Hydrogen resonance-assisted

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>. 

FIGURE 2. Resonance-assisted hydrogen bonding (RAHB)... 

Reimer-Tiemann reaction 685, 1071 Resole resins 1457, 1458, 1460 model compounds of 1476-1487 synthesis of 1475, 1476 Resol prepolymers 969 Resonance Assisted Hydrogen Bonding (RAHB) 552... 

S. J. Grabowski, w-Electron delocalisation for intramolecular resonance assisted hydrogen bonds, J. Phys. Org. Chem. 16, 797-802 (2003). 

Among the 0-H O intramolecular hydrogen bonds, those of (S-diketone enols are resonance assisted and characterized by a low barrier in the proton transfer pathway between two equivalent tautomeric forms, through the H-centred (C2v) conformation representing the transition state (Fig. 4). 

B3LYP/6-311++G(d,p) calculations have shown that in diketones with two resonance-assisted hydrogen bonds, as 2,4-dihydroxy-but-2-ene-4-diol, no rise of the resonance effect is noted and the Tr-delocalization energy of the closed-open conformations is greater than in the most stable conformers with two closed rings [210]. 

Gilli, G., Bellucci, F., Ferretti, V., and Bertolasi, V., Evidence for resonance-assisted hydrogen bonding from crystal-structure correlations on the enol form of p-diketone fragment, J. Am. 

Bertolasi, V., Gilli, P., Ferretti, V., and Gilli, G., Intramolecular O-H- -O hydrogen bonds assisted by resonance. Correlation between crystallographic data and H NMR chemical shifts, J. Chem. Soc. Perkin Trans. 2, 945-952 (1997). 

Varnali, T., and Hargittai, I., Geometrical consequences of resonance-assisted hydrogen bonding in 2-nitrovinyl alcohol and indication of a slight attractive O- -H interaction in 2-nitroethanol. An ab initio molecular ortbital investigation, J. Mol Struct. Theochem, 388, 315-319 (1996). 

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