Isomechanical Groups in Molecular Crystals and Role of Aromatic Interactions

We have measured mechanical properties in molecular crystals of a series of para-substituted benzoic acids using nanoindentation. Two linear correlations were found for these materials: first between the Young's modulus and the melting point per unit of volume, and second between the hardness and the crystal cohesive energy density. These correlations, however, were found to only hold within isomechanical groups. These are groups of materials with similar interaction strengths and dimensionalities. A close analysis of the intermolecular interactions present in these crystals revealed that structures with aromatic rings interacting through t-type interactions may achieve higher interaction dimensionalities and thus result in harder crystals than those with rings interacting through aromatic stacking. In these systems, thus, the dimensionality of the aromatic interactions in the crystals appears to play the major role in their mechanical behavior.