Joaquin Espinosa-Garcia

Potential energy surface, thermal, and state-selected rate coefficients, and kinetic isotope effects for Cl+CH4→HCl+CH3

A new potential energy surface is reported for the gas-phase reaction Cl+CH4→HCl+CH3. It is based on the analytical function of Jordan and Gilbert for the analog reaction H+CH4→H2+CH3, and it is calibrated by using the experimental thermal rate coefficients and kinetic isotope effects. The forward and reverse thermal rate coefficients were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200–2500 K. This surface is also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate coefficients. We find that excitation of C–H stretching modes and Cl–H stretching modes enhances the rate of both the forward and the reverse reactions, and excitation of the lowest frequency bending mode in the CH4 reactant also enhances the rate coefficient for the forward reaction. However, the vibrational excitation of t...

New analytical potential energy surface for the CH4+H hydrogen abstraction reaction: Thermal rate constants and kinetic isotope effects

A modified and recalibrated potential energy surface for the gas-phase CH4+H→CH3+H2 reaction and its deuterated analogs is reported and tested, which is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and is calibrated with respect to updated experimental and theoretical stationary point (reactants, products, and saddle point) properties, and experimental forward thermal rate constants. The forward and reverse rate constants are calculated using variational transition-state theory with multidimensional tunneling effect over a wide temperature range, 300–2000 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of the tunneling in this reaction. Five sets of kinetic isotope effects are also calculated. In general, they agree with experimental values within the experimental errors. This surface is then used to analyze dynamical features, such as reaction-path curvature, the couplin...

Analytical potential energy surface for the NH3+H↔NH2+H2 reaction: Application of variational transition-state theory and analysis of the equilibrium constants and kinetic isotope effects using curvilinear and rectilinear coordinates

The potential energy surface (PES) for the gas-phase NH3+H↔NH2+H2 reaction is constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. This surface is then used to calculate rate constants with variational transition-state theory over the temperature range 300–2000 K. While the forward rate constants agree with experimental results, the reverse ones are lower by factors of between 4 and 6. Since the same PES is used and these rates are related by detailed balance, this disagreement could indicate an uncertainty in the few available experimental studies for the reverse reaction. We also provide a detailed analysis of the equilibrium constants and of the kinetic isotope effects and compare the results of this analytical PES with earlier ab initio reaction-path calculations. Finally, for the vibrational frequency calculations, we analyze the consequen...

Analytical potential energy surface for the CH4+Cl→CH3+ClH reaction: Application of the variational transition state theory and analysis of the kinetic isotope effects

We present a potential energy surface for the CH4+Cl→CH3+Cl reaction, based on the analytical function J1 for the analog CH4+H→CH3+H2 reaction by Joseph et al. To calibrate the new surface we chose the reactant and product experimental properties as reference data. The forward and reverse rate constants were calculated using variational transition state theory with large curvature transmission coefficients over a wide temperature range, 200–1000 K. The variational effects were concluded to be small for this reaction, and good agreement with experimental rate constants was found in both forward and reverse reactions. The kinetic isotope effects (KIEs) at different temperatures for the forward and reverse reactions were also analyzed showing always a ‘‘normal’’ behavior. The factor analysis of the KIEs in the forward reactions indicated high vibrational and tunneling contributions at low temperatures.

Potential energy surface for a seven-atom reaction. Thermal rate constants and kinetic isotope effects for CH4+OH

The potential energy surface for the gas-phase CH4+OH→CH3+H2O reaction and its deuterated analogs was constructed with suitable functional forms to represent vibrational modes, and was calibrated by using the experimental thermal rate constants and kinetic isotope effects. On this surface, the forward and reverse thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200–2000 K, finding reasonable agreement with the available experimental data. We also calculated six sets of kinetic isotope effects and, in general, the theoretical results underestimate the few available experiments, with exception of the C-13 isotopic effect values which are overestimated. Finally, this surface is also used to analyze dynamical features, such as reaction-path curvature and coupling between the reaction coordinate and vibrational modes.

