Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane

Pressure Effects on the Relaxation of an Excited Ethane Molecule in High-Pressure Bath Gases

We use molecular dynamics to calculate the rotational and vibrational energy relaxation of C₂H₆ in Ar, Kr, and Xe bath gases over a pressure range of 10-400 atm and at temperatures of 300 and 800 K. The C₂H₆ is instantaneously excited by 80 kcal/mol randomly distributed into both vibrational and rotational modes. The computed relaxation rates show little sensitivity to the identity of the noble gas in the bath. Vibrational relaxation rates show a nonlinear pressure dependence at 300 K. At 800 K the reduced range of bath gas densities covered by the range of pressures does not yet show any nonlinearity in the pressure dependence. Rotational relaxation is characterized with two relaxation rates. The slower rate is comparable to the vibrational relaxation rate. The faster rate has a linear pressure dependence at 300 K but an irregular, nonlinear pressure dependence at 800 K. To understand this, a model was developed based on approximating the periodic box used in the molecular dynamics simulations by an equal-volume collection of cubes where each cube is sized to allow only single occupancy by the noble gas or the molecule. Combinatorial statistics then leads to a pressure- and temperature-dependent analytic distribution of the bath gas species the molecule encounters in a collision. This distribution, the dissociation energy of molecule/bath gas complexes and bath gas clusters, and the computed energy release per collision combine to show that only at 300 K is the energy release sufficient to dissociate likely complexes and clusters. This suggests that persistent and pressure-dependent clusters and complexes at 800 K may be responsible for the nonlinear pressure dependence of rotational relaxation.

Supercollisions of fast H-atom with ethylene on an accurate full-dimensional potential energy surface

The collisions transferring large portions of energy are often called supercollisions. In the H + C₂H₂ reactive system, the rovibrationally cold C₂H₂ molecule can be activated with substantial internal excitations by its collision with a translationally hot H atom. It is interesting to investigate the mechanisms of collisional energy transfer in other important reactions of H with hydrocarbons. Here, an accurate, global, full-dimensional potential energy surface (PES) of H + C₂H₄ was constructed by the fundamental invariant neural network fitting based on roughly 100 000 UCCSD(T)-F12a/aug-cc-pVTZ data points. Extensive quasi-classical trajectory calculations were carried out on the full-dimensional PES to investigate the energy transfer process in collisions of the translationally hot H atoms with C₂H₄ in a wide range of collision energies. The computed function of the energy-transfer probability is not a simple exponential decay function but exhibits large magnitudes in the region of a large amount of energy transfer, indicating the signature of supercollisions. The supercollisions among non-complex-forming nonreactive (prompt) trajectories are frustrated complex-forming processes in which the incoming H atom penetrates into C₂H₄ with a small C-H distance but promptly and directly leaves C₂H₄. The complex-forming supercollisions, in which either the attacking H atom leaves (complex-forming nonreactive collisions) or one of the original H atoms of C₂H₄ leaves (complex-forming reactive trajectories), dominate large energy transfer from the translational energy to internal excitation of molecule. The current work sheds valuable light on the energy transfer of this important reaction in the combustion and may motivate related experimental investigations.

Nonthermal rate constants for CH4 * + X → CH3 + HX, X = H, O, OH, and O2

Quasiclassical trajectories are used to compute nonthermal rate constants, k^*, for abstraction reactions involving highly-excited methane CH₄ ^* and the radicals H, O, OH, and O₂. Several temperatures and internal energies of methane, E_vib, are considered, and significant nonthermal rate enhancements for large E_vib are found. Specifically, when CH₄ ^* is internally excited close to its dissociation threshold (E_vib ≈ D₀ = 104 kcal/mol), its reactivity with H, O, and OH is shown to be collision-rate-limited and to approach that of comparably-sized radicals, such as CH₃, with k^* > 10^-10 cm³ molecule^-1 s^-1. Rate constants this large are more typically associated with barrierless reactions, and at 1000 K, this represents a nonthermal rate enhancement, k^*/k, of more than two orders of magnitude relative to thermal rate constants k. We show that large nonthermal rate constants persist even after significant internal cooling, with k^*/k > 10 down to E_vib ≈ D₀/4. The competition between collisional cooling and nonthermal reactivity is studied using a simple model, and nonthermal reactions are shown to account for up to 35%-50% of the fate of the products of H + CH₃ = CH₄ ^* under conditions of practical relevance to combustion. Finally, the accuracy of an effective temperature model for estimating k^* from k is quantified.

