Collision-Induced Intramolecular Energy Flow in Highly Excited Toluene

Abstract

The collision-induced relaxation of vibrationally excited molecules has been the subject of continuing interest for the past several decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] In recent years, furthermore, collisions involving large molecules have been studied actively, revealing valuable information on the rates and the mechanisms of vibrational energy processes. In particular, molecules vibrationally excited to near their dissociation threshold can undergo bond dissociation and vibrational relaxation, processes that play an important role in reaction dynamics. Recent studies 3,[5][6][7][8][9][10][11] show that when the excited molecule is a large organic molecule, the average amount of energy transfer per collision is not very large. The average energy transfer per collision between the highly vibrationally excited benzene and a noble gas atom is known to be about 30 cm, which is much smaller than benzene derivatives such as hexafluorobenzene or other hydrocarbons such as toluene and azulene.5,12-14 For example, for hexafluorobenzene + Ar, the measured value of the mean energy transfer per collision by the ultraviolet absorption method is −330 cm −1 , 13 whereas the calculated value using quasiclassical trajectory methods at 300 K is −150 cm . 14 These magnitudes are much larger in comparison to results for benzene colliding with argon. Among such large molecules, toluene is a particularly attractive molecule for studying collision-induced intramolecular energy flow and bond dissociation because of the presence of both methyl and ring CH bonds.In this paper, as an extension of th previous work in ref. 11, we study the collision-induced dynamics of highly vibrationally excited toluene interacting with argon using quasiclassical trajectory calculations. We now consider the collision system in which one CH (either CH methyl or CH ring ) vibration is in a highly excited state and the other in a ground state, while in Ref. 11 we considered both of CH methyl and CH ring bonds are in highly excited states. In the latter, it was not clear which CH bond is more effective for collisioninduced inter-and intra-molecular energy transfer in collision with Ar. Thus, we now elucidate which CH bond is more effective for collision-induced inter-and intra-molecular energy transfer, as considering and comparing the results of inter-and intra-molecular energy transfer dynamics when either CH methyl or CH ring is in highly excited states. Using the results obtained in the calculations, we discuss the relaxation of the excited CH vibration, and the time evolution of collision-induced intramolecular energy flow from the highly excited CH vibration. Thus, we elucidate the nature and mechanism of competition between methyl CH mode and ring CH mode in transferring energy to the incident atom. We set the initial vibrational energy of the methyl CH bond or the ring CH bond equal to the state just 0.10 eV below the dissociation threshold at 300 K. Interaction ModelThe present work follows the interaction model and numerical p...

The influence of the different elements of an organic molecule structure on the main kinetic parameters of its unimolecular reaction in the high-pressure region

“…The influence of the continuous-chain substitutes (1,2,3) in this case, except of a mobilization, is probably above all expressed in the reduction in the demobilization influence of the cyclobutate (4). So that the derivative for which both the influences mobilization and demobilization are counterbalanced lies near to these three derivatives (1,2,3) or even lower on the correlation line. This frontier derivative can have both its Arrhenius parameters maximum close to their estimation based on statistical theories (for example, RRKM), without the dynamical influence.…”

“…The main conclusion to be drawn from these studies [1,5] is that the relaxation process is dominated by the energy flow from the side chain (methyl group) to the benzene ring, but not in the opposite direction. The authors [5] write: "We do not find any evidence of energy flowing out of C-H ring and returning to C-H methyl .…”

“…The toluene molecule is a particularly attractive one to study because of the presence of both a benzene ring and a side chain. Most authors study the collision-induced dynamics of highly vibrationally excited toluene interacting with argon, using quasiclassical trajectory calculation [1,5] with the reproduced model Figure 1. Either CH methyl or CH ring has been set at its high-energy state (as a rule, a little bit below its dissociation threshold), while all other vibrational modes are in their ground state.…”

“…The present paper is devoted to the dynamics of energy mobilization inside an energized molecule in order that a reaction will proceed in the high-pressure region. Presently, the dynamics picture of the energy flow in chemical kinetics is studied practically exclusively by the method of collision-induced relaxation of a vibrationally excited molecule, which has been the subject of continuing interest for the past several decades [1][2][3][4][5][6]. The problem of the internal energy mobilization of a molecule is studied mainly in the fall-off region, where it plays a decisive role and determines in particular the transition in the high-pressure region.…”

“…For each energy surplus (E), many vibrations are performed at the random initial conditions, in order to calculate the average parameters of the detachment separately for light and for heavy atoms. The interaction model and numerical procedure is usually accepted now for computer calculation of collision-induced vibration [1,4]. The "molecule" vibrates until one of two atoms, the heavy or the light one, detaches from the diatomic molecule remaining.…”