Resonance-assisted O-H center dot center dot center dot O hydrogen bonding: Its role in the crystalline self-recognition of beta-diketone enols and its structural and IR characterization

The fact that hydrogen bonding is normally stronger than other nonbonding attractive forces can be exploited for the rational design of molecular crystals with known packing features and specific physical properties (crystal engineering). In the present paper the problem of obtaining homodromous molecular chains controlled by strong O-H...O interactions is investigated, particular attention being paid to beta-chains, that is, infinite hydrogen-bonded chains of beta-diketone enol fragments ...O=C-C=C-OH..., which are linked by stronger-than-usual resonance-assisted hydrogen bonds (RAHBs) and are intrinsically interesting as prototypes of a large family of switch ing proton bistate molecular devices. Accordingly, the crystal and molecular structures of thirteen new compounds containing the 1,3-cyclopentanedione and 1,3-cyclohexanedione fragment (or their heterocyclic analogues) were determined, and most of them were found to give the expected beta-chain packing pattern. Comparison with literature data makes it possible to identify seven fundamental beta-chain patterns, which can be shown to be selected by reason of the relative encumbrances of the substituents. Furthermore, a general analysis of all functional groups able to form strong O-H...O bonds reveals a semiquantitative correspondence between the O-H...O measurable parameters (O...O, H...O and O-H distances, and <(nu)over tilde>(O-H) IR stretching Frequencies) and the hydrogen bond energy E(HB), and a hierarchy of chemical functionalities that are well characterized by limited E(HB) ranges and that, in decreasing order of energy, can direct the crystal packing process.