First-principles calculation of adsorption of shale gas on CaCO3 (100) surfaces

Background

To demonstrate the adsorption strength of shale gas to calcium carbonate in shale matrix, the adsorption of shale gas on CaCO₃ (100) surfaces was studied using the first-principles method, which is based on the density functional theory (DFT).

Methods

The structures and electronic properties of CH₄, C₂H₆, CO₂ and N₂ molecules were calculated by the generalized gradient approximation (GGA), for a coverage of 1 monolayer (ML). Under the same conditions, the density of states (DOS) of CaCO₃ (100) surfaces before and after the adsorption of shale gas molecules at high-symmetry adsorption sites were compared.

Results

The results showed that the adsorption energies of CH₄, C₂H₆, CO₂ and N₂ on CaCO₃ (100) surfaces were between 0.2683 eV and -0.7388 eV. When a CH₄ molecule was adsorbed at a hollow site and its 2 hydrogen atoms were parallel to the long diagonal (H3) on the CaCO₃ (100) surface, it had the most stable adsorption, and the adsorption energy was only -0.4160 eV. The change of adsorption energy of CH₄ was no more than 0.0535 eV. Compared with the DOS distribution of CH₄ before adsorption, it shifted to the left overall after adsorption. At the same time, the partial density of states (PDOS) curves of CaCO₃ (100) surfaces before and after adsorption basically overlapped.

Conclusions

This work showed that the adsorption effect of shale gas on calcium carbonate is very weak, and the adsorption is physisorption at the molecular level.

Mixed quantum-classical theory for the collisional energy transfer and the rovibrational energy flow: application to ozone stabilization

A mixed quantum-classical approach to the description of collisional energy transfer is proposed in which the vibrational motion of an energized molecule is treated quantum mechanically using wave packets, while the collisional motion of the molecule and quencher and the rotational motion of the molecule are treated using classical trajectories. This accounts rigorously for quantization of vibrational states, zero-point energy, scattering resonances, and permutation symmetry of identical atoms, while advantage is taken of the classical scattering regime. Energy is exchanged between vibrational, rotational, and translational degrees of freedom while the total energy is conserved. Application of this method to stabilization of the van der Waals states in ozone is presented. Examples of mixed quantum-classical trajectories are discussed, including an interesting example of supercollision. When combined with an efficient grid mapping procedure and the reduced dimensionality approximation, the method becomes very affordable computationally.

Theoretical Study of the Dynamics of Ar Collisions with C2H6 and C2F6 at Hyperthermal Energy

We present a classical-trajectory study of the dynamics of high-energy (5-12 eV) collisions between Ar atoms and the C2H6 and C2F6 molecules. We have constructed the potential-energy surfaces for these systems considering separately the Ar-molecule interactions (intermolecular potential) and the interactions within the molecule (intramolecular potential). The intermolecular surfaces consist of pairwise empirical potentials derived from high-accuracy ab initio calculations. The intramolecular potentials for C2H6 and C2F6 are described using specific-reaction-parameters semiempirical Hamiltonians and are calculated "on the fly", i.e., while the trajectories are evolving. Trajectory analysis shows that C2F6 absorbs more energy than C2H6 and is more susceptible to collision-induced dissociation (CID). C-C bond-breakage processes are more important than C-H or C-F bond breakage at the energies explored in this work. Analysis of the reaction mechanism for CID processes indicates that, although C-C breakage is mostly produced by side-on collisions, head-on collisions are more efficient in producing C-F or C-H dissociation. Our results suggest that high-energy collisions between closed-shell species of the natural low-Earth-orbit environment and spacecraft can contribute to the observed degradation of polymers that coat spacecraft surfaces.

Energy transfer between azulene and krypton: Comparison between experiment and computation

Trajectory calculations of collisional energy transfer between excited azulene and Kr are reported, and the results are compared with recent crossed molecular beam experiments by Liu et al. [J. Chem. Phys. 123, 131102 (2005); 124, 054302 (2006)]. Average energy transfer quantities are reported and compared with results obtained before for azulene-Ar collisions. A collisional energy transfer probability density function P(E,E'), calculated at identical initial conditions as experiments, shows a peak at the up-collision branch of P(E,E') at low initial relative translational energy. This peak is absent at higher relative translational energies. There is a supercollision tail at the down-collision side of the probability distribution. Various intermolecular potentials are used and compared. There is broad agreement between experiment and computation, but there are some differences as well.

