Resonance-assisted hydrogen bonding

Alkorta, I., Elguero, J., Mo, O., Yanez, M., and Del Bene, J.E., Are resonance-assisted hydrogen bonds resonance assisted A theoretical NMR study, Chem. Phys. Lett., 411, 411-415 (2005). 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

The major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

The quantity 2 was defined as the difference in bond length of the two C—C bonds in the ring. This value varied from 6 pm up to 106 pm. Since and 2 linearly were related, as might be expected, the values of their sum, Q, was useful in discussing resonance-assisted hydrogen bonding. The value of Q was found to correlate well with " th other properties. 

Steiner, Th. (1998a). Structural evidence for resonance-assisted 0-H...S hydrogen bonding. Chem. Comm. 411-2. 

The pKa for dissociation of its proton is 12.3 and the hydrogen-bonded proton is probably located in the center of the bond with both amino groups sharing the charge.160 Enols can also form unusually strong "resonance-assisted" hydrogen bonds ... 

The relationship between tautomerism and aromaticity is strong both in carbocyclic chemistry (e.g., phenol) [37, 38] and in heterocyclic chemistry (e.g., 2-hydroxypy-ridine) [39, 40], We consider that the cycle should be formed by covalent bonds, thus excluding the possibility that rings comprising hydrogen bonds, the so-called resonance assisted hydrogen bonds (RAHB), could be aromatic [41 45],... 

Evidence for Resonance-Assisted Hydrogen Bonding from Crystal Structure Correlations on the Enol Form of the P-Diketone Fragment. 

Ab initio and density functional theory (DET) methods have been exploited to determine the structures and the interaction energies of 2/7-isoxazol-5-one B, and its dimer and trimer structures in the gas phase. For the cyclic trimer, the computed structural parameters resulted in excellent agreement with the X-ray determination of the supramolecular aggregate of 4-(2-methoxybenzyl)-3-phenyl-4//-isoxazol-5-one, involving very strong intermolecular H-bonds of the NH tautomeric form, interpreted in terms of the RAHB (resonance-assisted hydrogen bond) model (see Section 4.03.3.1) <2002HCA2364>. 

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