Role of the Vibrational and Translational Energies in the CN(v)+CH(ν, ν, ν and ν) Reactions. A Theoretical QCT Study.

Quasi-classical trajectory (QCT) calculations were conducted on the newly developed full-dimensional potential energy surface, PES-2023, to analyse two critical aspects: the influence of vibrational versus translational energy in promoting reactivity, and the impact of vibrational excitation within similar vibrational modes. The former relates to Polanyi's rules, while the latter concerns mode selectivity. Initially, the investigation revealed that independent vibrational excitation by a single quantum of ethane's symmetric and asymmetric stretching modes (differing by only 15 cm) yielded comparable dynamics, reaction cross-sections, HCN(v) vibrational product distributions, and scattering distributions.

Kinetic study of the CN + CH hydrogen abstraction reaction based on an analytical potential energy surface.

The temperature dependence of the thermal rate constants and kinetic isotope effects (KIE) of the CN + CH gas-phase hydrogen abstraction reaction was theoretically determined within the 25-1000 K temperature range, , from very low- to high-temperature regimes. Based on a recently developed full-dimensional analytical potential energy surface fitted to highly accurate explicitly correlated calculations, three different kinetic theories were used: canonical variational transition state theory (CVT), quasiclassical trajectory theory (QCT), and ring polymer molecular dynamics (RPMD) method for the computation of rate constants. We found that the thermal rate constants obtained with the three theories show a V-shaped temperature dependence, with a pronounced minimum near 200 K, qualitatively reproducing the experimental measurements.

The CN(X 2Σ+) + C2H6 reaction: Dynamics study based on an analytical full-dimensional potential energy surface.

The hydrogen abstraction reaction of the cyano radical with molecules of ethane presents some interesting points in the chemistry from ultra-cold to combustion environments especially with regard to HCN(v) product vibrational distribution. In order to understand its dynamics, a new analytical full-dimensional potential energy surface was developed, named PES-2023. It uses a combination of valence bond and mechanic molecular terms as the functional form, fitted to high-level ab initio calculations at the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ level on a reduced and selected number of points describing the reactive process.

Kinetics and dynamics study of the Cl(P) + CHOH reaction based on an analytical potential energy surface.

The reaction of chlorine atoms with methanol plays a central role in atmospheric and combustion processes and is a prototype of multi-channel reaction with two paths, HCl(,) + CHOH (R1) and HCl(,) + CHO (R2). In order to understand the kinetics and dynamics of the title reaction, using a valence-bond (VB) strategy we developed a new full-dimensional potential energy surface, named PES-2023, fitted to high-level calculations. Given that the (R2) path shows a noticeable barrier height, 12.

Current Status of the X + CH [X ≡ H, F(P), Cl(P), O(P), OH] Hydrogen Abstraction Reactions: A Theoretical Review.

This paper is a detailed review of the chemistry of medium-size reactive systems using the following hydrogen abstraction reactions with ethane, X + CH → HX + CH; X ≡ H, F(P), Cl(P), O(P) and OH, and focusing attention mainly on the theoretical developments. These bimolecular reactions range from exothermic to endothermic systems and from barrierless to high classical barriers of activation. Thus, the topography of the reactive systems changes from reaction to reaction with the presence or not of stabilized intermediate complexes in the entrance and exit channels.

Full-dimensional potential energy surface for the H + CHOH reaction. Theoretical kinetics and dynamics study.

The dynamics and kinetics of the abstraction reactions of hydrogen atoms with methanol have been studied using quasi-classical trajectory calculations and variational transition state theory with tunnelling corrections, based on a new analytical potential energy surface (PES). The new PES is a valence-bond/molecular mechanics (VB/MM) expression that provides us with the potential energy for any set of Cartesian coordinates. Two reaction channels are considered: hydrogen abstraction from the methyl group (R1) and hydrogen abstraction from the alcohol group (R2), R1 being much more likely to occur in the wide temperature range under study (250-1000 K), as expected from the lower barrier height.

Quasi-Classical Trajectory Study of the CN + NH Reaction Based on a Global Potential Energy Surface.

