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

Possibilities and limitations of iterative lineshape fitting approaches for the complete determination of magnitudes and orientations of NMR interaction tensors in a four-C-13-spin system from MAS NMR experiments are investigated. The availability of fast and numerically accurate computational methods is an important prerequisite. The model compound chosen for this investigation is the monoammonium salt of maleic acid. Various selectively and fully C-13-labeled versions of this compound permit a stepwise reduction of the number of unknown parameters, necessary to fully describe the four-C-13-spin system in the uniformly C-13-labeled maleate moiety, This stepwise procedure allows one to monitor reliability and accuracy of multiparameter fits of the four-C-13-spin system itself, as well as to characterize limitations and requirements for such fitting procedures. Satisfactory H-1-decoupling performance is an essential experimental requirement; TPPM decoupling yields n = 1, 2 rotational resonance C-13 MAS NMR lineshapes suitable for analysis by iterative lineshape fitting methods. It is demonstrated that assumptions about "typical" chemical shielding tensor orientations, even if not deviating much from the real orientations, lead to severe errors in internuclear distance determinations. (C) 1999 Academic Press.