Gas-Phase-like Inertial Rotation of a Platonic Solid in Reticular Amphidynamic Crystals of CUB-5: Molecular Dynamics and Experiment

Amphidynamic crystals are a type of condensed matter that blends two extremes of the dynamic spectrum: rigid components forming a static lattice and rapidly moving parts. Among them, ordered rotor arrays within metal-organic frameworks (MOFs) constitute a promising platform to explore uncharted territories, such as gas phase-like dynamics in the crystalline state. Through quantum mechanical (QM) calculations and molecular dynamics (MD) simulations we verified that nearly barrierless cubane rotators in CUB-5 display rotational dynamics that transitions from continuous or inertial at high temperature, to chaotic behavior, and ultimately to discrete jumps, as the temperature decreases from room temperature down to cryogenic conditions. ¹H NMR spin-lattice (<i>T</i>₁) relaxation measurements corroborate our theoretical predictions, with experimental rotational activation energy of 0.17 kcal/mol and an attempt frequency of 1.03 × 10¹² s^-1 that compare well with calculated values of 0.15 kcal/mol and 0.38 × 10¹² s^-1, respectively.