Crystal engineering and correspondence between molecular and crystal structures. Are 2- and 3-aminophenols anomalous?

Single-crystal neutron diffraction analyses of 2- and 3-aminophenols have been performed. In addition to O-H ... N and N-H ... O hydrogen bonds, both these structures contain previously unidentified N-H ...pi and C-H ... O hydrogen bonds. This unusual mutual recognition pattern is not found in 4-aminophenol and other related systems. Its presence hints that the optimization of the herringbone interactions, rather than the formation of N-H ... O hydrogen bonds, is the primary packing effect in these compounds. Our observations on these simple isomeric aminophenols indicate that it may not always be realistic to expect straightforward correspondences between molecular and crystal structure. The definition of the term ''supramolecular synthon'' recognizes this inadequacy and attempts to provide a more reliable basis for the description of crystal structures.