Resonance-assisted bonds

The theory of resonance-assisted coupling of H-bonding and torsions... 

According to (5.101) the rotation barrier varies linearly with the shift in resonance weights (or bond orders) on going from one resonance form to the other. Thus, resonance hybrids with particular sensitivity to intermolecular interactions (such as resonance-assisted H-bonds, Section 5.2.3) can be expected to exhibit strong... 

Figure 5.65 provides theoretical evidence that resonance-assisted H-bonding can serve as an effective mechanism for switching a methyl rotor from one preferred conformation to another, or for controlling the stiffness of torsional motions in alkylated amides. In particular, the torsional potentials of proteins (more specifically, the Ramachandran b angle at Ca) should be sensitive to N—H- O and related H-bonding interactions involving the amide backbone. In principle, this electronic... 

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... 

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