Trajectory dynamics study of collision-induced dissociation of the Ar + CH4 reaction at hyperthermal conditions: vibrational excitation and isotope substitution

We investigate the role of vibrational energy excitation of methane and two deuterated species (CD(4) and CH(2)D(2)) in the collision-induced dissociation (CID) process with argon at hyperthermal energies. The quasi-classical trajectory method has been applied, and the reactive Ar + CH(4) system has been modeled by using a modified version of the CH(4) potential energy surface of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339) and the Ar-CH(4) intermolecular potential function obtained by Troya (J. Phys. Chem. A 2005, 109, 5814). This study clearly shows that CID is markedly enhanced with vibrational excitation and, to a lesser degree, with collision energy. In general, CID increases by exciting stretch vibrational modes of the reactant molecule. For the direct dissociation of CH(4), however, the CID cross sections appear to be essentially independent of which vibrational mode is initially excited. In all situations studied, the CID cross sections are always greater for the Ar + CD(4) reaction than for the Ar + CH(4) one, the Ar + CH(2)D(2) being an intermediate situation. A detailed analysis of the energy transfer processes, including their relation with CID, is also presented.

Ab initioand direct quasiclassical-trajectory study of the F+CH4→HF+CH3 reaction

We present an electronic structure and dynamics study of the F+CH4-->HF+CH3 reaction. CCSD(T)/aug-cc-pVDZ geometry optimizations, harmonic-frequency, and energy calculations indicate that the potential-energy surface is remarkably isotropic near the transition state. In addition, while the saddle-point F-H-C angle is 180 degrees using MP2 methods, CCSD(T) geometry optimizations predict a bent transition state, with a 153 degrees F-H-C angle. We use these high-quality ab initio data to reparametrize the parameter-model 3 (PM3) semiempirical Hamiltonian so that calculations with the improved Hamiltonian and employing restricted open-shell wave functions agree with the higher accuracy data. Using this specific-reaction-parameter PM3 semiempirical Hamiltonian (SRP-PM3), we investigate the reaction dynamics by propagating quasiclassical trajectories. The results of our calculations using the SRP-PM3 Hamiltonian are compared with experiments and with the estimates of two recently reported potential-energy surfaces. The trajectory calculations using the SRP-PM3 Hamiltonian reproduce quantitatively the measured HF vibrational distributions. The calculations also agree with the experimental HF rotational distributions and capture the essential features of the excitation function. The results of the SRP semiempirical Hamiltonian developed here clearly improve over those using the two prior potential-energy surfaces and suggest that reparametrization of semiempirical Hamiltonians is a promising strategy to develop accurate potential-energy surfaces for reaction dynamics studies of polyatomic systems.

Experimental and Theoretical Investigations of the Inelastic and Reactive Scattering Dynamics of O(3P) + D2

This paper presents a combined experimental and theoretical study of the dynamics of O((3)P) + D(2) collisions, with emphasis on a center-of-mass (c.m.) collision energy of 25 kcal mol(-1). The experiments were conducted with a crossed-molecular-beams apparatus, employing a laser detonation source to produce hyperthermal atomic oxygen and mass spectrometric detection to measure the product angular and time-of-flight distributions. The novel beam source, which enabled these experiments to be conducted, contributed unique challenges to the experiments and to the analysis, so the experimental methods and approach to the analysis are discussed in detail. Three different levels of theory were used: (1) quasiclassical trajectories (QCT), (2) time-independent quantum scattering calculations based on high-quality potential surfaces for the two lower-energy triplet states, and (3) trajectory-surface-hopping (TSH) studies that couple the triplet surfaces with the lowest singlet surface using a spin-orbit Hamiltonian derived from ab-initio calculations. The latter calculations explore the importance of intersystem crossing in the dynamics. Both experiment and theory show that inelastically scattered O atoms scatter almost exclusively in the forward direction, with little or no loss of translational energy. For the reaction, O((3)P) + D(2) --> OD + D, the experiment shows that, on average, approximately 50% of the available energy goes into product translation and that the OD product angular distributions are largely backward-peaked. These results may be interpreted in light of the QCT and TSH calculations, leading to the conclusion that the reaction occurs mainly on triplet potential energy surfaces with, at most, minor intersystem crossing to a singlet surface. Reaction on either of the two low-lying reactive triplet surfaces proceeds through a rebound mechanism in which the angular distributions are backward-peaked and the product OD is both vibrationally and rotationally excited. The quantum scattering results are in good agreement with QCT calculations, indicating that quantum effects are relatively small for this reaction at a collision energy of 25 kcal mol(-1).

