Resonance-Assisted Hydrogen Bonds:  A Critical Examination. Structure and Stability of the Enols of β-Diketones and β-Enaminones

Abstract

The characteristics of the intramolecular hydrogen bond (IMHB) for a series of 40 different enols of beta-diketones and their nitrogen counterparts have been systematically analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In some cases, two tautomers may exist which are interconnected by a hydrogen shift through the IMHB. In tautomer a the HB donor group (YH) is attached to the six-membered ring, while in tautomer b the HB acceptor (X) is the one that is attached to the six-membered ring. We found that changing an O to a N favors the a tautomer when the atom is endo and the contrary when it is exo, while the presence of a double bond favors the a tautomers. As expected, the OH group behaves as a better HB donor than the NH2 group and the C=NH group as a better HB acceptor than the C=O group, although the first effect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH, C=NH] > [OH, C=O] > [NH2, C=NH] > [NH2, C=O]. For all those compounds in which the functionality exhibiting the IMHB is unsaturated (I-type), the IMHB is much stronger than in their saturated counterparts (II-type). However, when the systems of the II-type subset, which are saturated, are constrained to have the HB donor and the HB acceptor lying in the same plane and at the same distance as in the corresponding unsaturated analogue, the IMHB is of similar or even larger strength. Hence, we conclude that, at least for this series of unsaturated compounds, the resonance-assisted hydrogen bond effect is not the primary reason behind the strength of their IMHBs, which is simply a consequence of the structure of the sigma-skeleton of the system that keeps the HB donor and the HB acceptor coplanar and closer to each other.

The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspective

“…On the contrary, it is put forward that the reason behind the RAHB strengthening lies in the s-skeleton of the system. 32,33 Grabowski and coworkers used QTAIM analyses 34 to examine the nature of the stabilization in RAHBs. While they initially argued that the dissociation energy of intramolecular RAHBs depends mainly on the p-electron delocalisation, 35 they suggested in subsequent studies that the enhancement of electron delocalisation and the equalization of the length of the conjugated bonds do not correspond to the strengthening of the HBs.…”

Crystal Structure of Garciniaphenone and Evidences on the Relationship between Keto–Enol Tautomerism and Configuration

“…deviation of fitted atoms ¼ 0.0466 ). This result arises from the aromatic character of the chelate ring, which is stabilized by a resonance-assisted Hbond (RAHB) [24]. In this kind of H-bond, the multiple neighboring p-bonds are electronically delocalized, leading to an increase of the H-bonding strength by cooperativity [25].…”

Electronic and Molecular Structures of Heteroradialenes: A Combined Synthetic, Computational, Spectroscopic, and Structural Study Identifying IR Spectroscopy as a Simple but Powerful Experimental Probe

“…The substitution of the imino groups in H 3 13 Me 15 Me is greater than the value of 1.34Å in H8 and H9 and may be attributed to the difference of a resonance-assisted hydrogen bond and a not resonance-assisted hydrogen bond. The C ar -C(sp 3 ) bond length of 1.51Å perfectly matches the value for a C ar -C(sp 3 ) single bond of 1.51Å [129].…”