Quasiclassical trajectory study of formaldehyde unimolecular dissociation: H(2)CO-->H2 + CO, H + HCO

We report quasiclassical trajectory calculations of the dynamics of the two reaction channels of formaldehyde dissociation on a global ab initio potential energy surface: the molecular channel H(2)CO-->H(2) + CO and the radical H(2)CO-->H + HCO. For the molecular channel, it is confirmed that above the threshold of the radical channel a second, intramolecular hydrogen abstraction pathway is opened to produce CO with low rotation and vibrationally hot H(2). The low-j(CO) and high-nu(H(2) ) products from the second pathway increase with the total energy. The competition between the molecular and radical pathways is also studied. It shows that the branching ratio of the molecular products decreases with increasing energy, while the branching ratio of the radical products increases. The results agree well with very recent velocity-map imaging experiments of Suits and co-workers and solves a mystery first posed by Moore and co-workers. For the radical channel, we present the translational energy distributions and HCO rotation distributions at various energies. There is mixed agreement with the experiments of Wittig and co-workers, and this provides an indirect confirmation of their speculation that the triplet surface plays a role in the formation of the radical products.     

Quasiclassical trajectory calculations of the reaction C+C2H2-->l-C3H, c-C3H+H, C3+H2 using full-dimensional triplet and singlet potential energy surfaces

Full-dimensional, density functional theory (B3LYP/6-311g(d,p))-based potential energy surfaces (PESs) are reported and used in quasi-classical calculations of the reaction of C with C(2)H(2). For the triplet case, the PES spans the region of the reactants, the complex region (with numerous minima and saddle points) and the products, linear(l)-C(3)H+H, cyclic(c)-C(3)H+H and c-(3)C(3)+H(2). For the singlet case, the PES describes the complex region and products l-C(3)H+H, c-C(3)H+H and l-(1)C(3)+H(2). The PESs are invariant under permutation of like nuclei and are fit to tens of thousands of electronic energies. Energies and harmonic frequencies of the PESs agree well the DFT ones for all stationary points and for the reactant and the products. Dynamics calculations on the triplet PES find both l-C(3)H and c-C(3)H products, with l-C(3)H being dominant at the energies considered. Limited unimolecular reaction dynamics on the singlet PES find both products in comparable amounts as well as the C(3)+H(2) product.     

Quasi-classical trajectory calculations on the H + O2 reaction

Quasi-classical trajectory calculations have been performed on the H + O₂ system. Significant reaction probabilities are obtained when the initial energy is in rotation or vibration, or a combination of the two, but not when the initial energy is in translation. The opacity function shows a bimodal dependence on the impact parameter, with a small peak at 0.9 Å < b < 1.5 Å and a very prominent peak at 2.5 Å < b < 3.3 Å. The product scattering angles and product energy distributions also depend on b and to a limited extent on the initial energy distribution. The observations can be largely interpreted in terms of the nature of the motion on the potential energy surface, while the effects of rotational energy on the reaction follow qualitatively from statistical phase-space theory.     

Quasiclassical trajectory calculations of the OH+NO2 association reaction on a global potential energy surface

We report a full-dimensional potential energy surface (PES) for the OH+NO(2) reaction based on fitting more than 55,000 energies obtained with density functional theory-B3LYP6-311G(d,p) calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO(2), and the fragments OH+NO(2) and HO(2)+NO. Detailed comparison of the structures, energies, and harmonic frequencies of various stationary points on the PES are made with previous and present high-level ab initio calculations. Two hydrogen-bond complexes are found on the PES and confirmed by new ab initio CASPT2 calculations. Quasiclassical trajectory calculations of the cross sections for ground rovibrational OH+NO(2) association reactions to form HOONO and HONO(2) are done using this PES. The cross section to form HOONO is larger than the one to form HONO(2) at low collision energies but the reverse is found at higher energies. The enhancement of the HOONO complex at low collision energies is shown to be due, in large part, to the transient formation of a H-bond complex, which decays preferentially to HOONO. The association cross sections are used to obtain rate constants for formation of HOONO and HONO(2) for the ground rovibrational states in the high-pressure limit.     

