RESONANCE-ASSISTED HYDROGEN-BONDING .3. FORMATION OF INTERMOLECULAR HYDROGEN-BONDED CHAINS IN CRYSTALS OF BETA-DIKETONE ENOLS AND ITS RELEVANCE TO MOLECULAR ASSOCIATION

The beta-diketone enol (or enolone) HO-C=C-C=O fragment produced by enolization of beta-diketones is known to form strong intramolecular O-H...O hydrogen bonds where the decrease of the O...O contact distance (up to 2.40 angstrom) is correlated with the increased pi-delocalization of the O-C=C-C=O heteroconjugated system, the phenomenon has been interpreted by the resonance-assisted hydrogen-bonding (RAHB) model [Gilli, Bellucci, Ferretti & Bertolasi (1989). J. Am. Chem. Soc. 111. 1023-1028; Bertolasi, Gilli, Ferretti & Gilli (1991). J. Am. Chem. Soc. 113, 4917-4925]. When the intramolecular hydrogen bond is forbidden for steric reasons, molecules crystallize by forming hydrogen-bonded infinite chains of pi-delocalized enolone fragments (resonant beta-chains), i.e. they are hybrids of the canonical forms -OH...O=C-C=C-OH...O=C- <-- --> =+OH...-O-C=C-C=+OH...-O-C=. The occurrence of beta-chains in 14 crystals of enolone (2-en-3-ol-1-one) and eight of enediolone (2-en-2,3-diol-1-one) derivatives has been studied. The beta-chains were found to have the following properties: (i) O...O distances depend on the enediolone substituents and range from 2.69 angstrom in beta-ketoesters to 2.46 angstrom in beta-diketones; (iii) calculated hydrogen-bond energies are in the range 20-66 kJ mol-1; (iii) a strict intercorrelation between hydrogen-bond strengthening and pi-system delocalization is observed, in complete agreement with the RAHB model proposed previously. Beta-Chain morphologies are analyzed with the aim of determining crystal-engineering rules for the production of solid materials where systems of polar beta-chains can induce ferroelectric and second harmonic generation properties. The RAHB concept is generalized to other heteroconjugated systems such as carboxylic acids, amides, enamines (RN=CR-NHR) and enaminones (O=CR-CR=CR-NHR), and its possible relevance in biological processes such as base coupling in DNA and folding of proteins is briefly discussed.