High pressure crystal structure of nitroethane.

Abstract

Ambient temperature static high pressure compression of liquid nitroethane has been performed using the diamond anvil technique. The first transition to a crystalline powder around 4.2 GPa did not return reliable indexing solutions and could be the result of a mixture of phases. Subsequent cycling of the pressure apparatus with the same sample triggered a solid-solid transition around 4.9-4.3 GPa on the downstroke. Indexing returned a monoclinic structure with spacegroup P21 or P21/m which is consistent with the lowest enthalpy solution predicted from density functional theory (DFT) calculations performed in this study, namely, P21. Attempts to reproduce the first anomalous mixture of phases with other samples were not successful; rather, the numerically preferred monoclinic structure manifests itself after a liquid-solid transition around 4.3-3.6 GPa, a value consistent with the results of a previous study. The transition typically occurs more readily on the downstroke. Incomplete Debye rings resulting from preferred orientation complicates any Rietveld refinement, though the DFT simulations predict 2 molecules per unit cell. Unit cell volumes during the upstroke were within 3% of that predicted by DFT calculations. This departure increases to about 9% below that from DFT predictions during the downstroke suggesting the presence of residual stresses due to non-hydrostatic conditions. Finally, a sudden relief in d-spacing values around 3.7-4.3 GPa during the downstroke is thought to be due to the influence of intermolecular hydrogen bonding.