Solid-State ¹⁷O NMR Investigation of the Carbonyl Oxygen Electric-Field-Gradient Tensor and Chemical Shielding Tensor in Amides

Abstract

We have presented a systematic experimental and theoretical investigation of the carbonyl oxygen electric-field-gradient (EFG) tensor and chemical shielding (CS) tensor in crystalline amides. Three 17O-labled secondary amides, R1C[17O]-NHR2, have been synthesized: benzanilide (1), N-methylbenzamide (2), and acetanilide (3). Analysis of 17O magic-angle spinning (MAS) and stationary NMR spectra yields not only the magnitude but also the orientation of the carbonyl 17O EFG and CS tensors. For compounds 1−3, the carbonyl 17O quadrupolar coupling constant (QCC) and the span of the chemical shift tensor are found to be in the range of 8.5−8.97 MHz and 560−630 ppm, respectively. The largest 17O EFG component lies in the amide plane and is perpendicular to the CO bond, whereas the smallest component is perpendicular to the N−CO plane. For the carbonyl 17O CS tensor, the principal component with the largest shielding, δ33, is perpendicular to the amide plane, and the tensor component corresponding to the least shielding, δ11, is in the amide plane approximately 20° off the direction of the CO bond. Extensive quantum chemical calculations using density functional theory (DFT) have been performed for both isolated and hydrogen-bonded molecules of compounds 1−3. The calculated carbonyl 17O EFG and CS tensors from the latter molecular models are in reasonably good agreement with the experimental values. In particular, the B3LYP/D95** EFG calculations overestimate the carbonyl 17O QCC by approximately 0.5 MHz. The B3LYP/D95**/GIAO shielding calculations yield a linear correlation between the calculated and experimental data (slope = 1.125 and R2 = 0.9952). The quantum chemical calculations indicated that the intermolecular CO···H−N hydrogen-bonding interactions play an important role in determining the carbonyl oxygen EFG and CS tensors for an amide functional group.