Caged hydrocarbons exhibit interesting properties, rendering these materials suitable candidates for a variety of uses. Their carbon skeleton usually experiences a large strain unlike other, chained hydrocarbons – this prompts discussion about their use as energy storage media, precursors for pharmaceuticals or explosives. Despite this, relevant thermodynamic data for these compounds is relatively scarce or subject to significant inconsistencies. The primary aim of this work is to evaluate sublimation properties such as sublimation enthalpy and sublimation pressure using <i>ab initio</i> and density functional theory calculations. Calculations are performed both for the solid and gaseous phase and their results processed using statistical-mechanical tools. Furthermore, an <i>ab initio</i> fragment-based additive scheme is utilized to accurately assess the cohesion energy of the crystals. This scheme relies on the additivity of pairwise or higher-order interactions to calculate bulk properties using higher levels of theory, which would be otherwise unaffordable in bigger systems. Additionally, disorder in the solid phase of caged hydrocarbons is also studied. Carboxylated derivatives of caged hydrocarbons exhibit proton transfer in their respective solid phases, with nonsymmetrical potential surfaces thanks to the crystal lattice. This happens seemingly even at room temperatures, if the solid phase is stable. The corresponding energy barriers are studied using the dimer method to sample the potential energy landscape. Some caged hydrocarbons may also exhibit a form of rotational disorder, as their molecules possess symmetrical shape and zero dipole moment, enabling to rotate in the crystal lattice without significant energy penalty. Variation of the fragment-based additive scheme is also employed as a means to evaluate both absolute and relative energies of disordered (perturbed) configurations. These perturbations are thought of as point defects in a crystal lattice, where a single molecule is displaced from its equilibrium position. Both proton transfer and rotational disorder can potentially affect measurements of atomic positions, resulting in inaccurate or incomplete crystallographic data. The aforementioned strain energy of various caged hydrocarbons is also evaluated as the reaction enthalpy of the corresponding homodesmotic decomposition reaction and by comparison with a molecular equivalent constructed from non-strained group increments.

Sublimation, strain energies and local disorder of crystalline caged hydrocarbons: A theoretical study

ACS Spring 2022

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