Abstract
The principal values of the nitrogen nuclear magnetic resonance shielding tensor for trans-azobenzene have been measured and compared with theoretical values recently reported for trans-diazene. The shielding anisotropy for the two crystallographically nonequivalent molecules in trans-azobenzene is 925 and 880 ppm respectively, about 40% smaller than the value calculated for trans-diazene. Interestingly, the isotropic shielding constants for the two nonequivalent molecules are the same with in experimental error. The most shielded component of the 15N shielding tensor, σ33, is approximately perpendicular to the molecular plane, as it is in other planar π-systems; σ11, the least shielded component, is directed in the same general direction as the N—Cipso bond. The theoretical calculations overestimate the paramagnetic contribution to σ11. The magnitude and relative orientation of the nitrogen shielding tensor in trans-azobenzene are also compared with the corresponding phosphorus values in a diphosphene. Keywords: 15N chemical shielding anisotropy, solid state NMR, trans-azobenzene.
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