A quantum-mechanical study of chemical reaction and charge-transfer processes in the (Ar + H2)+ system

A three-dimensional quantum-mechanical study of the (Ar + H₂⁺ system within the reactive infinite-order sudden approximation is presented. All four possible channels for chemical reaction and charge transfer were treated simultaneously. The various cross sections deviate by at most 50% from recent trajectory surface hopping results.     

Quasiclassical trajectory study of the Cl + Hl → HCl + I reaction

Monte Carlo quasiclassical trajectory calculations have been carried out for the reaction Cl + Hl → HCl + I for 300, 1000, and 2000 K. A semi-empirical potential-energy surface (London equation) was obtained by “transfering” parameters from surfaces computed for other reaction systems. The computed results are in general accord with experimental measurements. Thermal rate coefficients, differential scattering cross sections, and product vibrational and rotational distributions were computed for the three temperatures. Angular scattering distributions are in agreement with experiment only at elevated temperatures.     

Quasiclassical trajectory study of the collision-induced dissociation of CH3SH+ + Ar

Quasiclassical trajectory calculations were carried out to study the dynamics of energy transfer and collision-induced dissociation (CID) of CH(3)SH(+) + Ar at collision energies ranging from 4.34 to 34.7 eV. The relative abundances calculated for the most relevant product ions are found to be in good agreement with experiment, except for the lowest energies investigated. In general, the dissociation to form CH(3)(+) + SH is the dominant channel, even though it is not among the energetically favored reaction pathways. The results corroborate that this selective dissociation observed upon collisional activation arises from a more efficient translational to vibrational energy transfer for the low-frequency C-S stretching mode than for the high-frequency C-H stretching modes, together with weak couplings between the low- and high-frequency modes of vibration. The calculations suggest that CID takes place preferentially by a direct CH(3)(+) + SH detachment, and more efficiently when the Ar atom collides with the methyl group-side of CH(3)SH(+).     

Quasiclassical trajectory study of fast H-atom collisions with acetylene

Translationally hot H collisions with the acetylene are investigated using quasiclassical trajectory calculations, on a recent full-dimensional ab initio-based potential energy surface. Three outcomes are focused on: non-reactive energy transfer via prompt collisions, non-reactive energy transfer via the formation of the vinyl complex, and reactive chemical H-atom exchange, also via complex formation. The details of these outcomes are presented and correlated with the collision lifetime. Large energy transfer is found via complex formation, which can subsequently decay back to reactants, a non-reactive event, or to new products, a reactive event. For the present system, these two events are experimentally indistinguishable.     

Quasiclassical trajectory study of the CH3++HD-->CH2D++H2 Reaction

A full dimensional ab initio potential energy surface for the CH5+ system based on coupled cluster electronic structure calculations and capable of describing the dissociation of methonium ion into methyl cation and molecular hydrogen (J. Phys. Chem. A 2006, 110, 1569) is used in quasiclassical trajectory calculations of the reaction CH3++HD-->CH2D++H2 for low collision energies of relevance to astrochemistry. Cross sections for the exchange are obtained at several relative translational energies and a fit to the energy dependence of the cross sections is used to obtain the rate constant at temperatures between 10 and 50 K. The calculated rate constant at 10 K agrees well with the previously reported experimental value. Internal energy distributions of the products are presented and discussed in the context of zero-point energy "noncompliance".     

