Polymorphism of methylchloromethane compounds VIII.: Low-temperature phases of t-butyl chloride

The experimentally determined crystal structure of ordered t-butyl chloride in its lowest temperature Phase III state has been found to agree with that predicted by the theoretical calculations of Chen and Bartell [J. Phys. Chem. 97 (1993) 10 645]. An analysis of the volume changes (Delta V) which occur during the phase transition, by resolving Delta V into components parallel and perpendicular to the threefold molecular axis, sheds light on the mechanism of the phase transition. However, the crystalline phase reported by Ohtani and Hasabe [Chem. Lett. (1986)1283] to exist between 217.7 and 219.5 K has been shown to actually be a manifestation of heterophase pretransition activity as the orientationally-disordered Phase I begins to form while the bulk of the sample still exists in the partially-disordered Phase II. Synchrotron radiation low-temperature X-ray powder diffraction data obtained approximately at every 0.3 K between 215 and 220 K show that the intensity of the Phase I (111) reflection (2 theta similar to 13.29 degrees, lambda = 1.15032 Angstrom) increases as the intensity of the Phase II (110) reflection (2 theta similar to 13.25 degrees) decreases in the temperature region 2-3 K below the transition temperature. This is explained in terms of the transition being initiated simultaneously in several microregions of the sample in accordance with the description given by Ubbelohde [The Molten State of Matter, 1978]. A labeling scheme for uniquely identifying polymorphic phases is introduced so as to prevent the confusion (in the scientific literature and subsequent data bases) which was caused by the same label being assigned to two different crystalline phases. This proposed scheme includes the phase identifier, the temperature of formation, whether it is formed as the temperature is being raised or lowered, the pressure of formation, and the label of the immediately preceding phase. (C) 1999 Elsevier Science B.V. All rights reserved.