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. The 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 initio fragment-based calculation of
the cohesive energy of crystalline cubane at MP2C-F12 and CCSD(T)
levels. 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 the related anharmonic
contributions to the thermodynamic properties arising from these dynamic
degrees of freedom is presented and validated. Finally, the question
of whether the molecular steric strain is compensated by stronger
crystal cohesion is addressed.