A comparison of two-dimensional and three-dimensional trajectory calculations in thermal HBr2 and H2Br systems

Two-dimensional (2D) and three-dimensional (3D) quasiclassical trajectory calculations on H + Br₂ at 300°K and H + HBr at 1000°K are reported. Angular scattering, energy disposal, and impact parameter distributions for reactive collisions are compared after removal of phase-space factors (dimensionality bias) as a means of examining the similarities and differences in the dynamic bias in 2D and 3D. Qualitatively, for all reactive processes studied, the 3D trajectory calculated distributions are reproduced by the phase-space adjusted 2D trajectory data. Thus the surprisal of these angular scattering, energy disposal, and impact parameter distributions is dimensionally invariant, and the same dynamic bias appears in 2D and 3D. A systematic method for converting 2D reaction probabilities and maximum reactive impact parameters into 3D rate coefficients is presented. We find that trajectory calculated 3D rate coefficients may in general differ markedly from those derived from 2D trajectory data. In particular, the surprisal associated with rate coefficients depends on dimensionality for the H + HBr → H₂ + Br reaction, but is invariant for the H′ + HBr → H′Br + Br and H + Br₂ → HBr + Br reactions.     

A surface hopping quasiclassical trajectory study of the H2+ + H2 and (H2 + D2)+ systems

In this work we use a complete surface hopping quasiclassical trajectory method to determine cross sections for the reactions H₂⁺ + H₂ → H₃⁺ + H and the isotopic variants (H₂⁺ + D₂ and D₂⁺ + H₂). Initial translational energies ranged between 0.5 and 6 eV. The vibrational quantum number (v⁺) of the charged diatom is either 0 or 3. Comparing these results with our previous results with a partial treatment of surface hopping, we find essentially no change for v⁺ = 0 and reductions in cross sections of up to 30% for v⁺ = 3 trajectories.     

Cross sections of the O+ + H2 --> OH+ + H ion-molecule reaction and isotopic variants (D2, HD): quasiclassical trajectory study and comparison with experiments

A dynamics study [cross section and microscopic mechanism versus collision energy (E(T))] of the reaction O+ + H2 --> OH+ + H, which plays an important role in Earth's ionosphere and interstellar chemistry, was conducted using the quasiclassical trajectory method, employing an analytical potential energy surface (PES) recently derived by our group [R. Martinez et al., J. Chem. Phys. 120, 4705 (2004)]. Experimental excitation functions for the title reaction, as well as its isotopic variants with D2 and HD, were near-quantitatively reproduced in the calculations in the very broad collision energy range explored (E(T) = 0.01-6.0 eV). Intramolecular and intermolecular isotopic effects were also examined, yielding data in good agreement with experimental results. The reaction occurs via two microscopic mechanisms (direct and nondirect abstraction). The results were satisfactorily interpreted based on the reaction probability and the maximum impact parameter dependences with E(T), and considering the influence of the collinear [OHH]+ absolute minimum of the PES on the evolution from reactants to products. The agreement between theory and experiment suggests that the reaction mainly occurs through the lowest energy PES and nonadiabatic processes are not very important in the wide collision energy range analyzed. Hence, the PES used to describe this reaction is suitable for both kinetics and dynamics studies.     

Quasiclassical trajectory study of the C(1D) + H2 reaction and isotopomeric variants: kinetic isotope effect and CD/CH branching ratio

The recently proposed ab initio single-sheeted double many-body expansion potential energy for the methylene molecule has been used to perform quasiclassical trajectory (QCT) calculations for the title reaction. Thermal and initial state-specific (v = 0, j = 0) rate constants for the C((1)D) + H(2)/HD/D(2) reactions have been obtained over a wide range of temperatures. Cross sections for the reaction C((1)D) + H(2) and its deuterated isotopes have also been calculated, as well as the CD/CH branching ratios for the C((1)D) + HD reaction. It is found that the CD + H product channel in the C((1)D) + HD reaction is preferred relative to the CH + D channel. The estimated rate constants are predicted to be in the order k(H2) > k(HD) > k(D2) and the calculated cross sections and rate constants compared with available theoretical and experimental data.     

A case study in ultracold reactive scattering: D + H2

We report theoretical results for reaction and vibrational quenching of the ultracold collision D + H(2) (v, j = 0) for a wide range of initial vibrationally excited states v. The v-dependence of the zero-temperature limit of the reaction rate coefficient shows two distinct regimes: a barrier dominated regime for 0 ≤ v ≤ 4, and a barrierless regime for v ≥ 5. We also present detailed distributions over the rovibrational states of the products. We find an approximate conservation of the internal vibrational energy; namely, the branching ratios always favor the highly excited final states, which have vibrational energies similar to that of the entrance channel.     

