Crystal Structure of Nitromethane up to the Reaction Threshold Pressure

Abstract

Angle dispersion X-ray diffraction (AXDX) experiments on nitromethane single crystals and powder were performed at room temperature as a function of pressure up to 19.0 and 27.3 GPa, respectively, in a membrane diamond anvil cell (MDAC). The atomic positions were refined at 1.1, 3.2, 7.6, 11.0, and 15.0 GPa using the single-crystal data, while the equation of state (EOS) was extended up to 27.3 GPa, which is close to the nitromethane decomposition threshold pressure at room temperature in static conditions. The crystal structure was found to be orthorhombic, space group P2(1)2(1)2(1), with four molecules per unit cell, up to the highest pressure. In contrast, the molecular geometry undergoes an important change consisting of a gradual blocking of the methyl group libration about the C-N bond axis, starting just above the melting pressure and completed only between 7.6 and 11.0 GPa. Above this pressure, the orientation of the methyl group is quasi-eclipsed with respect to the NO bonds. This conformation allows the buildup of networks of strong intermolecular O...H-C interactions mainly in the bc and ac planes, stabilizing the crystal structure. This structural evolution determines important modifications in the IR and Raman spectra, occurring around 10 GPa. Present measurements of the Raman and IR vibrational spectra as a function of pressure at different temperatures evidence the existence of a kinetic barrier for this internal rearrangement.

X-ray crystallography of simple molecular solids up to megabar pressures: application to solid oxygen and carbon dioxide

“…In some cases, the quality may still be enough for structural refinement in the 20 GPa range, but hardly above. For example, X-ray diffraction data from a single crystal of nitromethane grown from the melt at 1 GPa, 300 K were collected up to 15 GPa and could be used to refine the atomic positions, but at higher pressure, the crystal quality became too poor to obtain reliable peak intensities [26]. Helium has been shown to remain quasi-hydrostatic well above its freezing pressure (11 GPa at 300 K) [27,28], and is thus considered as the best pressure transmitting medium (PTM) for samples in a DAC.…”

A Molecular Dynamics Study of Classical Vibrational Spectra in Hydrostatically Compressed Crystalline Nitromethane

“…The results are shown in Figure 6; panels a, b, and c correspond to the CN stretching mode, a CH 3 deformation mode, and the NO 2 asymmetric stretching mode, respectively, as determined from the normal-mode eigenvectors of a single molecule in the explicit crystal field (calculated as described in section II.C). These are also the modes considered by Citroni et al (see Figure 6 in ref 24). In order to facilitate comparisons to the experimental data, the widths of the intervals in the frequency domain (i.e., abscissa intervals in Figure 6) are approximately the same as in Figure 6 of ref 24.…”

“…Liu et al obtained isothermal compression curves that are much stiffer than those obtained from both experiment [24][25][26] and earlier simulations 68 for pressures greater than 2 GPa; states on the Hugoniot locus as determined using the iterative approach due to Kress et al 62 similarly indicate an unrealistically strong interatomic repulsion that arises from the use of Lennard-Jones potentials for the nonbonded interactions in the force field model of Alper et al…”

“…25 It has now been established that there are only two phases, both with the same space group, between zero pressure and 40 GPa near room temperature. 24 Given the large number of published journal articles for nitromethane, not to mention a sizable body of work that appears only in conference proceedings (see, for example, ref 46), in the following paragraphs we briefly summarize only a few theoretical and experimental studies of particular relevance to the present work.…”

Understanding the desensitizing mechanism of olefin in explosives: shear slide of mixed HMX-olefin systems

“…It is efficient to find good desensitizers to partly overcome the said intrinsic conflict between energy and sensitivity of CHNO explosives. To expose the structure-property relationship of explosive compounds and desensitizers applied in practical formulations is helpful to find new more appropriate ones [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. We have discussed the mechanism of some applicable desensitizers versus external mechanical stimuli including crystalline graphite and TATB [23,24], and the reason for low mechanical sensitivities of some explosives from a viewpoint of molecular stacking [25].…”