sp3)C7-H7AC•••C3' 2.789(3) 2.726(2) 2.667(2) 2.668(6) x-1, y, z (sp2)C6-H6•••C5' 2.706(2) 2.661(2) 2.642(2) 2.632(5) x-0.5, y, 0.5-z

Crystal structures of organic compounds with all intermolecular contacts longer than the sum of van der Waals radii can be classified as a loose state of solids. The survey of Cambridge Structural Database (CSD) revealed about 450 of such crystals. Specific features responsible for the loose arrangement are low magnitudes of electrostatic potential on the molecular surface and the concentric distribution of net atomic charges, reducing the contribution of electrostatic forces to the crystal cohesion interactions. Additionally, in the loose structures a partial mismatch between the requirement of molecular close-packing with electrostatic and specific directional interactions has been observed. Consequently, the cohesion forces are dominated by dispersion interactions, and their contribution is much larger compared to Coulombic and polarization energy. In the most loose compound presently deposited in the CSD, bis(trichlorosilyl)acetylene, the shortest of all contacts is by 0.256 Å longer than the sum of van der Waals radii, according to Bondi.

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There are clearly two maxima in the distribution of the shortest intermolecular contacts, referred to the atomic van der Waals radii, in crystals of organic compounds. Accordingly, the crystals of organic compounds can be classified into those governed by strong cohesion forces (such as OHhalogen bonds, and dispersion forces). In about 1/3 of all known structures of organic compounds, there are strong cohesion interactions, while in 2/3 of structures the shortest contacts are associated with weak interactions. Characteristic properties of organic compounds depend on these either strong or weak cohesion forces. The distributions of the shortest intermolecular contacts of specific types, such as OHand Br•••Br can be approximated by the Gaussian functions. These Gaussian functions, with mean distance and standard deviation characteristic of specific interactions, can be used for predicting molecular arrangements and for validating crystal structures.

Benzocaine (BZC), an efficient and highly permeable anaesthetic and an active pharmaceutical ingredient of many commercially available drugs, was studied under high pressure up to 0.78 GPa. As a result, new BZC polymorph (IV) was discovered. The crystallization of polymorph (IV) can be initiated by heating crystals of polymorph (I) at a pressure of at least 0.45 GPa or by their compression to 0.60 GPa. However, no phase transition from polymorph (I) to (IV) was observed. Although polymorph (IV) exhibits the same main aggregation motif as in previously reported BZC polymorphs (I)–(III), i.e. a hydrogen-bonded ribbon, its molecular packing and hydrogen-bonding pattern differ considerably. The N—H...N hydrogen bonds joining parallel BZC ribbons in crystals at ambient pressure are eliminated in polymorph (IV), and BZC ribbons become positioned at an angle of about 80°. Unfortunately, crystals of polymorph (IV) were not preserved on pressure release, and depending on the decompression protocol they transformed into polymorph (II) or (I).