Density functional theory studies of transannular ~1H-~1H J-coupling in half-cage alcohols and rigid 1,3- and 1,4-diols: conformational dependencies and implications for intramolecular hydrogen bond detection in carbohydrates

摘要

Density functional theory (DFT) methods were used to investigate the conformational dependence of transannular H-H coupling constants in half-cage alcohols and cage diols. Finite perturbation theory (FPT) was used to obtain the Fermi contact (FC) contributions to scalar coupling constants in three half-cage alcohols and the OH…OH coupling between hydroxyl groups sharing 1,4- and 1,3-intramolecular hydrogen bonds. Transannular CH…OH coupling constants in the half-cage alcohols (inner OH) were obtained as a function of the dihedral angle defining the OH orientation. In comparison with the parent compound, chlorine substituents substantially modify the conformational dependence of the transannular H-H coupling and lead to observable coupling in the hexachloro compound. This is an unusual case, wherein the conformationally averaged coupling constants are quite different than those for the minimum-energy conformations. A -1.0 Hz scalar coupling is predicted between the very crowded protons in the half-cage acetate (outer OAc) in comparison with the experimental magnitude <1 Hz. In model 1,4- and 1,3-diols the coupling constants were obtained as functions of the analogous dihedral angles. The primary effects of hydrogen bonding are the stabilization of conformations in which the positive and negative contributions effectively concel, thereby leading to small, observed values (e.g. <=0.3 Hz). The DF/FPT data for transannular coupling constants are consistent with a sum-over-states analysis for the direct mechanisms involving the two bonds containing the coupled nuclei rather than indirect mechanisms associated with oxygen lone pairs. Discussed here are the prospects of using OH…OH scalar coupling for structural studies of carbohydrates.