Orientational Disorder in Crystalline Disubstituted Benzenes and Its Implications for Sublimation and Polymorphism

Derivatives of benzene with multiple similar substituents possess a large potential to exhibit static disorder of their crystalline phases. Since multiple orientations of these molecules in such crystals can lie close in terms of the electronic energy, appreciable contributions to the thermodynamic properties of the crystals and their phase behavior, such as polymorphism or sublimation, can arise due to related configurational effects. Four target materials, always containing two ethynyl, cyano, fluoro, carbonyl, or nitro substituents attached to a benzene ring, are selected for a combined computational and experimental study. Calorimetry, vapor pressure measurements, density functional theory, and ab initio calculations are employed to derive and to interpret the residual and configurational entropy terms relevant for these materials and to trace the disorder to particular molecular interactions. Impact of the local disorder on the sublimation equilibrium and polymorphism is demonstrated computationally. Comparison of calculated and experimental sublimation data reveals that interactions of ethynyl- and cyano-derivatives of benzene can be described satisfactorily within the current PBE-D3 framework. On the other hand, such an accuracy is not reached for the nitro-derivative of benzene.