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

Abstract

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.

Mode‐specific pressure effects on the relaxation of an excited nitromethane molecule in an argon bath

“…None of these above studies decomposed the decay curves into vibrational or rotational normal mode‐specific decays. In our previous work (henceforth RWP), 19 we described an approximate method to do this decomposition using only the archived positions and velocities of the molecule as a function of time. This method requires the calculation of the vibrational, rotational, and Coriolis kinetic energies of the molecule.…”

Pressure Effects on the Relaxation of an Excited Ethane Molecule in High-Pressure Bath Gases

“…These results support a previous conclusion on the nitromethane molecule where after an initial very rapid relaxation of the rotational energy the rotational energy then proceeds to relax in lockstep with the vibrational relaxation. 53,54 IV. DISCUSSION In Figure 5, the initial rotational decay rate at 800 K has structure as a function of pressure not well represented by a linear dependence on pressure that well represents the 300 K results.…”

“…10,59 In the case of ethane that would be the CH 3 rotation. However, similar to CH 3 NO 2 , 53 it is expected that the CH 3 symmetric stretches will couple with the CH 3 rotation, making these modes also "gateway" modes.…”

“…Fitting of the Decay Curves. Following the recommendation from our previous works on nitromethane, 12,53,54 we used the Lendvay−Schatz (LS) function, 55 eq 2, to fit the vibrational energy decay curves and the ExpLS function, eq 3, to fit the rotational energy decay curves.…”

Mode‐to‐Mode Collision Energy Transfer from Vibrationally Excited C6F6 to NO/N2 Mixed Bath with the Development of New Potential Energy Functions

“…These studies include a recently developed bath model where a single excited molecule is surrounded by many thermalized bath molecules in a simulation box and IET was studied between the excited and the bath molecules. [40][41][42][43][44][45][46][47][48][49] In a seminal work, [40] the IET was studied for C 6 F 6 * + N 2 system, where a highly vibrationally excited hexafluorobenzene (HFB) was placed in a N 2 bath with 1000 N 2 molecules equilibrated at a temperature of 300 K. In that study, a single collisional bath density (pressure) was obtained which represented gas phase bi-molecular collisional limit. Also studied was the mode specific energy transfer efficiencies for both the excited molecule as well as the bath.…”