LOW-TEMPERATURE PHASE OF t-BUTYL CHLORIDE (tBC): CRYSTAL STRUCTURE AND, NOMENCLATURE. Reuben Rudman, Adelphi University, Chemistry Dept., Garden City, NY 11530, USA rudman@sable.adelphi.edu

tBC, with a pseudo-spherical molecular envelope, forms an orientationally-disordered, plastic crystalline phase as it is cooled below its freezing point. Although plastic crystals often have only one solid-solid phase transition (between the disordered and ordered phases), tBC has long been known to form a partially-ordered crystalline phase prior to forming the ordered phase [R. Rudman and B.Post, Mol. Cryst. 5, 95-110 (1968)]. These three tBC phases have been labeled Phases I, II, and III in dozens of articles in the scientific literature. However, S. Ohtani and T. Hasabe (OH) [Chem. Lett. 1986, 1283-1286] reported a newly discovered phase, stable over a 1.8K range (217.7K - 219.5K), between the historic Phases I and II. This phase, which was identified on the basis of differential thermal analysis and not characterized further, was labeled by them Phase II, with the historic Phases II and III relabeled as Phase III and Phase IV, respectively. This relabeling will lead to a great deal of confusion in the literature and subsequent databases. |

Single-crystal X-ray diffraction data have been obtained for the disordered and partially ordered phases of tBC (Rudman & Post, loc. cit.). However, because single crystals of tBC shatter as they transform to the low-temperature ordered phase (our Phase III), the crystal structure of this ordered phase has not been experimentally investigated previously. In 1993, J. Chen and L. S. Bartell [J.Chem.Phys. 97, 10645-8] published a theoretical Molecular Dynamics study of this phase which they identify, following OH, as Phase IV. They determined the unit cell, space group, and crystal structure of this phase using computational methods and stated that they verified their proposed structure by indexing neutron powder diffraction data (not published).

This paper will report on the results of a low-temperature synchrotron X-ray powder diffraction investigation of the various phases of tBC. The crystal structure of the ordered phase, an investigation of the newly reported phase (stable only over a 1.8K temperature range), and recommendations for the proper nomenclature of these various phases will be presented.