Solid-state ¹⁷O NMR and computational studies of C -nitrosoarene compounds

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), SnCl ₂(CH₃)₂([¹⁷O]NODMA)₂, ZnCl₂([¹⁷O]NODMA)₂, and [¹⁷O] NODMAÂHCl. The ¹⁷O quadrupole coupling constants (C _Q) 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 (Î₁₁-Î₃₃) ranging from just 350 ppm in [ ¹⁷O]NODMAÂHCl 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 Ï bond order of the Nî-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.