Pressure effects on the relaxation of an excited nitromethane molecule in an argon bath

Abstract

Classical molecular dynamics simulations were performed to study the relaxation of nitromethane in an Ar bath (of 1000 atoms) at 300 K and pressures 10, 50, 75, 100, 125, 150, 300, and 400 atm. The molecule was instantaneously excited by statistically distributing 50 kcal/mol among the internal degrees of freedom. At each pressure, 1000 trajectories were integrated for 1000 ps, except for 10 atm, for which the integration time was 5000 ps. The computed ensemble-averaged rotational energy decay is ∼100 times faster than the vibrational energy decay. Both rotational and vibrational decay curves can be satisfactorily fit with the Lendvay-Schatz function, which involves two parameters: one for the initial rate and one for the curvature of the decay curve. The decay curves for all pressures exhibit positive curvature implying the rate slows as the molecule loses energy. The initial rotational relaxation rate is directly proportional to density over the interval of simulated densities, but the initial vibrational relaxation rate decreases with increasing density relative to the extrapolation of the limiting low-pressure proportionality to density. The initial vibrational relaxation rate and curvature are fit as functions of density. For the initial vibrational relaxation rate, the functional form of the fit arises from a combinatorial model for the frequency of nitromethane "simultaneously" colliding with multiple Ar atoms. Roll-off of the initial rate from its low-density extrapolation occurs because the cross section for collision events with L Ar atoms increases with L more slowly than L times the cross section for collision events with one Ar atom. The resulting density-dependent functions of the initial rate and curvature represent, reasonably well, all the vibrational decay curves except at the lowest density for which the functions overestimate the rate of decay. The decay over all gas phase densities is predicted by extrapolating the fits to condensed-phase densities.

Pressure effects on the relaxation of an excited hydrogen peroxyl radical in an Argon bath

“…The initial vibrational decay rate begins to show a non-linear dependence on densities higher than ∼0.002 Å -3 corresponding to ∼250 atm at 800 K. This deviation implies the breakdown of the isolated binary collision model. The density dependence can be reasonably well represented by a combinatorial collision model proposed Rivera-Rivera et al 7 that represents the effect of interfering collisions of multiple bath gas atoms with the molecule. Rotational relaxation rates remain proportional to density over the full range of pressures studied.…”

“…This method is free of the IBC assumption and any number of atoms as dictated by pressure can be involved in an inelastic collision with the molecule. There are only a recent few examples [6,7] of this approach in dense gases because of the computational expense of following bath gas collisions that largely serve only to maintain the thermal state of the bath. Only one of these examples, Rivera-Rivera et al [7] (hereafter identified as RWST), explicitly considered IBC breakdown and concluded that for nitromethane in an Ar bath at 300K it occurs at ~100 atm, a pressure well within the operating range of engines.…”

“…There are only a recent few examples [6,7] of this approach in dense gases because of the computational expense of following bath gas collisions that largely serve only to maintain the thermal state of the bath. Only one of these examples, Rivera-Rivera et al [7] (hereafter identified as RWST), explicitly considered IBC breakdown and concluded that for nitromethane in an Ar bath at 300K it occurs at ~100 atm, a pressure well within the operating range of engines. The HO 2 system has had its potential energy surface, dynamics, and kinetics theoretically studied multiple times but never in the context of IBC breakdown.…”

Bath Model for N2 + C6F6 Gas-Phase Collisions. Details of the Intermolecular Energy Transfer Dynamics

“…27 A simulation similar to this one for C 6 F 6 + N 2 was recently reported for nitromethane in an Ar bath. 28 This work reports additional details and new extensions of the previous bath model simulations of IET for C 6 F 6 + N 2 . In the previous study, the sensitivity of IET to the form of the C 6 F 6 + N 2 intermolecular potential was investigated for a high N 2 bath density, and in the current work this investigation is extended to a low bath density.…”

Collisional energy transfer in polyatomic molecules at high temperatures: Master equation analysis of vibrational relaxation of shock-heated alkanes

“…These studies revealed that the energy transfer rate depends on the initial energy, bath temperature, and type of third-body collider. Computationally, classical trajectory calculations have been used to simulate collisions of polyatomic molecules with bath gases, revealing some fundamental mechanisms and trends of collisional energy transfer (see, e.g., [15][16][17][18][19][20][21][22] and references therein). Recent studies have demonstrated that the pressure-dependent rate constants for some unimolecular reactions can be quantitatively reproduced through master equation modeling using the energy transfer parameters obtained from classical trajectory simulations [23][24][25].…”