Crystal and pVT data and thermodynamics of the phase transitions of 2-methyl-2-nitropropane

Abstract

The lattice symmetry and lattice parameters of the low-temperature solid forms II and I of 2-methyl-2-nitropropane (TBN), (CH₃)₃CNO₂, have been determined by X-ray powder diffraction and found to be of the orthorhombic and face-centred cubic type, respectively. The pVT data have been determined in the temperature range 228 to 353 K up to ca. 300 MPa, as well as the volume changes accompanying the phase transitions. The volume changes allow the calculation, via the Clausius–Clapeyron equation of the corresponding entropy changes. The volume changes on melting decrease markedly with increasing pressure, whereas those at the solid–solid transitions (II → I and III → II) exhibit a much less pronounced pressure dependence. With regard to the entropy changes, only the II → I transition shows a small dependence (decrease) on increasing pressure. The two mean parts of the entropy changes (configurational and volume-dependent terms) are discussed for each transition.