Trajectory Study of Supercollision Relaxation in Highly Vibrationally Excited Pyrazine and CO2

Classical trajectory calculations were performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) and CO(2), which were conducted using a high-resolution transient infrared absorption spectrometer. The goal here is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results. In the trajectory calculations, Newton's laws of motion are used for the molecular motion, isolated molecules are treated as collections of harmonic oscillators, and intermolecular potentials are formed by pairwise Lennard-Jones potentials. The calculations qualitatively reproduce the observed energy partitioning in the scattered CO(2) molecules and show that the relative partitioning between bath rotation and translation is dependent on the moment of inertia of the bath molecule. The simulations show that the low-frequency modes of the vibrationally excited pyrazine contribute most to the strong collisions. The majority of collisions lead to small DeltaE values and primarily involve single encounters between the energy donor and acceptor. The large DeltaE exchanges result from both single impulsive encounters and chattering collisions that involve multiple encounters.

Quasiclassical trajectory study of energy transfer and collision-induced dissociation in hyperthermal Ar + CH4 and Ar + CF4 collisions

We present a study of energy transfer in collisions of Ar with methane and perfluoromethane at hyperthermal energies (E(coll) = 4-10 eV). Quasiclassical trajectory calculations of Ar + CX(4) (X = H, F) collisions indicate that energy transfer from reagents' translation to internal modes of the alkane molecule is greatly enhanced by fluorination. The reasons for the enhancement of energy transfer upon fluorination are shown to emerge from a decrease in the hydrocarbon vibrational frequencies of the CX(4) molecule with increasing the mass of the X atom, and to an increase of the steepness of the Ar-CX(4) intermolecular potential. At high collision energies, we find that the cross section of Ar + CF(4) collisions in which the amount of energy transfer is larger than needed to break a C-F bond is at least 1 order of magnitude larger than the cross sections of Ar + CH(4) collisions producing CH(4) with energy above the dissociation limit. In addition, collision-induced dissociation is detected in short time scales in the case of the fluorinated species at E(coll) = 10 eV. These results suggest that the cross section for degradation of fluorinated hydrocarbon polymers under the action of nonreactive hyperthermal gas-phase species might be significantly larger than that of hydrogenated hydrocarbon polymers. We also illustrate a practical way to derive intramolecular potential energy surfaces for bond-breaking collisions by improving semiempirical Hamiltonians based on grids of high-quality ab initio calculations.

Trajectory Dynamics Study of the Ar + CH4Dissociation Reaction at High Temperatures: the Importance of Zero-Point-Energy Effects

Large-scale classical trajectory calculations have been performed to study the reaction Ar + CH4--> CH3 +H + Ar in the temperature range 2500 < or = T/K < or = 4500. The potential energy surface used for ArCH4 is the sum of the nonbonding pairwise potentials of Hase and collaborators (J. Chem. Phys. 2001, 114, 535) that models the intermolecular interaction and the CH4 intramolecular potential of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339), which has been modified to account for the H-H repulsion at small bending angles. The thermal rate coefficient has been calculated, and the zero-point energy (ZPE) of the CH3 product molecule has been taken into account in the analysis of the results; also, two approaches have been applied for discarding predissociative trajectories. In both cases, good agreement is observed between the experimental and trajectory results after imposing the ZPE of CH3. The energy-transfer parameters have also been obtained from trajectory calculations and compared with available values estimated from experiment using the master equation formalism; in general, the agreement is good.

Classical trajectory study of collisions of Ar with alkanethiolate self-assembled monolayers: Potential-energy surface effects on dynamics

We have investigated collisions between Ar and alkanethiolate self-assembled monolayers (SAMs) using classical trajectory calculations with several potential-energy surfaces. The legitimacy of the potential-energy surfaces is established through comparison with molecular-beam data and ab initio calculations. Potential-energy surfaces used in previous work overestimate the binding of Ar to the SAM, leading to larger energy transfer than found in the experiments. New calculations, based on empirical force fields that better reproduce ab initio calculations, exhibit improved agreement with the experiments. In particular, polar-angle-dependent average energies calculated with explicit-atom potential-energy surfaces are in excellent agreement with the experiments. Polar- and azimuthal-angle-dependent product translational energies are examined to gain deeper insight into the dynamics of Ar+SAM collisions.