| Popis: |
A comparative study of molecular balances by NMR spectroscopy indicates that noncovalent functional-group interactions with an arene dominate over those with an alkene, and that a p-facial intramolecular hydrogen bond from a hydroxy group to an arene is favored by approximately 1.2 kJ mol � 1 . The strongest interaction observed in this study was with the cyano group. Analysis of the series of groups CH2CH3 ,C H=CH2 ,CCH, and CN shows a correlation between conformational free-energy differences and the calcu- lated charge on the C a atom of these substituents, which is indicative of the electrostatic nature of their p interactions. Changes in the free-energy differences of conformers show a linear dependence on the solvent hydrogen bond acceptor parameter b. The vital role played by noncovalent interactions, especially the contribution of aromatic rings to chemical and biological recognition, continues to be a subject of intense research activity. (1) Detailed understanding and quantifiable estimates of the strength, distance, and geometric dependence of such intermolecular forces are essential, not only for the under- standing of protein-ligand interactions and hence for drug design, but also for the synthesis of new asymmetric ligands, catalysts, and sensors. Phenomena such as p stacking, (2) the behavior of an aromatic ring as a hydrogen-bond acceptor, (3) or cation-p interactions (4) have been studied extensively. Although a variety of techniques have contributed to provide valuable insight, the use of designed molecular balances has proven particularly useful for estimating the very small interaction energies involved. Moreover, such balances also allow the often dominant influence of solvation to be explored. The molecular torsion balance pioneered by Wilcox and co-workers (2d, 5) has provided the basic framework for many elegant studies, which exemplify the quantitative power of this approach, and the results from a significant number of new molecular balances (6) have been summarized in an insightful review by Mati and Cockroft. (7) |
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