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 17 O-labled secondary amides, R 1 C[ 17 O]-NHR 2, have been synthesized: benzanilide ( 1 ), N -methylbenzamide ( 2 ), and acetanilide ( 3 ). Analysis of 17 O magic-angle spinning (MAS) and stationary NMR spectra yields not only the magnitude but also the orientation of the carbonyl 17 O EFG and CS tensors. For compounds 1 − 3, the carbonyl 17 O 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 17 O EFG component lies in the amide plane and is perpendicular to the C O bond, whereas the smallest component is perpendicular to the N−C O plane. For the carbonyl 17 O CS tensor, the principal component with the largest shielding, δ 33, is perpendicular to the amide plane, and the tensor component corresponding to the least shielding, δ 11, is in the amide plane approximately 20° off the direction of the C O 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 17 O 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 17 O 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 R 2 = 0.9952). The quantum chemical calculations indicated that the intermolecular C O···H−N hydrogen-bonding interactions play an important role in determining the carbonyl oxygen EFG and CS tensors for an amide functional group.