Orthogonal contacts between benzene rings: A special type of specific intermolecular interactions

The crystal structures of benzene and some of its derivatives are analyzed in detail. The structures are characterized by orthogonal contacts between the benzene cycles (OBzC). Although the orthogonal contacts conflict with the close packig law, they are an important structure-forming factor, being thus a specific type of intermolecular interactions. The OBzC system, which is inherent in the orthorhombic crystals of benzene, remains invariable in 2-aminophenol and 2-amino-4-chlorophenol crystals, notwithstanding other specific interactions (H-bonding and halogen...halogen contacts). OBzC are also observed in para- and ortho-cresol crystals. A classification of OBzC is given, and an especially effective projection method (rectangular projection) is suggested.