Combined NMR three-bond scalar coupling measurements and QM calculations to calculate OH-rotamer equilibrium of polyalcohols.

A combined but independently applied NMR and QM procedure has been used to investigate the conformational properties of the exchangeable hydroxyl protons of polyalcohols. In this study, to demonstrate the applicability of such a strategy, we investigated a simple monosaccharide, i.e. alpha- and beta-anomers of a D-glucopyranoside derivative. The redundant set of experimental vicinal homonuclear and heteronuclear scalar couplings involving the OH-protons obtained for both anomers of our model compound were simultaneously analyzed to yield the preferred OH-rotamer populations and moreover to parametrize a new Karplus-type equation for (3)J(C(i-1)OH(i)) coupling. The populations of the lowest energy conformers and the conformational-averaged coupling constants were independently calculated using the QM approach in both vacuum and chloroform. The similarity of the estimated rotamer populations obtained by two very different techniques and the similarity of the experimental and calculated coupling constants suggest that these approaches can be used in conjunction and in a fully integrated way to determine a more accurate atomic level description of molecular conformers.