Collisional Intermolecular Energy Transfer from a N2 Bath at Room Temperature to a Vibrationlly "Cold" C6F6 Molecule Using Chemical Dynamics Simulations

Post-Transition State Direct Dynamics Simulations on the Ozonolysis of Catechol in an N2 Bath and Comparison with Gas-Phase Results

Chemical dynamics simulations on the post-transition state dynamics of ozonolysis of catechol are performed in this article using a newly developed QM + MM simulation model. The reaction is performed in a bath of N2 molecules equilibrated at 300 K. Two bath densities, namely, 20 and 324 kg/m3, are considered for the simulation. The excitation temperatures of a catechol-O3 moiety are taken as 800, 1000, and 1500 K for each density. At these new excitation temperatures, the gas-phase results are also computed to compare the results and quantify the effect of surrounding molecules on this reaction. Like the previous findings, five reaction channels are observed in the present investigation, producing CO2, CO, O2, small carboxylic acid (SCA), and H2O. The probabilities of these products are discussed with the role of bath densities. Results from the gas-phase simulation and density of 20 kg/m3 are very similar, whereas results differ significantly at a higher bath density of 324 kg/m3. The rate constants for the unimolecular channel at each temperature and density are also calculated and reported. The QM + MM setup used here can also be used for other chemical reactions, where the solvent effect is important.

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.

Chemical dynamics simulations of energy transfer in CH4 and N2 collisions

Chemical dynamics simulations have been performed to study the energy transfer from a hot N₂ bath at 1000 K to CH₄ fuel at 300 K at different bath densities ranging from 1000 kg m⁻³ to 30 kg m⁻³. At higher bath densities, the energy transfer from the bath to the fuel was rapid and as the density was decreased, the energy transfer rate constant decreased. The results show that in combustion systems with CH₄ as a prototype fuel, the super pressure regimes control the fuel heating and combustion processes.

Chemical dynamics simulations have been performed to study the energy transfer from a hot N₂ bath at 1000 K to CH₄ fuel at 300 K at different bath densities ranging from 1000 kg m⁻³ to 30 kg m⁻³.

Comparison of intermolecular energy transfer from vibrationally excited benzene in mixed nitrogen-benzene baths at 140 K and 300 K

Gas phase intermolecular energy transfer (IET) is a fundamental component of accurately explaining the behavior of gas phase systems in which the internal energy of particular modes of molecules is greatly out of equilibrium. In this work, chemical dynamics simulations of mixed benzene/N₂ baths with one highly vibrationally excited benzene molecule (Bz^*) are compared to experimental results at 140 K. Two mixed bath models are considered. In one, the bath consists of 190 N₂ and 10 Bz, whereas in the other bath, 396 N₂ and 4 Bz are utilized. The results are compared to results from 300 K simulations and experiments, revealing that Bz^*-Bz vibration-vibration IET efficiency increased at low temperatures consistent with longer lived "chattering" collisions at lower temperatures. In the simulations, at the Bz^* excitation energy of 150 kcal/mol, the averaged energy transferred per collision, ⟨ΔE_c⟩, for Bz^*-Bz collisions is found to be ∼2.4 times larger in 140 K than in 300 K bath, whereas this value is ∼1.3 times lower for Bz^*-N₂ collisions. The overall ⟨ΔE_c⟩, for all collisions, is found to be almost two times larger at 140 K compared to the one obtained from the 300 K bath. Such an enhancement of IET efficiency at 140 K is qualitatively consistent with the experimental observation. However, the possible reasons for not attaining a quantitative agreement are discussed. These results imply that the bath temperature and molecular composition as well as the magnitude of vibrational energy of a highly vibrationally excited molecule can shift the overall timescale of rethermalization.

A Competition between Dissociation Pathway and Energy Transfer Pathway: Unimolecular Dissociation of a Benzene-Hexafluorobenzene Complex in Nitrogen Bath

The unimolecular dissociation of a benzene-hexafluorobenzene complex at 1000, 1500, and 2000 K is studied inside a bath of 1000 N₂ molecules kept at 300 K using chemical dynamics simulation. Three bath densities of 20, 324, and 750 kg/m³ are considered. The dissociation dynamics of the complex at a 20 kg/m³ bath density is found to be similar to that in the gas phase, whereas the dynamics is drastically different at higher bath densities. The microcanonical/canonical dissociation rate constants for the three bath densities are calculated and fitted to the Arrhenius equation. The activation energies are found to be similar to the gas-phase one. However, the pre-exponential factor is lower and decreases with the increase in bath density. The vibrational degree of freedom of the complex more effectively participates in the collisional energy transfer to the N₂ bath, whereas the translational and rotational degrees of freedom of N₂ receive the transferred energy. The energy transfer efficiency increases with the increase in bath density. The time scale of the energy transfer pathway is more than that of the dissociation pathway, and negligible direct dissociation of the complex is observed from the simulation at the highest bath density.

Normal mode analysis on the relaxation of an excited nitromethane molecule in argon bath

In our previous work [Rivera-Rivera et al., J. Chem. Phys. 142, 014303 (2015)], classical molecular dynamics simulations followed the relaxation, in a 300 K Ar bath at a pressure of 10-400 atm, of nitromethane (CH₃NO₂) instantaneously excited by statistically distributing 50 kcal/mol among all its internal degrees of freedom. Both rotational and vibrational energies decayed with nonexponential curves. The present work explores mode-specific mechanisms at work in the decay process. With the separation of rotation and vibration developed by Rhee and Kim [J. Chem. Phys. 107, 1394 (1997)], one can show that the vibrational kinetic energy decomposes only into vibrational normal modes, while the rotational and Coriolis energies decompose into both vibrational and rotational normal modes. The saved CH₃NO₂ positions and momenta were converted into mode-specific energies whose decay was monitored over 1000 ps. The results identify vibrational and rotational modes that promote/resist energy lost and drive nonexponential behavior.

Pressure effects on the vibrational and rotational relaxation of vibrationally excited OH (ν, J) in an argon bath

Quasi-classical molecular dynamics simulations were used to study the energy relaxation of an initially non-rotating, vibrationally excited (ν = 4) hydroxyl radical (OH) in an Ar bath at 300 K and at high pressures from 50 atm to 400 atm. A Morse oscillator potential represented the OH, and two sets of interaction potentials were used based on whether the Ar-H potential was a Buckingham (Exp6) or a Lennard-Jones (LJ) potential. The vibrational and rotational energies were monitored for 25 000-90 000 ps for Exp6 trajectories and 5000 ps for LJ trajectories. Comparisons to measured vibrational relaxation rates show that Exp6 rates are superior. Simulated initial vibrational relaxation rates are linearly proportional to pressure, implying no effect of high-pressure breakdown in the isolated binary collision approximation. The vibrational decay curves upward from single-exponential decay. A model based on transition rates that exponentially depend on the anharmonic energy gap between vibrational levels fits the vibrational decay well at all pressures, suggesting that anharmonicity is a major cause of the curvature. Due to the competition of vibration-to-rotation energy transfer and bath gas relaxation, the rotational energy overshoots and then relaxes to its thermal value. Approximate models with adjustable rates for this competition successfully reproduced the rotational results. These models show that a large fraction of the vibrational energy loss is initially converted to rotational energy but that fraction decreases rapidly as the vibrational energy content of OH decreases. While simulated rates change dramatically between Exp6 and LJ potentials, the mechanisms remain the same.