Hydrogen bonds selectivities

Molecular Interactions in Molecular Recognition Molecular recognition occurs due to various molecular interactions such as electrostatic interaction and hydrogen bonding. Selective and efficient recognition is sometimes achieved by cooperative contributions from these interactions. 

Figure 12.8 X-Ray structure of the first Fca peptide conjugate 47 showing two intramolecular hydrogen bonds. Selected hydrogen bond distances (A) and angles (°) N1—052 2.812(3), N51—02 2.914(3), Nl—H1N—052 169(3), H51—H51N—02 157(3). (Redrawn from Barisic et al.58)...

It would be wrong to assume that non-ionic stabilized emulsions are immune to the effect of added electrolytes. The addition of electrolytes to non-ionic stabilized emulsions can cause pronounced effects on stability. In solutions of non-ionic surfactants, the addition of electrolytes generally causes a dehydration of the ethylene oxide chains by disruption of hydrogen bonds. Selected salts have been shown to exhibit interaction with polyethylene oxide ethers, reducing their solvation and producing more compact molecular conformations [127,128]. 

Ge Y, Lilienthal RR, Smith DK (1996) Electrochemically-controlled hydrogen bonding. Selective recognition of urea and amide derivatives by simple redox-dependent receptors. J Am Chem Soc 118 3976-3977... 

Figure 2.6. The tetrahedral structures of ice (a), (fc) are planes through sheets of selected oxygen nuclei (open circles), hydrogen nuclei (shotm in the insert as solid circles) are not shown in the main drawing. The insert illustrates the overlap of oxygen line pairs and the hydrogen nuclei, thus forming the hydrogen bonds (dotted lines)...

Fig. 7. Snapshots of rupture taken (A) at the start of the simulation (zcant = 0), (li) at ZcB.nl = 2.8 A, (C) at Zcnm = 4.1 A, (D) at Zcnm = 7.1 A, and (E) at Zcant = 10.5 A. The biotin molecule is drawn as a ball-and-stick model within the binding )ocket (lines). The bold dashed lines show hydrogen bonds, the dotted lines show selected water bridges.

The solvent effect on the diastereofacial selectivity in the reactions between cyclopentadiene and (lR,2S,5R)-mentyl acrylate is dominated by the hydrogen bond donor characteristics of the solvent... 

Table 1.2. Relative rate constants of some selected Diels-Alder reactions in water compared to organic solvents of different hydrogen bond donor capacities.

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

In conclusion, the special influence of water on the endo-exo selectivity seems to be a result of the fact that this solvent combines in it three characteristics that all favour formation of the endo adduct (1) water is a strong hydrogen bond donor, (2) water is polar and (3) water induces hydrophobic interactions. 

Table 1,5, Donor scales (Dg, DN and DNbulk) of some selected solvents, as well as acceptor number (AN) and hydrogen bond donor capacities (a).

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. 

What is the effect of water on the rate and selectivity of the Lewis-acid catalysed Diels-Alder reaction, when compared to oiganic solvents Do hydrogen bonding and hydrophobic interactions also influence the Lewis-acid catalysed process Answers to these questions will be provided in Chapter 2. 

Metal hydrides reduce preferably polar double bonds, whereas catalytic hydrogenation is somewhat selective for non-polar double bonds. Selective protection of amino groups in amino acids. 

The most common hydrophobic adsorbents are activated carbon and siUcahte. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel stmcture of siUcahte must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, siUcahte has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

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