The effects of interplay between quadrupolar, dipolar and shielding tensors on magic-angle spinning NMR spectra: shapes of spinning sidebands

The shapes of spinning sidebands for 13C magic-angle spinning spectra of solids affected by residual dipolar splittings to 14N are found to be spinning-speed dependent. Simple theory for the case of co-axial tensors indicates that centreband splittings at low spinning speeds yield values of xD (x and D are quadrupolar and dipolar coupling constants respectively) directly. The case of the cyanide carbon resonances for the coordination polymer [ᵵ(CH3)3Pb∼4Ru(CN)6]∞ is considered in detail. Computer simulations of the sideband bandshapes fit the experimental results well. The quadrupole coupling constants for the three cyanide nitrogen sites lie in the range -1·9 to -2·5 MHz. The shielding anisotropy for the cyanide carbon giving the highest-frequency resonance is found to be Δσ = ± 350 ppm.