STRUCTURE ELUCIDATION OF AMIDE BONDS WITH DIPOLAR CHEMICAL-SHIFT NMR-SPECTROSCOPY

The structure of the amide bonds of gluconamide has been elucidated an d compared to acetanilide by the combined application of C-13 and N-15 double-and triple-resonance solid-state NMR spectroscopy. The length of the amide bond has been determined from the dipolar spectrum using a SEDOR type experiment, and the orientation of the principal axis sys tems of both the C-13 and N-15 chemical shift tensors have been determ ined by employing dipolar chemical shift NMR spectroscopy in conjuncti on with CSA spectroscopy. The groups exhibit for amide bonds typical a pproximately 120 degrees bond angles between -CO, -CN, -CR, and -NC, - NH, -NR. Comparing the structure of the gluconamide with the correspon ding structure of the acetanilide, two major differences are visible: the orientations of the CSA tensors in the amide plane with respect to the CN-bond direction are different (12 degrees for the C-13 tensor a nd 10 degrees for the N-15 tensor), and the directions of least shield ing and intermediate shielding are interchanged in the gluconamide as compared to the acetanilide. Since the chemical shielding tensors of t he N-15 are strongly influenced by hydrogen bonding, these different o rientations are an indication of the different hydrogen bond structure of the gluconamide as compared to the acetanilide.