M. Monge-Palacios

Ab initio based potential energy surface and kinetics study of the OH + NH3 hydrogen abstraction reaction

A full-dimensional analytical potential energy surface (PES) for the OH + NH3 → H2O + NH2 gas-phase reaction was developed based exclusively on high-level ab initio calculations. This reaction presents a very complicated shape with wells along the reaction path. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction path, and points on the reaction swath) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the coupled-cluster single double triple (CCSD(T)) = FULL/aug-cc-pVTZ//CCSD(T) = FC/cc-pVTZ single point level, which represents a severe test of the new surface. As a first application, on this analytical PES we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K. The forward rate constants reproduce the experimental measurements, while the reverse ones are slightly underestimated. However, the detailed analysis of the experimental equilibrium constants (from which the reverse rate constants are obtained) permits us to conclude that the experimental reverse rate constants must be re-evaluated. Another severe test of the new surface is the analysis of the kinetic isotope effects (KIEs), which were not included in the fitting procedure. The KIEs reproduce the values obtained from ab initio calculations in the common temperature range, although unfortunately no experimental information is available for comparison.

Dynamics study of the OH + NH3 hydrogen abstraction reaction using QCT calculations based on an analytical potential energy surface

To understand the reactivity and mechanism of the OH + NH3 → H2O + NH2 gas-phase reaction, which evolves through wells in the entrance and exit channels, a detailed dynamics study was carried out using quasi-classical trajectory calculations. The calculations were performed on an analytical potential energy surface (PES) recently developed by our group, PES-2012 [Monge-Palacios et al. J. Chem. Phys. 138, 084305 (2013)]. Most of the available energy appeared as H2O product vibrational energy (54%), reproducing the only experimental evidence, while only the 21% of this energy appeared as NH2 co-product vibrational energy. Both products appeared with cold and broad rotational distributions. The excitation function (constant collision energy in the range 1.0-14.0 kcal mol(-1)) increases smoothly with energy, contrasting with the only theoretical information (reduced-dimensional quantum scattering calculations based on a simplified PES), which presented a peak at low collision energies, related to quantized states. Analysis of the individual reactive trajectories showed that different mechanisms operate depending on the collision energy. Thus, while at high energies (E(coll) ≥ 6 kcal mol(-1)) all trajectories are direct, at low energies about 20%-30% of trajectories are indirect, i.e., with the mediation of a trapping complex, mainly in the product well. Finally, the effect of the zero-point energy constraint on the dynamics properties was analyzed.

Role of vibrational and translational energy in the OH + NH3 reaction: a quasi-classical trajectory study.

Issues such as mode selectivity and Polanyi rules are connected to the effects of vibrational and translational energy in dynamics studies. Using the heavy-light-heavy OH(ν) + NH3(ν) gas-phase reaction, these effects were analyzed by performing quasi-classical trajectory calculations, at low and high collision energies (3.0 and 10.0 kcal mol(-1)), based on an analytical potential energy surface developed by our group. While the independent vibrational excitation of the NH3(ν) modes increases the reactivity by a factor of ∼1.1-2.8 with respect to the vibrational ground-state at both collision energies, OH(ν) stretching acts as a spectator mode. With respect to mode selectivity, we find a different behavior for both reactants. Thus, while the OH(ν) vibrational excitation is maintained in the products, indicating a certain degree of mode selectivity, the vibrational excitation of the NH3(ν) modes is not retained in the products; furthermore, the independent excitation of the N-H asymmetric and symmetric stretch modes leads to similar reaction probabilities, indicating negligible mode selectivity. For this early transition state reaction, translational energy is more effective in driving the reaction than an equivalent amount of energy in vibration, thus extending the validity of Polanyi rules to this polyatomic system. Finally, these results were interpreted on the basis of the existence of little or negligible intramolecular vibrational redistribution in the reactants before collision, while the nonconservation of the zero-point energy has a strong influence.

QCT and QM calculations of the Cl(2P) + NH3 reaction: influence of the reactant well on the dynamics

A detailed dynamics study, using both quasi-classical trajectory (QCT) and reduced-dimensional quantum mechanical (QM) calculations, was carried out to understand the reactivity and mechanism of the Cl((2)P) + NH(3)→ HCl + NH(2) gas-phase reaction, which evolves through deep wells in the entry and exit channels. The calculations were performed on an analytical potential energy surface recently developed by our group, PES-2010 [M. Monge-Palacios, C. Rangel, J. C. Corchado and J. Espinosa-Garcia, Int. J. Quantum. Chem., 2011], together with a simplified model surface, mod-PES, in which the reactant well is removed to analyze its influence. The main finding was that the QCT and QM methods show a change of the reaction probability with collision energy, suggesting a change of the atomic-level mechanism of reaction with energy. This change disappeared when the mod-PES was used, showing that the behaviour at low energies is a direct consequence of the existence of the reactant well. Analysis of the trajectories showed that different mechanisms operate depending on the collision energy. Thus, while at high energies (E(coll) > 5 kcal mol(-1)) practically all trajectories are direct, at low energies (E(coll) < 3 kcal mol(-1)) the trajectories are indirect, i.e., with the mediation of a trapping complex in the entry and/or the exit wells. The reactant complex allows repeated encounters between the reactants, increasing the reaction probability at low energies. The differential cross section results reinforce this change of mechanism, showing also the influence of the reactant well on this reaction. Thus, the PES-2010 surface yields a forward-backward symmetry in the scattering, while when the reactant well is removed with the mod-PES the shape is more isotropic.

