Intramolecular RAHBs

An even stronger case of intramolecular RAHB coupling is provided by the maleate ion (HOOCCH=CHCOO-), whose H-bonded and open conformers are shown in Fig. 5.22. Skeletal bond lengths and bond orders of these conformers are compared in Table 5.18. As shown in Fig. 5.22, the H-bonded conformer is favored in this case by more than 26 kcalmol-1, which is indicative of a powerful intramolecular no aoH interaction (estimated second-order stabilization 104 kcalmol-1) that is sufficient to overcome the severe steric repulsion of the extremely short H- -O nonbonded distance ( 1.3 A).56... 

As was mentioned in the introduction, H-bonds exist within species where TT-electron delocahzation occurs, among them in RAHBs. These types of systems were analyzed early on. However, the systematization of such interactions was given in detail by Gilli and coworkers [5, 9). The authors characterized the features of homonuclear O-H O intramolecular RAHBs. For example, H-bonds in malonaldehyde and its simple derivatives may be classified as RAHBs since a system of conjugated double and single bonds are contained there (Scheme 1). 

Figure 2. The dependence between the A-parameter and the O- O distance (in A). The enol-keto form corresponds to the A-values close to unity and the keto-enol form to that value close to zero. The full equalization of CC and CO bonds is for A-values equal to 0.5. Full circles designate the systems with intramolecular RAHBs while empty circles the intermolecular RAHBs. The g-par-ameter is also indicated. (Reprinted with permission from Ref. [5]. Copyright 1989 American Chemical Society.)...

One can see that the formation of six-membered rings and intramolecular RAHBs should be preferred in crystals. Figure 3 presents an example of such species in the crystal structure of benzylacetone [19]. 

It is worth mentioning that there are numerous studies of intramolecular RAHBs. For example, one can refer to the sound studies of Buemi and Zuccarello [13, 31]. There are investigations of homonuclear and heteronuclear intramolecular RAHBs [13, 31, 32], and not only were O-H- - -O, N-H- - -O, and O-H- -N studied, but also other more exotic systems such as O-H- - -S... 

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. 

Keywords Resonance-assisted hydrogen bond (RAHB) electron density intramolecular... 

The characteristics of the intramolecular O-H- O RAHBs summarized by Gilli and coworkers are as follows ... 

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