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

We report the first solid-state O-17 NMR determination of the O-17 quadrupole coupling (QC) tensor and chemical shift (CS) tensor for four O-17-labeled C-nitrosoarene compounds: p-[O-17]nitroso-N,N-dimethylaniline ([O-17]NODMA), SnCl2(CH3)(2)([O-17]NODMA)(2), ZnCl2([O-17]NODMA)(2), and [O-17]NODMA center dot HCl. The O-17 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 O-17 CS tensor in these compounds exhibits remarkable sensitivity toward the nitroso bonding scheme with the chemical shift anisotropy (delta(11) - delta(33)) ranging from just 350 ppm in [O-17]NODMA center dot HCl to over 2800 ppm in [O-17]NODMA. This latter value is among the largest O-17 chemical shift anisotropies reported in the literature. These extremely anisotropic O-17 NMR interactions make C-nitrosoarene compounds excellent test cases that allow us to assess the detection limit of solid-state O-17 NMR. Our results suggest that, at 21.14 T, solid-state O-17 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 O-17 QC and CS tensors for these challenging molecules. By combining quantum chemical calculations with experimental solid-state O-17 NMR results, we are able to determine the O-17 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 O-17 shielding tensor in C-nitrosoarene compounds. We also perform a Townes Dailey analysis for the observed O-17 QC tensors and show that O-17 CS and QC tensors are intrinsically related through the pi bond order of the N=O bond. Furthermore, we are able for the first time to examine the parallelism between individual O-17 and N-15 CS tensor components in C-nitrosoarenes.