Geminal 13C–1H NMR Spin-Coupling Constants in Furanose Rings: New Empirical Correlations with Conformation

Density functional theory (DFT) calculations have been used to develop a new approach to interpreting geminal (two-bond) ²J_CCH NMR spin-coupling constants in saccharides containing aldofuranosyl (five-membered) rings. In the biologically important β-d-ribofuranosyl and 2-deoxy-β-d-ribofuranosyl (2-deoxy-β-d-erythro-pentofuranosyl) rings that were used as models, many of the ²J_CCH values associated with coupling pathways involving an endocyclic C–C bond depend linearly on P/π, a measure of ring conformation. In most cases, the endocyclic C–C bond is present in the coupling pathway. In other cases, the ²J_CCH value depends linearly on either an adjacent C–C bond torsion angle or shows no systematic relationship with any endocyclic C–C bond torsion angle. In the latter case, secondary (remote) structural effects, defined as those that primarily affect C–C or C–H bond lengths in the C–C–H coupling pathway, cause the ²J_CCH value to behave with less predictability. These effects apparently cancel and lead to linearity involving an adjacent C–C bond in some cases. These findings provide a new conceptual framework to understand and exploit the dependencies of geminal ¹³C–¹H NMR spin-couplings on furanose ring conformation. They also reveal the effect of exocyclic C–O bond torsion angles on the magnitudes and signs of ²J_CCH values in saccharides, a complication that remains to be addressed before ²J_CCH values can be used quantitatively in single- and multi-state MA’AT modeling of redundant NMR J-values in furanosyl rings.