Chemical dynamics simulations are performed to study the unimolecular dissociation of the benzene (Bz)−hexafluorobenzene (HFB) complex at five different temperatures ranging from 1000 to 2000 K, and the results are compared with that of the Bz dimer at common simulation temperatures. Bz−HFB, in comparison with Bz dimer, possesses a much attractive intermolecular interaction, a very different equilibrium geometry, and a lower average quantum vibrational excitation energy at a given temperature. Six low-frequency modes of Bz−HFB are formed by Bz + HFB association which are weakly coupled with the vibrational modes of Bz and HFB. However, this coupling is found much stronger in Bz−HFB compared to the same in the Bz dimer. The simulations are done with very good potential energy parameters taken from the literature. Considering the canonical (TST) model, the unimolecular dissociation rate constant at each temperature is calculated and fitted to the Arrhenius equation. An activation energy of 5.0 kcal/mol and a pre-exponential factor of 2.39 × 10 12 s −1 are obtained, which are of expected magnitudes. The responsible vibrational mode for dissociation is identified by performing normal-mode analysis. Simulations with random excitations of high-frequency Bz and HFB modes and lowfrequency inter-Bz−HFB vibrational modes of the Bz−HFB complex are also performed. The intramolecular vibrational energy redistribution (IVR) time and the unimolecular dissociation rate constants are calculated from these simulations. The latter shows good agreement with the same obtained from simulation with random excitation of all vibrational modes.

The unimolecular dissociation of a benzene− hexafluorobenzene complex at 1000, 1500, and 2000 K is studied inside a bath of 1000 N 2 molecules kept at 300 K using chemical dynamics simulation. Three bath densities of 20, 324, and 750 kg/ m 3 are considered. The dissociation dynamics of the complex at a 20 kg/m 3 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 gasphase 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 N 2 bath, whereas the translational and rotational degrees of freedom of N 2 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.

Chemical dynamics simulations are performed to study the association of benzene (Bz) and hexafluorobenzene (HFB) followed by the ensuing dissociation of the Bz–HFB complex. The calculations are done for 1000, 1500, and 2000 K with an impact parameter (b) range of 0–10 Å at each temperature. Almost no complexes are observed to form at b = 8 and 10 Å. Following three different methods of calculation of the temperature-dependent association rate constant k asso(T), the values obtained are 1.67 × 10–10, 1.86 × 10–10, and 2.05 × 10–10 cm3/molecule·s with a standard deviation of approximately 0.1 × 10–10 cm3/molecule·s for T = 1500 K. Among those values of k asso(T), the middle one is obtained by considering a relative translational energy of 3RT/2 at T = 1500 K, and the same is followed to calculate k asso(T) at 1000 and 2000 K. The Arrhenius parameters, using the k asso(T) values at three temperatures, are 0.203 × 10–10 cm3/molecule·s for the pre-exponential factor and −5.79 kcal/mol for the activation energy. The absolute value of the latter is similar to the Bz + HFB association energy of 5.93 kcal/mol. The ensuing dissociation dynamics of the complex is significantly different from the unimolecular dissociation dynamics, and an exponential function fits the N(t – t 0)/N(t 0) curves comparatively well. The ensuing dissociation is also observed to be independent of time for a statistically large sample size.

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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 300 K simulations and experiments, revealing that Bz*-Bz vibration-vibration (V-V) 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, < Ec>, for Bz*-Bz collisions is found ~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 < Ec>, 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 time scale of rethermalization.

Chemical dynamics simulations are performed to study collisional intermolecular energy transfer (IET) from highly vibrationally excited C6F6 to NO/N2 mixed baths equilibrated at 300 K. Two baths with a respective total of 200 and 1000 molecules are considered. The simulations are performed with very accurate intramolecular and intermolecular potential energy parameters either taken from literature or developed. A new simulation methodology is implemented to prepare a three‐component bath system. There is a rise in temperature during the IET dynamics in the smaller bath. The rotational ΔT is observed as 85 K in this bath and is much higher than 20 K obtained at 600 ps from the large bath simulation. However, both the simulations show less average energy transfer as compared to the one obtained from pure N2 bath simulation done previously [J. Chem. Phys. 2014, 140, 194103]. The deviation is less for the large bath simulation. The rotational degree of freedom for NO is found less effective towards IET compared to N2, whereas, the center‐of‐mass translational and vibrational modes behave similar to those of N2.

Role of environment (N2 molecules) on the association followed by ensuing dissociation reaction of benzene + benzene system is studied here with the help of a new code setup. Chemical...

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.