Resonance-assisted hydrogen bond systems

P. Gilli, V. Ferretti, V. Bertolasi, and G. Gilli, Evidence for resonance-assisted hydrogen bonding. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H- O system by crystal structure correlation methods, J. Am. Chem. Soc. 116, 909-915 (1994). 

Abstract In this chapter we discuss the influence of ir-electron delocalization on the properties of H-bonds. Hence the so-called resonance-assisted hydrogen bonds (RAHBs) are characterized since such systems are mainly classified in the literature as those where TT-electron delocalization plays a very important role. Both the intramolecular and intermolecular RAHBs are described. RAHBs are often indicated as very strong interactions thus, their possible covalent nature is also discussed. Examples of the representative crystal structures as well as the results of the ab initio and DFT calculations are presented. Additionally the RAHB systems, and the other complexes where rr-electron delocalization effects are detectable, are characterized with the use of the QTAIM (Quantum Theory Atoms in Molecules ) method. The decomposition scheme of the interaction energy is applied to expand the knowledge of the nature of the RAHBs. 

For resonance-assisted hydrogen bond (RAHB) systems such as enaminones, it was found that the intrinsic one-bond deuterium isotope effects increased with the hydrogen bond strength, being typically larger than 1 ppm [27]. For intermolecular cases, the behavior is very different as, AN(D) decreases as the distance to the hydrogen partner decreases [28, 29]. For a geometric description, see Limbach et al. [20]. 

Scheme 8.1 Molecular fragments that are known or expected to include resonance-assisted hydrogen bonding (RAHB). The proton transfer in these systems will be facilitated by the extensive electron conjugation. Adapted from [3],...

The effect of weak forces on the equilibrium constant for the diaza-Cope rearrangement suggests that the anion effect is the strongest followed the resonance-assisted hydrogen-bond, steric, conjugation, and electronic effects. These weak forces are said to be additive." An intramolecular Fischer indole synthesis with a double bond in the tether allows a tandem [3,3]-sigmatropic rearrangement access to tricyclic benzo[cindole systems (Scheme 1). ... 

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