Magnitudes and orientations of interaction tensors determined from rotational resonance MAS NMR lineshapes of a four-C-13-spin system

Possibilities and limitations of iterative lineshape fitting approaches for the complete determination of magnitudes and orientations of NMR interacti on tensors in a four-C-13-spin system from MAS NMR experiments are investig ated. The availability of fast and numerically accurate computational metho ds is an important prerequisite. The model compound chosen for this investi gation is the monoammonium salt of maleic acid. Various selectively and ful ly C-13-labeled versions of this compound permit a stepwise reduction of th e number of unknown parameters, necessary to fully describe the four-C-13-s pin system in the uniformly C-13-labeled maleate moiety, This stepwise proc edure allows one to monitor reliability and accuracy of multiparameter fits of the four-C-13-spin system itself, as well as to characterize limitation s and requirements for such fitting procedures. Satisfactory H-1-decoupling performance is an essential experimental requirement; TPPM decoupling yiel ds n = 1, 2 rotational resonance C-13 MAS NMR lineshapes suitable for analy sis by iterative lineshape fitting methods. It is demonstrated that assumpt ions about "typical" chemical shielding tensor orientations, even if not de viating much from the real orientations, lead to severe errors in internucl ear distance determinations. (C) 1999 Academic Press.