Theoretical study of the CH4+F→CH3+FH reaction. I. Abinitio reaction path

Using ab initio information, the reaction path for the CH4+F→CH3+FH reaction was traced and the coupling between the reaction coordinate and normal modes was analyzed along it. The FH product may be vibrationally excited due to the nonadiabatic flow of energy between the reaction coordinate and this bound mode, manifest in the large peak in the coupling term after the saddle point. It was concluded that the variational effects were due only to entropic effects. The rate constants were calculated for the temperature range 100–500 K using the variational transition state theory with different levels of calculation to calibrate the reaction path. Agreement was found with the experimental values when using the QCI/b3 shifted curve, avoiding the errors associated with the use of the single‐point calculation.

Theoretical study of the CH4+F→CH3+FH reaction. II. Semiempirical surfaces

We present two semiempirical surfaces for the CH4+F→CH3+FH reaction. One is based on the PM3 semiempirical molecular orbital theory, using parameters specifically calculated for this reaction (SRP method), and the other is based on the analytic function J1 for the CH4+H→CH3+H2 reaction, slightly modified (MJ1 surface). To calibrate the first surface we chose as reference data the reactant and product experimental properties, while to fit the second, we also used ab initio calculated saddle‐point information. Experimental rate constants were not used in the calibration because of their uncertainty. Because of the flattening of these surfaces in the saddle‐point zone, the variational effects are important and the location of the transition state is concluded to be due to entropy effects. The kinetic isotope effects (KIEs) at different temperatures were also analyzed showing reasonable agreement with the experimental value for both surfaces. The factor analysis of the KIEs indicates an inverse tunneling cont...

The hydrogen abstraction reaction H + CH4. I. New analytical potential energy surface based on fitting to ab initio calculations.

Based exclusively on high-level ab initio calculations, a new full-dimensional analytical potential energy surface (PES-2014) for the gas-phase reaction of hydrogen abstraction from methane by an oxygen atom is developed. The ab initio information employed in the fit includes properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, points on the reaction path, and points on the reaction swath, taking especial caution respecting the location and characterization of the intermediate complexes in the entrance and exit channels. By comparing with the reference results we show that the resulting PES-2014 reproduces reasonably well the whole set of ab initio data used in the fitting, obtained at the CCSD(T) = FULL/aug-cc-pVQZ//CCSD(T) = FC/cc-pVTZ single point level, which represents a severe test of the new surface. As a first application, on this analytical surface we perform an extensive dynamics study using quasi-classical trajectory calculations, comparing the results with recent experimental and theoretical data. The excitation function increases with energy (concave-up) reproducing experimental and theoretical information, although our values are somewhat larger. The OH rotovibrational distribution is cold in agreement with experiment. Finally, our results reproduce experimental backward scattering distribution, associated to a rebound mechanism. These results lend confidence to the accuracy of the new surface, which substantially improves the results obtained with our previous surface (PES-2000) for the same system.

THEORETICAL KINETICS STUDY OF THE O(3P) + CH4/CD4 HYDROGEN ABSTRACTION REACTION: THE ROLE OF ANHARMONICITY, RECROSSING EFFECTS AND QUANTUM MECHANICAL TUNNELING

Using a recently developed full-dimensional accurate analytical potential energy surface [Gonzalez-Lavado, E., Corchado, J. C., and Espinosa-Garcia, J. J. Chem. Phys. 2014, 140, 064310], we investigate the thermal rate coefficients of the O((3)P) + CH4/CD4 reactions with ring polymer molecular dynamics (RPMD) and with variational transition-state theory with multidimensional tunneling corrections (VTST/MT). The results of the present calculations are compared with available experimental data for a wide temperature range 200-2500 K. In the classical high-temperature limit, the RPMD results match perfectly the experimental data, whereas VTST results are smaller by a factor of 2. We suggest that this discrepancy is due to the harmonic approximation used in the present VTST calculations, which leads to an overestimation of the variational effects. At low temperatures the tunneling plays an important role, which is captured by both methods, although they both overestimate the experimental values. The analysis of the kinetic isotope effects shows a discrepancy between both approaches, with the VTST values smaller by a factor about 2 at very low temperatures. Unfortunately, no experimental results are available to shed any light on this comparison, which keeps it as an open question.

The SiH4+H→SiH3+H2 reaction: Potential energy surface, rate constants, and kinetic isotope effects

The potential energy surface for the gas-phase SiH4+H→SiH3+H2 reaction and its deuterated analogs was constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. Using this surface, the rate constants were calculated with variational transition-state theory over the temperature range 200–1000 K, finding good agreement with experiments. We also provide a detailed analysis of the kinetic isotope effects and a comparison with the scarce experimental results.