Are the amide bonds in N -acyl imidazoles twisted? A combined solid-state ¹⁷O NMR, crystallographic, and computational study

We report synthesis of ¹⁷O-labeling and solid-state ¹⁷O NMR measurements of three N-acyl imidazoles of the type R-C(¹⁷O)-Im: R = p-methoxycinnamoyl (MCA-Im), R = 4-(dimethylamino)benzoyl (DAB-Im), and R = 2,4,6-trimethylbenzoyl (TMB-Im). Solid-state ¹⁷O NMR experiments allowed us to determine for the first time the ¹⁷O quadrupole coupling and chemical shift tensors in this class of organic compounds. We also determined the crystal structures of these compounds using single-crystal X-ray diffraction. The crystal structures show that, while the C(O)-N amide bond in DAB-Im exhibits a small twist, those in MCA-Im and TMB-Im are essentially planar. We found that, in these N-acyl imidazoles, the ¹⁷O quadrupole coupling and chemical shift tensors depend critically on the torsion angle between the conjugated acyl group and the C(O)-N amide plane. The computational results from a plane-wave DFT approach, which takes into consideration the entire crystal lattice, are in excellent agreement with the experimental solid-state ¹⁷O NMR results. Quantum chemical computations also show that the dependence of ¹⁷O NMR parameters on the Ar-C(O) bond rotation is very similar to that previously observed for the C(O)-N bond rotation in twisted amides. We conclude that one should be cautious in linking the observed NMR chemical shifts only to the twist of the C(O)-N amide bond.