Precise 1H nuclear magnetic resonance spectral parameters are reported for salicyladehyde and its 3-fluoro and 5-fluoro derivatives in nonpolar solutions. Such data are also given for the 2-mercapto, 2-methylthio, and 2-methoxy derivatives of benzaldehyde. Comparison of the long-range coupling constants in the various compounds and their conformers shows a large perturbation of their magnitudes by hydrogen bond formation. For the salicylaldehyde system, the perturbation is particularly large for couplings involving the aldehyde proton and protons or fluorine nuclei placed ortho to the hydroxyl group. For example, 5Jt (F, CHO) is reduced by about 50%. The perturbation, as expected, is much smaller for coupling constants of nuclei remote from the site of the hydrogen bond. In 2-mercaptobenzaldehyde the long-range coupling constants are also sensitive to hydrogen bond formation, those involving the sulfhydryl proton markedly so compared to the hydroxyl proton in salicylaldehyde. The strength of the [Formula: see text] bond is discussed. It is argued that the reference conformer for the mercapto compound in such a discussion is less easily defined than for salicylaldehyde because [Formula: see text] are similar to [Formula: see text] energies. The experimental data for the CCl4 solutions imply a free energy of formation of the [Formula: see text] bond of 4.8(5) kJ/mol at 300 K. Molecular orbital computations on the four planar conformers of each salicylaldehyde and 2-mercaptobenzaldehyde with the 6-31 G**(5D) basis are reported. For salicylaldehyde, the [Formula: see text] arrangement is taken as the reference conformer, with a computed energy of 25.7 kJ/mol relative to the hydrogen-bonded structure. For 2-mercaptobenzaldehyde, the [Formula: see text] and [Formula: see text] conformers are calculated to be isoenergetic, at 5.1 kJ/mol relative to the hydrogen-bonded conformer. Hence either arrangement serves as a reference structure in computations of the strength of the hydrogen bond. The computations are consistent with the experimental results for solutions of the molecules under discussion. An appendix gives the computed geometries of the eight planar conformers, as well as some atomic charges, allowing a rationalization of the relative energies of the conformers.
The peptideZ-Aib-Aib-Aib-L-Ala-OtBu