The rate constant for radiative association of HF: comparing quantum and classical dynamics.

Radiative association for the formation of hydrogen fluoride through the A(1)Π → X(1)Σ(+) and X(1)Σ(+) → X(1)Σ(+) transitions is studied using quantum and classical dynamics. The total thermal rate constant is obtained for temperatures from 10 K to 20,000 K. Agreement between semiclassical and quantum approaches is observed for the A(1)Π → X(1)Σ(+) rate constant above 2000 K. The agreement is explained by the fact that the corresponding cross section is free of resonances for this system. At temperatures below 2000 K we improve the agreement by implementing a simplified semiclassical expression for the rate constant, which includes a quantum corrected pair distribution. The rate coefficient for the X(1)Σ(+) → X(1)Σ(+) transition is calculated using Breit-Wigner theory and a classical formula for the resonance and direct contributions, respectively. In comparison with quantum calculations the classical formula appears to overestimate the direct contribution to the rate constant by about 12% for this transition. Below about 450 K the resonance contribution is larger than the direct, and above that temperature the opposite holds. The biggest contribution from resonances is at the lowest temperature in the study, 10 K, where it is more than four times larger than the direct. Below 1800 K the radiative association rate constant due to X(1)Σ(+) → X(1)Σ(+) transitions dominates over A(1)Π → X(1)Σ(+), while above that temperature the situation is the opposite.

Bond and mode selectivity in the OH + NH2D reaction: a quasi-classical trajectory calculation

A state-to-state dynamics study was performed to analyze the effects of vibrational excitation on the dynamics of the OH + NH2D gas-phase reaction, which are connected to issues such as bond and mode selectivity. This reaction can evolve along two channels: H-abstraction, H2O(ν) + NHD(ν); and D-abstraction, HOD(ν) + NH2(ν). Based on an analytical potential energy surface previously developed by our group, quasi-classical trajectory calculations and subsequent normal mode analysis were performed. While vibrational excitation of the NH-sym mode of NH2D slightly favours H-abstraction over the D-abstraction, vibrational excitation of the ND mode shows that there is no clear preference for the H- or D-abstraction. These results show that this reaction does not exhibit bond selectivity, suggesting a breakdown of the spectator model. For H-abstraction, vibrational excitation of the non-reactive ND mode is partially retained in the NHD product; and for D-abstraction, excitation of the non-reactive NH mode is also partially retained in the products, indicating that this reaction exhibits mode selectivity only partially. In sum, we rule out bond and mode selectivity for this reaction. All these results were interpreted on the basis of strong coupling between modes along the reaction path, a behaviour which seems to be more the general tendency than the exception in polyatomic reactions.

Theoretical Kinetic Study of the Unimolecular Keto–Enol Tautomerism Propen-2-ol ↔ Acetone. Pressure Effects and Implications in the Pyrolysis of tert- and 2-Butanol

The need for renewable and cleaner sources of energy has made biofuels an interesting alternative to fossil fuels, especially in the case of butanol isomers, with its favorable blend properties and low hygroscopicity. Although C4 alcohols are prospective fuels, some key reactions governing their pyrolysis and combustion have not been adequately studied, leading to incomplete kinetic models. Enols are important intermediates in the combustion of C4 alcohols, as well as in atmospheric processes. Butanol reactions kinetics is poorly understood. Specifically, the unimolecular tautomerism of propen-2-ol ↔ acetone, which is included in butanol combustion kinetic models, is assigned rate parameters based on the tautomerism vinyl alcohol ↔ acetaldehyde as an analogy. In an attempt to update current kinetic models for tert- and 2-butanol, a theoretical kinetic study of the titled reaction was carried out by means of CCSD(T,FULL)/aug-cc-pVTZ//CCSD(T)/6-31+G(d,p) ab initio calculations, with multistructural torsional anharmonicity and variational transition state theory considerations in a wide temperature and pressure range (200-3000 K; 0.1-108 kPa). Results differ from vinyl alcohol ↔ acetaldehyde analogue reaction, which shows lower rate constant values. It was observed that decreasing pressure leads to a decrease in rate constants, describing the expected falloff behavior. Tunneling turned out to be important, especially at low temperatures. Accordingly, pyrolysis simulations in a batch reactor for tert- and 2-butanol with computed rate constants showed important differences in comparison with previous results, such as larger acetone yield and quicker propen-2-ol consumption.