Hydrogen-bonded donor-acceptor pairs

Figure 21. Hydrogen-bonded donor-acceptor pairs featuring (a) amidinium-carboxylate, (b) carboxylate-amidinium and (c) biscarboxylate interfaces.

As points of reference, we will take two well-established hydrogen-bond donor/ acceptors, H2O and NH3. Their computed gas-phase Vs,max and Vs,mm are in Table 5, along with the same data for all of the molecules that have been discussed hydroxylamine (5), dimethylhydroxylamine (6), acetoxime (7), acetohydroxamic acid (8), and the isomeric pairs of oximes examined in the last section. Finally, we included an additional hydroxamic acid, 11, to see the effects of the strongly electron-withdrawing cyano group. 

Molecular properties dipole moment, polarizability Chemical properties Acidity (including the abilities as proton donor, hydrogen-bond donor, electron pair acceptor, and electron acceptor)a) ... 

In the context of the foregoing discussion, the hydrogen-bonded donor-acceptor supramolecules of Figure 1 meld the constructs of both inter- and intramolecular electron transfer. From the standpoint of intermolecular electron transfer, these systems represent a preformed precursor complex held together by noncovalent interactions from the standpoint of intramolecular electron transfer, these systems provide a fixed-distance donor-acceptor pair in which charge transport occurs along a noncovalent pathway. 

Fig. 24.1 Structure of the B-DNA double helix. Enlarged Lewis structures depict the single strand and the canonical base pairs (AT and GC) with their hydrogen bond donor/acceptor pattern inside the minor and major groove indicated by arrows...

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

Figure 7.4 The edges of the base pairs in DNA that ate in the major groove are wider than those in the minor groove, due to the asymmetric-attachment of the base pairs to the sugar-phosphate backbone (a). These edges contain different hydrogen bond donors and acceptors for potentially specific interactions with proteins (b).

Figure 7.7 Color codes for the recognition patterns at the edges of the base pairs in the major (a) and minor (b) grooves of B-DNA. Hydrogen-bond acceptors are red hydrogen-bond donors are blue. The methyl group of thymine is yellow, while the corresponding H atom of cytosine is white.

---