Structure elucidation of amide bonds with dipolar chemical shift NMR spectroscopy

The structure of the amide bonds of gluconamide has been elucidated and 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 systems of both the C-13 and N-15 chemical shift tensors have been determined by employing dipolar chemical shift NMR spectroscopy in conjunction with CSA spectroscopy. The groups exhibit for amide bonds typical approximately 120 degrees bond angles between -CO, -CN, -CR, and -NC, -NH, -NR. Comparing the structure of the gluconamide with the corresponding 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 and 10 degrees for the N-15 tensor), and the directions of least shielding and intermediate shielding are interchanged in the gluconamide as compared to the acetanilide. Since the chemical shielding tensors of the N-15 are strongly influenced by hydrogen bonding, these different orientations are an indication of the different hydrogen bond structure of the gluconamide as compared to the acetanilide.