Selective Deuteration of Neopentyl Glycol and its Effect on the Plastic Crystal Transformation

Transitions between rotationally ordered and disordered states in globular small molecules are associated with large entropy changes, and thus, hold promise as solid-state barocaloric refrigerants. However, the relationships between elements of the molecular structure, and the corresponding thermodynamic properties of the phase transformation between ordered and disordered states remain unresolved. We hypothesize that more spherical molecules, as measured by their rotational moments of inertia, exhibit larger relative increases in rotational degrees of freedom as they transition to rotationally disordered states. We probe this by isolating the impact of rotational moments of inertia from more dominant factors, including intermolecular bonding, through selective deuteration of different functional groups of the model plastic crystal molecule neopentyl glycol. We demonstrate a decrease in the phase transition temperature of up to approximately 3 K associated with the existence of deuterated methyl groups and explain this change in terms of relative changes in rotational moments of inertia of the compounds. This observation places bounds on the role of rotational moments of inertia in thermodynamic aspects of the phase transformation, and introduces a vector for subtle tuning of the transition point for cooling applications.