A bicyclic receptor was synthesized and evaluated for its ability to bind alkali halide salts and polar neutral molecules in organic solvents. The receptor design is relatively straightforward in the sense that it is a combination of a dibenzo-18-crown-6 and a bridging 1,3-phenyldicarboxamide. In the presence of 1 mol equiv of metal cation, chloride affinities are enhanced in the order: K + (9-fold enhancement) > Na + (8-fold enhancement) . Cs + (no enhancement). An X-ray crystal structure shows that the receptor binds sodium chloride as a solvent-shared ion pair. The receptor has very weak affinity for acetonitrile, nitromethane, or acetone in chloroform solvent, whereas the association constant for dimethylsulfoxide is 160 M -1 at 295 K. An X-ray crystal structure shows that the dimethylsulfoxide is bound deeply in the receptor cavity and forms hydrogens bonds to the receptor via a bridging water molecule. There is also evidence for CH-O interactions. Solid-liquid extraction studies show that the receptor can dissolve and associate with urea, primary amides, and primary sulfonamides in CDCl 3 but does not dissolve amino acids.

The barrier for rotation about an N-alkylcarbamate C(carbonyl)-N bond is around 16 kcal/mol. In the case of an N-phenylcarbamate, the rotational barrier is lowered to 12.5 kcal/mol, but with N-(2-pyrimidyl)carbamates the barriers are so low (<9 kcal/mol) that the syn and anti rotamers cannot be observed as separate signals by 500 MHz NMR spectroscopy at 183 K. X-ray and computational data show that the N-(2-pyrimidyl) carbamates have C(carbonyl)-N bonds that are on average 0.03 A longer than for related N-phenylcarbamates. The computational results trace the origin of the effect to increased single bond character for the C(carbonyl)-N bond due to the increased electron-withdrawing ability of the pyrimidyl ring.

The cis/trans ratios for six model secondary amides were determined by 1H NMR in a range of solvent systems. The trans to cis equilibrium in chloroform is only slightly affected by addition of the hydrogen bond donor, trifluorethanol, but the cis rotamer is stabilized by an average of 0.7 kcal mol−1 when acetic acid is used as an intermolecular donor–acceptor template. Conversely, amide interaction with anionic hydrogen bond acceptors decreases the percentage of cis rotamer. 15N NMR spectroscopy was used to determine the effect of hydrogen bonding on the trans amide structure. The direction and the magnitude of 15N complex‐induced‐shifts indicate that both hydrogen bond donors and acceptors raise the secondary amide rotational barrier by increasing the CN bond order. The relationship of these results to protein structure is discussed. Copyright © 2001 John Wiley & Sons, Ltd.

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