Electron-impact dissociative excitation of H 2 + : low energy studies

Electron impact dissociative excitation cross sections for vibrationally excited H ₂ ⁺ ions have been measured over the energy range from 0.01–35 eV. They are found to blend smoothly with previous crossed beam measurements and to match theory based upon the Born Approximation.     

Quasiclassical trajectory study of the reaction H+CH4(nu3 = 0,1)-->CH3+H2 using a new ab initio potential energy surface

Detailed quasiclassical trajectory calculations of the reaction H+CH4(nu3 = 0,1)-->CH3 + H2 using a slightly updated version of a recent ab initio-based CH5 potential energy surface [X. Zhang et al., J. Chem. Phys. 124, 021104 (2006)] are reported. The reaction cross sections are calculated at initial relative translational energies of 1.52, 1.85, and 2.20 eV in order to make direct comparison with experiment. The relative reaction cross section enhancement ratio due to the excitation of the C-H antisymmetric stretch varies from 2.2 to 3.0 over this energy range, in good agreement with the experimental result of 3.0 +/- 1.5 [J. P. Camden et al., J. Chem. Phys. 123, 134301 (2005)]. The laboratory-frame speed and center-of-mass angular distributions of CH3 are calculated as are the vibrational and rotational distributions of H2 and CH3. We confirm that this reaction occurs with a combination of stripping and rebound mechanisms by presenting the impact parameter dependence of these distributions and also by direct examination of trajectories.     

New calculations on the ion-molecule processes C2H2(+) + H2 --> C2H3(+) + H and C2H2(+) + H2 --> C2H4+

New high-level quantum chemical calculations have been undertaken to understand the rates and mechanisms of the reactive and associative channels for the reactants C2H2(+) + H2. The reactive channel, which produces C2H3(+) + H, has been shown to be slightly endothermic, confirming earlier calculations at a somewhat lower level and in agreement with some recent experimental work. The associative channel, leading to C2H4+, has been shown to proceed via a transition state with negative energy relative to the reactants, so that association is predicted to be efficient. This result is in conflict with an earlier theoretical study but in agreement with low-temperature experimental measurements.     

Quasiclassical trajectory study of the reaction NH(X~3∑~-)+H→N(~4S)+H_2

The dynamics of the NH + H→N+H2 reaction has been investigated by means of the 3D quasiclassical trajectory approach by using the LEPS potential energy surface.The calculated rate coefficient is in good agreement with the experimental value.The reaction was found to occur via a direct channel.The product H2 has a cold excitation of rotational state,but has a reverse distribution of the vibrational state with a peak at v=1.Based on the potential energy surface and the trajectory analysis,the reaction mechanism has been explained successfully.     

A state-to-state dynamical study of the Br + H(2) reaction: comparison of quantum and classical trajectory results

We present a detailed theoretical investigation of the dynamics corresponding to the strongly endothermic Br + H(2) (v = 0-1, j = 0) → H + HBr reaction in the 0.85 to 1.9 eV total energy range. State-averaged and state-to-state results obtained through time-independent wave packet (TIWP) and time-independent quantum mechanical (TIQM) calculations and quasiclassical trajectories (QCT) are compared and analyzed. The agreement in the results obtained with both quantum mechanical results is very good overall. However, although QCT calculations reproduce the general features, their agreement with the QM results is sometimes only qualitative. The analysis of the mechanism based on state-averaged results turns out to be deceptive and conveys an oversimplified picture of the reaction consistent with a direct-rebound mechanism. Consideration of state-to-state processes, in contrast, unveils the existence of multiple mechanisms that give rise to a succession of maxima in the differential cross section (DCS). Such mechanisms correlate with different sets of partial waves and display similar collision times when analyzed through the time-dependent DCS.