Based on a combination of valence-bond and molecular mechanics functions which were fitted to high-level ab initio calculations, we constructed an analytical full-dimensional potential energy surface, named PES-2020, for the hydrogen abstraction title reaction for the first time. This surface is symmetrical with respect to the permutation of the three hydrogens in ammonia, it presents numerical gradients and it improves the description presented by previous theoretical studies. In order to analyze its quality and accuracy, stringent tests were performed, exhaustive kinetics and dynamics studies were carried out using quasi-classical trajectory calculations, and the results were compared with the available experimental evidence.

VTST and RPMD kinetics study of the nine-body X + CH (X ≡ H, Cl, F) reactions based on analytical potential energy surfaces.

Thermal rate constants of nine-atom hydrogen abstraction reactions, X + C2H6 → HX + C2H5 (X ≡ H, Cl, F) with qualitatively different reaction paths, have been investigated using two kinetics approaches - variational transition state theory with multidimensional tunnelling (VTST/MT) and ring polymer molecular dynamics (RPMD) - and full dimensional analytical potential energy surfaces. For the H + C2H6 reaction, which proceeds through a noticeable barrier height of 11.62 kcal mol-1, kinetics approaches showed excellent agreement between them (with differences less than 30%) and with the experiment (with differences less than 60%) in the wide temperature range of 200-2000 K.

The hydrogen abstraction reaction H + CH→ H(v,j) + CH. Part II. Theoretical kinetics and dynamics study.

Two important issues motivated the present study: the role of the tunnelling contribution at low temperatures and the role of the alkyl fragment in the dynamics. Using a recently developed full-dimensional analytical potential energy surface (PES), named PES-2018 (Part I), kinetics and dynamics studies were performed. The kinetics study was performed using the variational transition-state theory with multidimensional tunnelling over the temperature range of 200-2000 K.

The hydrogen abstraction reaction H + CH→ H(v,j) + CH. Part I. A full-dimensional analytical potential energy surface based on ab initio calculations.

Using as input data high-level structure electronic calculations, a new full-dimensional analytical potential energy surface (PES), named PES-2018, was developed for the title reaction, which is a valence bond/molecular mechanics based surface that depends on a set of adjustable parameters. The title reaction is practically thermoneutral, -0.18 kcal mol, with a high barrier, 11.

Quasi-Classical Trajectory Dynamics Study of the Cl(P) + CH → HCl(v,j) + CH Reaction. Comparison with Experiment.

To understand and simulate the dynamics behavior of the title reaction, QCT calculations were performed on a recently developed global analytical potential energy surface, PES-2017. These calculations combine the classical description of the dynamics with pseudoquantization in the reactants and products to perform a theoretical/experimental comparison on the same footing. Thus, in the products a series of constraints are included to analyze the HCl(v = 0,j) product, which is experimentally detected.

Full-dimensional analytical potential energy surface describing the gas-phase Cl + CH reaction and kinetics study of rate constants and kinetic isotope effects.

Within the Born-Oppenheimer approximation a full-dimensional analytical potential energy surface, PES-2017, was developed for the gas-phase hydrogen abstraction reaction between the chlorine atom and ethane, which is a nine body system. This surface presents a valence-bond/molecular mechanics functional form dependent on 60 parameters and is fitted to high-level ab initio calculations. This reaction presents little exothermicity, -2.

Simulation of the experimental imaging results for the OH + CHD reaction with a simple and accurate theoretical approach.

The OH + CHD reaction is among the largest one ever studied at the high-resolution level permitted by imaging techniques [B. Zhang et al., J.

Kinetics study of the CN + CH hydrogen abstraction reaction based on a new ab initio analytical full-dimensional potential energy surface.

We have developed an analytical full-dimensional potential energy surface, named PES-2017, for the gas-phase hydrogen abstraction reaction between the cyano radical and methane. This surface is fitted using high-level ab initio information as input. Using the PES-2017 surface, a kinetics study was performed via two theoretical approaches: variational transition-state theory with multidimensional tunnelling (VTST-MT) and ring polymer molecular dynamics (RPMD).

Theoretical Study of the Pair-Correlated F + CHD(v = 0,ν = 1) Reaction: Effect of CH Stretching Vibrational Excitation.

