Hydrogen-Bonded Crystals of Exceptional Dielectric Properties

Hydrogen bonds are well known for governing the molecular association and crystal packing, as well as for their contribution to the chemical and physical properties of substances. They affect the properties of materials as versatile as amphibole minerals, KDP-type (—OH⋯O= bonded) ferroelectrics or biopolymers (e.g. cellulose). While the classical examples of such materials usually concentrate on the —OH⋯O= bonds, recently analogical and completely new features were observed in —NH⋯N= bonded substances. The key factor for understanding these new properties is the structural coupling between molecular arrangements and H-atom position in the NH⋯N hydrogen bonds. In this paper the very weak NH⋯N involving N_sp3 nitrogen atoms in secondary and tertiary amines has been compared. Clear preferences for the molecular orientation depending on the H-atomposition have been revealed for the small, weakly interacting molecules for the molecular crystals of aziridine, C₂H₅N, and difluoroamine, HNF₂. This coupling explains the microscopic mechanisms accompanying reorientations or inversions of the ammonium groups and molecular rearrangements, which are essential for understanding dynamics of protons in H-bonds, or spontaneous polarization and phase transitions in NH⋯N hydrogen-bonded materials. The directional preferences of NH⋯N hydrogen bonds can be employed for engineering transformable crystal structures, for predicting physical properties of crystals built of small molecules, for analyzing the molecular associations and conformations of larger molecules and biopolymers, or validating structural determinations. On the other hand, specific features of molecular structure, conformation or molecular arrangement in aggregates and crystal structure can compete with the subtle orientational preferences induced by NH⋯N bonds. Such systematic conformational preferences in molecular aggregates exist in the structures of piperazine, piperidine and morpholine.