Solid-State 17 O NMR and Computational Studies of C-Nitrosoarene Compounds

We report the first solid-state 17 O NMR determination of the 17 O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for four 17 O-labeled C-nitrosoarene compounds: p-( 17 O)nitroso-N,N- dimethylaniline (( 17 O)NODMA), SnCl2(CH3)2(( 17 O)NODMA)2, ZnCl2(( 17 O)NODMA)2, and ( 17 O)NODMA · HCl. The 17 O 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 17 O 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 ( 17 O)NODMA · HCl to over 2800 ppm in ( 17 O)NODMA. This latter value is among the largest 17 O chemical shift anisotropies reported in the literature. These extremely anisotropic 17 O NMR interactions make C-nitrosoarene compounds excellent test cases that allow us to assess the detection limit of solid-state 17 O NMR. Our results suggest that, at 21.14 T, solid- state 17 O 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 17 OQ C and CS tensors for these challenging molecules. By combining quantum chemical calculations with experimental solid-state 17 O NMR results, we are able to determine the 17 O 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 17 O shielding tensor in C-nitrosoarene compounds. We also perform a Townes-Dailey analysis for the observed 17 O QC tensors and show that 17 O CS and QC tensors are intrinsically related through the π bond order of the NdO bond. Furthermore, we are able for the first time to examine the parallelism between individual 17 O and 15 NC S tensor components in C-nitrosoarenes.