the B3LYP/6-311+G(3df,2p) level of theory. Their energetics have been further refined with the G2M method. The results show that the H-abstraction process is the most favorable channel in each reaction and the barriers predicted in decreasing order are OClO > ClO > Cl > ClO3 > ClO4. All reactions were found to occur by hydrogen-bonding complexes; the rate constants for these complex metathetical processes have been calculated in the temperature range 200-2000 K by the microcanonical VTST and/or RRKM theory (for ClO4 + NH3) with Eckart tunneling and multiple reflection corrections. The predicted rate constants are in good agreement with the available experimental data.
SourceAvailable from: Zheng-Zhe Lin[Show abstract] [Hide abstract]
ABSTRACT: A simple model based on the statistics of individual atoms [Europhys. Lett. 94 40002 (2011)] or molecules [Chin. Phys. Lett. 29 080504 (2012)] was used to predict chemical reaction rates without empirical parameters, and its physical basis was further investigated both theoretically and via MD simulations. The model was successfully applied to some reactions of extensive experimental data, showing that the model is significantly better than the conventional transition state theory. It is worth noting that the prediction of the model on ab initio level is much easier than the transition state theory or unimolecular RRKM theory.Chinese Physics B 04/2014; 23(5):050501. DOI:10.1088/1674-1056/23/5/050501 · 1.39 Impact Factor
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ABSTRACT: We present for the first time an analytical potential energy surface (PES) for the reaction of hydrogen abstraction from ammonia by a chlorine atom. It has a very complicated shape with various maxima and minima. The functional form used in the development of the PES considered the stretching and bending nuclear motions, and the parameters in the calibration process were fitted to reproduce exclusively high-level ab initio electronic structure calculations obtained at the CCSD(T) = FULL/aug-cc-pVTZ//CCSD(T) = FC/cc-pVTZ single point level. Thus, the surface is completely symmetric with respect to the permutation of the three ammonia hydrogen atoms, and no experimental information is used in the process. The ab initio information used in the fit includes a wide spectrum of properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, intermediate complexes in the entry and exit channels, points on the reaction path, and points on the reaction swath. By comparison with the reference results, we show that the resulting PES reproduces not only the ab initio data used in the fitting procedure but also other thermochemical and kinetics results computed at the same ab initio level, which were not used in the fit—equilibrium constants, rate constants, and kinetic isotope effects. This represents a severe test for the new surface. As a first application, we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200–2000 K, on this analytical PES. The forward rate constants reproduce the sparse experimental measurements, while the reverse ones reproduce the change of activation energy with temperature reported in another theoretical study, although unfortunately there are no experimental data for comparison. Finally, we analyze the influence of the intermediate complexes and the spin–orbit correction on the kinetics results. In summary, these results indicate that the PES adequately describes this reaction, and the reasonable agreement with experiment lends further confidence to this new surface. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011International Journal of Quantum Chemistry 04/2012; 112(8). DOI:10.1002/qua.23165 · 1.17 Impact Factor
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ABSTRACT: The kinetics and mechanisms for N2H4 + NOx, (x = 1-3) reactions and the related reverse reactions have been investigated by ab initio molecular orbital theory based on the CCSD(T)/CBS/ICCSD/6-31G(d,p), CCSD(T)//B3LYP and CCSD(T)//BH&HLYP methods with the 6-311++G(3df,2p) basis set. These reactions are important to the propulsion chemistry of the N2H4-N2O4 propellant system. The results show that the reactions of N2H4 with NO and NO2 producing N2H3 + HNO and N2H3 + c-HONO by H-abstraction with 33.7 and 10.3 kcal/mol barriers, respectively, are dominant. For the N2H4 + NO3(D-3h) reaction via two pre-reaction van der Waals complexes with 0.5 kcal/mol and -1.6 kcal/mol binding energies produces HNO3 + N2H3 by H-abstraction and t-HONO + N2H3O by concerted O- and H-atom transfers, respectively. The predicted enthalpies of formation of various products at 0 K are in good agreement with available experimental data within reported errors. Furthermore, the rate constants for the forward and some key reverse reactions have been predicted in the temperature range 300-2000 K with tunneling corrections using transition state theory (for direct abstraction) and variational Rice-Ramsperger-Kassel-Marcus theory (for association! decomposition) by solving the master equation.Computational and Theoretical Chemistry 10/2014; 1046:73–80. DOI:10.1016/j.comptc.2014.07.011 · 1.37 Impact Factor