Theoretical Studies of Solid Nitromethane

Report No. ARL-RP-14
Authors: Sorescu, Dan C.; Rice, Betsy M.; Thompson, Donald L.
Date/Pages: February 2001; 26 pages
Abstract: A classical potential to simulate the dynamics of a nitromethane crystal as a function of temperature and pressure is described. The intramolecular part of the potential was taken as superposition of bond stretching, bond bending, and torsional angles terms. These terms were parametrized on the basis of the geometric and spectroscopic (vibrational frequencies and eigenvectors) data obtained using ab initio molecular orbital calculations performed at the B3LYP/6-3 1 G level on an isolated molecule. The intermolecular potential used is of the Buckingham 6-exp form plus charge-charge Coulombic interactions and has been previously developed by us (Sorescu, D. C.; Rice, B. M.; Thompson, D. L. J: Phys. Chem. 1997, BlOl, 798) to simulate crystals containing nitramine molecules and several other classes of nitro compounds. The analyses performed using constant pressure and temperature molecular dynamics simulations and molecular packing calculations indicate that the proposed potential model is able to reproduce accurately the changes of the structural crystallographic parameters as functions of temperature or pressure for the entire range of values investigated. In addition, the calculated bulk modulus of nitromethane was found in excellent agreement with the corresponding experimental results. Moreover, it was determined that the present potential predicts correctly an experimentally observed 450 change in methyl group orientation in the high-pressure regime relative to the low-temperature configuration. The analysis of the linear expansion coefficients and linear compression data indicate anisotropic behavior for the unit cell edges.
Distribution: Approved for public release
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Last Update / Reviewed: February 1, 2001