Solid-State 17O NMR and Computational Studies of C-Nitrosoarene Compounds
Gang Wu, Jianfeng Zhu, Xin Mo, Ruiyao Wang and Victor Terskikh
We report the first solid-state 17O NMR determination of the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for four 17O-labeled C-nitrosoarene compounds: p-[17O]nitroso-N,N-dimethylaniline ([17O]NODMA), SnCl2(CH3)2([17O]NODMA)2, ZnCl2([17O]NODMA)2, and [17O]NODMA·HCl. The 17O quadrupole coupling constants (CQ) observed in these C-nitrosoarene compounds are on the order of 10−15 MHz, among the largest values found to date for organic compounds. The 17O CS tensor in these compounds exhibits remarkable sensitivity toward the nitroso bonding scheme with the chemical shift anisotropy (δ11 − δ33) ranging from just 350 ppm in [17O]NODMA·HCl to over 2800 ppm in [17O]NODMA. This latter value is among the largest 17O chemical shift anisotropies reported in the literature. These extremely anisotropic 17O NMR interactions make C-nitrosoarene compounds excellent test cases that allow us to assess the detection limit of solid-state 17O NMR. Our results suggest that, at 21.14 T, solid-state 17O NMR should be applicable to all oxygen-containing organic functional groups. We also show that density functional theory (DFT) calculations can reproduce reasonably well the experimental 17O QC and CS tensors for these challenging molecules. By combining quantum chemical calculations with experimental solid-state 17O NMR results, we are able to determine the 17O QC and CS tensor orientations in the molecular frame of reference for C-nitrosoarenes. We present a detailed analysis illustrating how magnetic field-induced mixing between individual molecular orbitals (MOs) contributes to the 17O shielding tensor in C-nitrosoarene compounds. We also perform a Townes−Dailey analysis for the observed 17O QC tensors and show that 17O CS and QC tensors are intrinsically related through the π bond order of the N═O bond. Furthermore, we are able for the first time to examine the parallelism between individual 17O and 15N CS tensor components in C-nitrosoarenes.