The low-temperature crystal structure of neopentane.

Neopentane [tetramethylmethane, C(CH3)4] is an ideal tetrahedral molecule, with 4 ̅3m symmetry, being the simplest alkane to contain a quaternary carbon and the lightest homologue in the series X(CH3)4, where X may be any one of the group IV elements, C, Si, Ge, Sn or Pb. Neopentane solidifies below 256.6 K in an orientationally-disordered cubic-close-packed plastic crystal. The structure and properties of this plastic crystalline phase have been extensively studied. At 140 K, neopentane transforms into an unknown structure that has been described as cubic, tetragonal and hexagonal. Using perdeuterated neopentane our goals are: (1) to determine the nature of the low–temperature phase of neopentane; (2) to produce a complete structure refinement; (3) to characterise the phase transition (including any hysteresis / frozen-in disorder) in to/from the higher-temperature phase.