Some stereochemical aspects of two-bond 13C–1H spin–spin coupling. Sign determination in natural abundance 13C nuclear magnetic resonance spectra

Abstract

The magnitudes and signs of two-bond 13C–1H coupling constants (2J) in the array O—13CA—CB—1H, appear to be determined by the relative orientations of the O—13CA and CB—1H bonds, as shown with a variety of carbohydrate derivatives. The data have been observed from natural abundance lH-coupled 13C nmr spectra, and signs of 2J have been determined by the 'selective population transfer' technique. When CB bears one oxygen atom, the observed 2J values range from −6 to 0 Hz, whereas a range of 0 to +8 Hz is found when CB bears two oxygen atoms. By contrast, for arrays in which CB bears one oxygen atom and CB two, the 2J values vary from −6 to +6 Hz. In general, the results confirm earlier indications that the larger the torsional angle between the O—13CA and CB—1H bonds, the more positive is the value of 2J. These findings are supported by calculations for model compounds, i.e., propane-1,1,2-triol and butane-2,3-diol.