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.

Motivated by photodissociation experiments in which non-RRKM nanosecond lifetimes of the ethyl radical were reported, we have performed a classical trajectory study of the dissociation and isomerization of C2H5 over the energy range 100-150 kcal/mol. We used a customized version of the AIREBO semiempirical potential (Stuart, S. J.; et al. J. Chem. Phys. 2000, 112, 6472-6486) to more accurately describe the gas-phase decomposition of C2H5. This study constitutes one of the first gas-phase applications of this potential form. At each energy, 10,000 trajectories were run and all underwent dissociation in less than 100 ps. The calculated dissociation rate constants are consistent with RRKM models; no evidence was found for nanosecond lifetimes. An analytic kinetics model of isomerization/dissociation competition was developed that incorporated incomplete mode mixing through a postulated divided phase space. The fits of the model to the trajectory data are good and represent the trajectory results in detail through repeated isomerizations at all energies. The model correctly displays single exponential decay at lower energies, but at higher energies, multiexponential decay due to incomplete mode mixing becomes more apparent. At both ends of the energy range, we carried out similar trajectory studies on CD2CH3 to examine isotopic scrambling. The results largely support the assumption that a H or a D atom is equally likely to dissociate from the mixed-isotope methyl end of the molecule. The calculated fraction of products that have the D atom dissociation is ∼20%, twice the experimental value available at one energy within our range. The calculated degree of isotopic scrambling is non-monotonic with respect to energy due to a non-monotonic ratio of the isomerization to dissociation rate constants.

Molecular dynamics simulations were used to study the effect of pressure on the vibrational deactivation of HO 2 embedded in an Ar bath gas at 800 K and at pressures ranging from 10 atm to 400 atm. The time dependent decay of vibrational energy is found to be poly-exponential for all of the simulated pressures. Plots of the relaxation rate constants as a function of density show deviation from the expected linear dependence at ~250 atm. A combinatorial multi-bath-gas collisional model suggests this deviation is due to the breakdown in the isolated binary collision approximation. Comparisons to studies with similar findings and additional considerations for understanding this behavior are discussed.

The classical dynamics and rates of isomerization and dissociation of HO2 have been studied using two potential energy surfaces (PESs) based on interpolative fittings of ab initio data: An interpolative moving least-squares (IMLS) surface [A. Li, D. Xie, R. Dawes, A. W. Jasper, J. Ma, and H. Guo, J. Chem. Phys. 133, 144306 (2010)] and the cubic-spline-fitted PES reported by Xu, Xie, Zhang, Lin, and Guo (XXZLG) [J. Chem. Phys. 127, 024304 (2007)]. Both PESs are based on similar, though not identical, internally contracted multi-reference configuration interaction with Davidson correction (icMRCI+Q) electronic structure calculations; the IMLS PES includes complete basis set (CBS) extrapolation. The coordinate range of the IMLS PES is limited to non-reactive processes. Surfaces-of-section show similar generally regular phase space structures for the IMLS and XXZLG PESs with increasing energy. The intramolecular vibrational energy redistribution (IVR) at energies above and below the threshold of isomerization is slow, especially for O-O stretch excitations, consistent with the regularity in the surfaces-of-section. The slow IVR rates lead to mode-specific effects that are prominent for isomerization (on both the IMLS and XXZLG) and modest for unimolecular dissociation to H + O2 (accessible only on the XXZLG PES). Even with statistical distributions of initial energy, slow IVR rates result in double exponential decay for isomerization, with the slower rate correlated with slow IVR rates for O-O vibrational excitation. The IVR and isomerization rates computed for the IMLS and XXZLG PESs are quantitatively, but not qualitatively, different from one another with the largest differences ascribed to the ~2 kcal/mol difference in the isomerization barrier heights. The IMLS and XXZLG results are compared with those obtained using the global, semi-empirical double-many-body expansion DMBE-IV PES [M. R. Pastrana, L. A. M. Quintales, J. Brandão, and A. J. C. Varandas, J. Chem. Phys. 94, 8073 (1990)], for which the surfaces-of-section display more irregular phase space structure, much faster IVR rates, and significantly less mode-specific effects in isomerization and unimolecular dissociation. The calculated IVR results for all three PESs are reasonably well represented by an analytic, coupled three-mode energy transfer model.

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