UNUSUAL AB SPECTRA IN HIGH-RESOLUTION MAGIC-ANGLE-SPINNING NMR-STUDIES OF SOLIDS

Phosphorus-31CP-MAS spectra of Cd(NO3)2(PPh3)2 have been obtained as a function of spinning frequency. Although the two 31P nuclei are crystallographically equivalent and have the same isotropic chemical shifts in the solid state, they exhibit spinning-rate-dependent MAS spectra which have been analyzed to obtain the value of 2J(P, P). At high spinning rates, the spectra are analogous to "A2" spectra in isotropic solutions, while at slower spinning rates, the spectra are more characteristic of strongly coupled "AB" solution spectra. The AB spectra are unusual in that δA = δB and J(A, B) is given by the splitting between the alternate peaks in the four-peak multiplet as opposed to the splitting between the outer and adjacent inner lines. This assignment was confirmed by a 2D CP-MAS J-resolved experiment. The unusual spinning-rate-dependent MAS lineshapes result from recoupling of the J interaction between the two crystallographically equivalent nuclei via anisotropic interactions, i.e., weak homonuclear dipolar coupling and differences in the orientation dependence of the chemical-shift tensors. Such spinning-rate-dependent MAS lineshapes are predicted to be a more frequent observation at higher applied magnetic fields. © 1993 by Academic Press, Inc.