Pairs and Electrostatic Donor-Acceptor Interactions

9 kj/mol, regardless of whether the latter is attractive (as in AAA-DDD combinations) or repulsive (as in the more frequently occurring non-hornogenous combinations). These increments have been derived from the analysis of 58 complexes 

The acceptor and donor strength of many functions besides fhose of fhe amide type have been characterized by the analysis of associations between simple molecules, such as, e.g., phenols and anilines, for which fhousands of experimental data exist, mosfly measured in chloroform or in carbon tetrachloride [94, 95]. Although these data are hampered by less well-defined structures compared to supramolecular complexes, they not only give a fairly consistent basis for the prediction of hydrogen-bonded associations but also can be used, e.g., for crown efher and cryptand complexes with alkali or ammonium ligands [32]. 

Hydrogen bonds also play an important role in anion binding, both in proteins [96] and in recently developed artificial receptors [97]. Systematic association measurements wifh model amides (Fig. 2.19) in chloroform show binding increments (Tab. 2.1) between a single amide group and different anions, which are approximately additive [98]. The AG values for chloride complexation increase from monodentate to bidentate to tridentate hosts (Fig. 2.19, 1-3), i.e., from 6 to 12 to 18 kj/mol, respectively. Noticeable deviation from additivity is observed if an an- 

The affinities observed for complexes between amides and anions are remarkably parallel to those found for the interactions between such anions and carbohydrate models. The data in Tab. 2.1 show the same affinity increase in the sequence r Br cr RCOO [100] the carboxylate is again a particularly strong acceptor as the result of two geometrically matching hydrogen bonds with vicinal diols. The formation of two almost linear and parallel hydrogen bonds is also responsible for the efficiency of the guanidinium residue for carboxylate complexation in artificial receptors [101] as well as in proteins (cf. Chapter 6) [102]. 

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