Hydrogenbond Acceptor

Stabilizing the folded state also benefits from optimizing hydrogenbonding interactions, with water providing donor and acceptor groups to polar side chains and main chain C=0 and N—H groups left exposed on the surface of the folded structure. Direct evidence for such... 

Further studies by the same authors have demonstrated that PFj acts a hydrogen bond-acceptor template in the assembly of several other interwoven structures. In an extensive study aimed at using a combination of hydrogenbonding motifs to self-assemble pseudorotaxanes into more complex structures it was discovered that PFj assists on the organization of the components that yield the final superstructure [74]. Particularly, it was found that this anion dictates the orientation of the two carboxylic acid groups of the [3]pseudorotaxanes 58 and 59 (see Schemes 27 and 28) when these groups are co-directional with respect to each other the formation of discrete hydrogen-bonded dimers is observed. 

In addition, rearranging Eq. (21) and combining with Eq. (19) illustrates that i = k" y. Therefore, a two parameter (one term) model would require that a plot of the enthalpies of adduct formation of one acid versus the enthalpies of adduct formation of another acid for the same series of bases be linear with a zero intercept. The enthalpies of adduct formation for 12 and phenol with a wide series of bases does not give rise to such a plot as can be seen in Fig. 5. These acids have very different C/E ratios and their enthalpies of adduct formation cannot be correlated by a one term model. Furthermore, a one term model could riot incorporate systems in which reversals in donor-acceptor strength are observed 32). However, it is possible to correlate enthalpies of adduct formation for acids with very similar C/E ratios such as hydrogenbonding acids using a one term equation. Correlations restricted to one particular type of acid are, of-course, only a subset of the overall E and C correlation. 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

In crystal structure analyses where the hydrogen atoms are not directly observed, as in macromolecules and structures with disordered water molecules, the signature of the three-center hydrogen bond is a triangle of potential donor or acceptor nonhydrogen atoms at distances in the range observed for weak hydrogenbonding interactions, i.e., 2.8 to 3.5 A. 

Selected crystal structures of thymidine and its derivatives. The hydrogenbonding patterns of thymidine and its derivatives are comparable to those of the uridine derivatives, but relatively simple and they have a uniformity which is rare. In three of the four structures, infinite chains involving a- and rr-cooperativity occur through the adjacent donor and acceptor groups of the thymine ring. 

A m or challenge to completing a practical model and description of mobile-phase effects in LSC is the further elucidation of hydrogenbonding effects. This will involve a more fundamental classification of solutes and solvents in terms of their proton-donor and proton-acceptor properties, so that values of can be estimated as a function of the molecular structures of solute X and solvent C. It will also require a more precise description of the adsorbate-surface bonding that occurs in the adsorbed monolayer, so that values of can likewise be rationalized and predicted. 

Figure 6 shows the synthesis, templated by donor-acceptor and hydrogenbonding interactions, of [2]catenane 8 + [23]. Reaction of the dication with the... 

On the basis of Monte Carlo simulations [40] and molecular orbital calculations [26a], hydrogen bonding was proposed as the key factor controlling the variation of the acceleration for Diels-Alder reactions in water. Experimental differences of rate acceleration in water-promoted cycloadditions were recently observed [41]. Cycloadditions of cyclopentadiene with acridizinium bromide, acrylonitrile and methyl vinyl ketone were investigated in water and in ethanol for comparison (Scheme 3). Only a modest rate acceleration of 5.3 was found with acridizinium bromide, which was attributed to the absence of hydrogenbonding groups in the reactants. The acceleration factor reaches about 14 with acrylonitrile and 60 with methyl vinyl ketone, which is the best hydrogen-bond acceptor [41]. 

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