Title: 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 similar to 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 themore » 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.« less

Authors:

Rivera-Rivera, Luis A. ^[1]; Wagner, Albert F.  ^[2];

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Sewell, Thomas D. ^[1]; Thompson, Donald L. ^[1]

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+ Show Author Affiliations

1. Univ. of Missouri, Columbia, MO (United States). Dept. of Chemistry
2. Argonne National Lab. (ANL), Argonne, IL (United States). Chemical Sciences and Engineering Division

Publication Date:

Mon Jan 05 00:00:00 EST 2015

Research Org.:

Argonne National Lab. (ANL), Argonne, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); US Army Research Laboratory (USARL)

OSTI Identifier:

1393974

Alternate Identifier(s):

OSTI ID: 1228458

Grant/Contract Number:  

AC02-06CH11357; W911NF-09-1-0199

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Chemical Physics

Additional Journal Information:

Journal Volume: 142; Journal Issue: 1; Journal ID: ISSN 0021-9606

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

74 ATOMIC AND MOLECULAR PHYSICS; classical trajectory; energy relaxation; high pressure; nitromethane; Rotational dynamics; Molecular dynamics; chemical compounds; crystal structure; Equilibrium thermodynamics; zero point energy; collision theories; gas phase; regression analysis; phase transitions