Dynamic disorder, Strain and Sublimation of Crystalline Caged Hydrocarbons from First Principles

Caged hydrocarbons exhibit diverse molecular and material properties thanks to a large variability of the three-dimensional carbon backbone of such molecules. High molecular symmetry of caged hydrocarbons predetermines these materials to pack very efficiently in crystal lattices that belong to highly symmetric space groups, as well as to easily form plastic solid phases with highly pronounced dynamic disorder in the vicinity of the melting temperature. This work aims at resolving the literature debate about the two contradictory values of experimental sublimation enthalpy for cubane, being a typical state of the art for such uncommon molecules. For this purpose, we use density functional theory (DFT) powered quasi-harmonic protocol, further fortified with the ab inito fragment-based calculation of the cohesive energy of crystalline cubane at the CCSD(T) level. Further, this work presents a viable first-principles treatment of dynamic disorder of molecules via their hindered rotations in the crystal lattice. A protocol for assessment of the energetic and entropic aspects of this local disorder, as well as to the related anharmonic contributions to the thermodynamic properties arising from these dynamic degrees of freedom is presented and validated. Finally, the question, whether the molecular steric strain is compensated by stronger crystal cohesion, is addressed.