Orientation Effects and the Sign of Two-bond 13C–1H Coupling

Signs of two bond (2J) 13C–1H coupling in 13C–enriched carbohydrates have been determined from proton spectra by the double irradiation technique. Instances of both positive and negative 2J values are given and are characterized in terms of the relative orientations of the coupled proton and of oxygen appended to the 13C nucleus; e.g., an oxygen anti to the proton appears to make a positive contribution to 2J and a gauche oxygen a negative one. Theoretical calculations, using ethanol and 1,1-ethanediol as model compounds, are in accord with these experimental observations.