A molecular dynamics study of the rotator phase of t-butyl bromide

Molecular dynamics calculations are reported for the cubic plastic-crystal phase of t-butyl bromide. An intermolecular potential model has been constructed that yields a stable rotator phase, reasonable thermodynamic properties and a neutron structure factor in fair accord with experimental observations. The calculated orientational distribution functions show that the C—Br and C—Me bonds align preferentially along the (110) crystal axes and avoid the (111) directions; there is only a small difference between these two distributions. The fluctuations in collective orientational parameters and their relation to the transition to an ordered phase are discussed in terms of a generalised Landau model. We find that the primary order parameters are probably certain L= 3 variables, suggesting that the ordering process is analogous to that in carbon tetrabromide, rather than to that found in t-butyl cyanide.