Intermolecular RAHBs

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. 

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.)...

It is always of great interest to extend the understanding of the nature of interaction of RAHBs. For this purpose it is possible to apply the scheme of the decomposition of interaction energy. Such an approach was described in Chaps. 4 and 5 of this volume and may be applied for intermo-lecular interactions. The intermolecular RAHBs will be analyzed in the next sections. 

There have been other investigations of intermolecular RAHBs such as of the pyrrole-2-carboxylic acid (PCA). The infrared and Raman spectroscopic studies of this eompound indicate the formation of cyclic dimer spedes in the solid state [47]. These studies are supported by the results of DFT and ab initio calculations. Three types of dimers of PCA are possible. Figure 8 shows the molecular graphs of those speeies. 

To provide more insight into the nature of heteronuclear intermolecular RAHBs, ab initio calculations at the MP2/6-311++G(Quantum Theory Atoms in Molecules ) calculations were performed for the formamide dimer (Fig. 13) and its simple fluoro derivatives... 

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>. 

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]. 

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