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) ² J _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 ² J _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 ² J _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 ² J _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 ² J _CCH values in saccharides, a complication that remains to be addressed before ² J _CCH values can be used quantitatively in single- and multi-state MA'AT modeling of redundant NMR J-values in furanosyl rings.