MOLECULAR-DYNAMICS STUDIES OF SOLID-STATE PHASE-TRANSITION .1. INFERENCE OF LOW-TEMPERATURE PHASE OF TERT-BUTYL CHLORIDE

A molecular dynamics simulation was carried out on a 188-molecule cluster of tert-butyl chloride to investigate whether it would transform upon cooling from the known tetragonal phase III to phase IV, a phase of unknown structure. The cluster was found to exist stably in the tetragonal phase at an appropriately low temperature and to undergo a change to a new form when cooled. The transition was shown to be reversible and to occur at a temperature nearly 100 deg lower than in the bulk, as would be expected for such a small crystal. The new form corresponded to a monoclinic structure, space group P2(1)/m, no. 11. Powder diffraction patterns calculated from the molecular coordinates, after minor structural refinement, agreed convincingly with experimental electron and neutron patterns which had previously resisted indexing. This demonstrates the utility of molecular dynamics techniques in the study of solid-state structures in cases where transitions are facile.