The F + CHD(v) reaction is a benchmark system in polyatomic reactions. Theoretical/experimental comparisons have been reported in recent years that present some controversies, specifically the role of the reactant CH stretching vibrational excitation, CHD(ν = 1), on the reactivity of both isotope channels, DF(v) + CHD(v') and HF(v) + CD(v'). However, in many cases, these comparisons are not made on an equal footing.

Product Translational and Vibrational Distributions for the OH/OD + CH4/CD4 Reactions from Quasiclassical Trajectory Calculations. Comparison with Experiment.

For the OH + CH4/CD4 hydrogen abstraction reactions, the methyl radical (CH3 and CD3) product translational distributions and the water (H2O and HOD) product vibrational distributions experimentally reported by Liu's group are reproduced by quasi-classical trajectory (QCT) calculations on an analytical full-dimensional potential energy surface when a quantum spirit is included in the analysis. Our simulations correctly predict: (i) the vibrational excitation of the water product, (ii) the inversion of the water vibrational population, and (iii) the propensity of transfer from reactant kinetic energy to product translational energy. These reactions therefore present a marked isotopic effect.

Final state-resolved mode specificity in HX + OH → X + H2O (X = F and Cl) reactions: a quasi-classical trajectory study.

The state-to-state dynamics of the title reactions are investigated using a quasi-classical trajectory method on recently developed accurate global potential energy surfaces. Although both produce the H2O product, these two reactions have very different characteristics in the reaction energy, barrier location, and barrier height. It is shown that the H2O product is moderately excited in its three vibrational modes in the HF + OH reaction, but its stretching modes are highly excited in the HCl + OH reaction.

Quasi-classical trajectory study of the vibrational and translational effects on the O((3)P) + CD4 reaction.

The effects of vibrational excitation and translational energy, connected to mode selectivity and Polanyi's rules, are important issues in dynamics studies. To analyze these effects on the O((3)P) + CD4 reaction, an exhaustive dynamics study was performed using quasi-classical trajectory calculations on a full-dimensional analytical potential energy surface. The independent excitation of the C-D symmetric or asymmetric stretch modes leads to reactions with similar reaction cross sections and product scattering distributions, mode selectivity being discarded.

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.

The hydrogen abstraction reaction O(3P) + CH4: a new analytical potential energy surface based on fit 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.

CO2 vibrational state distributions from quasi-classical trajectory studies of the HO + CO → H + CO2 reaction and H + CO2 inelastic collision.

Quasi-classical trajectory studies have been carried out for the HO + CO → H + CO2 reaction and H + CO2 inelastic collision on a recently developed global potential energy surface based on a large number of high-level ab initio points. The CO2 vibrational state distributions for these processes have been determined using an original normal-mode analysis method. It was found that the CO2 product of the reaction is highly excited in both the Fermi-linked bending and symmetric stretching modes, but little population was found in the antisymmetric stretching mode.

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.

Quasi-classical trajectory calculations of the hydrogen abstraction reaction H + NH3.

On a new potential energy surface (PES-2009) recently developed by our group describing the H + NH(3) hydrogen abstraction reaction, we perform an exhaustive state-to-state dynamics study using quasi-classical trajectory (QCT) calculations at collision energies between 15 and 50 kcal mol(-1). The reaction cross section is very small, corresponding to a large barrier height and reproducing other theoretical measurements. Most of the available energy appears as product translational energy ( approximately 50%) with the H(2) diatomic product being vibrationally cold (v' = 0, 1).

A five-dimensional quantum dynamics study of the F(2P) + CH4 reaction.

By applying the semirigid vibrating rotor target (SVRT) model to the title reaction, five-dimensional wave packet quantum dynamics calculations have been carried out on the new potential energy surface PES-2006 [Espinosa-Garcia et al., J. Phys.

Product vibrational distributions in polyatomic species based on quasiclassical trajectory calculations.

By including anharmonicity and Coriolis coupling terms, we have improved our earlier quasi-classical method for vibrational mode analysis in polyatomic species, which was based on a harmonic approach. Because accurate methods have been developed only for diatomic and triatomic systems, the new algorithm was tested against accurate methods for diatomic molecules, and against the semiclassical fast Fourier transform (FFT) method for triatomic species, finding excellent agreement. The new algorithm is designed to be used with dynamics studies based on quasi-classical trajectory (QCT) calculations, and it is general for any polyatomic species.