Isolation of Geometric Contributions to Plastic Crystal Transformations Through Selective Deuteration of Neopentyl Glycol

Plastic crystals exhibit a degree of structural order between an ordered crystal, in which molecules have fixed position and orientation, and a liquid, in which molecules can rotate and translate in every direction. Small molecules within plastic crystals are fixed on a rigid lattice, but still maintain rotational degrees of freedom. While it is understood that the strength of intermolecular bonding, and in particular, the degree of hydrogen bonding, plays a critical role in governing the thermodynamics of the solid-solid transformation, the role of other geometric parameters remains unclear. This information remains critical to identify novel plastic crystal molecules, and to engineer the properties of known compounds.
To resolve this question, we have investigated a series of selectively-deuterated neopentyl glycol (NPG) compounds which have identical chemical interactions, but span a spectrum of molecular sphericity and molecular weights. By selectively deuterating either hydroxyl groups (C(CH₃)₂(CH₂OD)₂), hydroxy methyl groups (C(CH₃)₂(CD₂OH)₂), or methyl groups (C(CD₃)₂(CH₂OH)₂), or combinations thereof, we produce a series of compounds which undergo identical solid-solid transformations between a high symmetry cubic structure and a low-symmetry monoclinic structure, but which have subtle differences in molecule geometry and size. We measure the resulting enthalpy and entropy of transformation, as well as the heat capacities of these compounds, and report the relationship between molecular geometry and the resulting thermodynamics of the solid-solid transformations. These results clarify the role of molecule geometry on the resulting rotational degrees of freedom in the plastic crystalline phase, and may help design future novel plastic crystal molecules.