Orientational Dependance of Vicinal and Geminal 13C–1H Coupling

The effect of changes in the relative orientation of vicinal and geminal 13C and 1H nuclei, and in the disposition of adjacent substituents, on coupling between these nuclei has been examined with a variety of 13C-labelled carbohydrates. Compounds synthesized for this purpose include derivatives of D-glucose-1- and -6-13C and L-idose-6-13C, and related lactones, and the measurements have been made with p.m.r. spectra of these compounds. Vicinal 13C-1H coupling, both through C—C and C—O bonds shows an orientational dependance analogous to that for protons: i.e., dihedral angles of 60–100° are associated with smaller coupling (0–3 Hz) than angles of 140–180° (4.5–5.5 Hz); sp2 carbon produces splittings of almost twice these values. The observed spacing is smaller when an oxygen atom is in the plane of the coupling pathway than when out of plane, both for vicinal and geminal 13C–1H interactions, and the presence of an extra oxygen also may have a notable impact on the geminal interaction. A study of rotational isomerism in these compounds indicates that the characteristics of coupling between 13C and 1H nuclei may fruitfully be compared with those of H–H coupling in the analysis of such dynamic systems.