Evidence for resonance-assisted hydrogen bonding from crystal-structure correlations on the enol form of the .beta.-diketone fragment

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Abstract

The geometry variations caused by inter- and intramolecular hydrogen bond (HB) formation on the molecular fragment acacH (HOCR=CR—CR=0) contained in the enol form of acetylacetone and other related β-diketones and β-keto esters have been studied by crystal structure correlation methods on 25 X-ray or neutron crystallographically determined molecules containing such a fragment. acacH can form either a single intramolecular HB closing a 6-membered ring or intermolecular HBs connecting the groups head-to-tail in infinite linear arrays. Experimental data show that there is a strong correlation between the strength of the HB formed (measured by both the do.oand do-Hvalues) and the delocalization of the system of conjugated double bonds. The effect is qualitatively interpreted in terms of a mechanism of synergistic interplay of resonance and HB formation which is called resonance-assisted hydrogen bonding (RAHB), and it is shown that the proposed model is in agreement with all present NMR and IR data and with the most recent ab initio quantum mechanical calculations reported for malonaldehyde. For more complex molecules a semiempirical model has been developed to evaluate both the energy of the HB formed (E1) and the height of the barrier for proton transfer (E2); with reference to water, where E1= 20 and E2= 40 kJ mol−1, E1and E2are calculated to be 53.4 and 35 kJ mol−1in acetylacetone and 82.5 and 27.5 kJ mol−1in hexamethylacetylacetone, showing that very strong HB's with central hydrogen position are favored by substituents of relevant steric hindrance. These conclusions are generalized to take into account the possible role played by RAHB in a variety of heteroconjugated systems. © 1989, American Chemical Society. All rights reserved.

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