Iterative lineshape fitting of MAS NMR spectra:: A tool to investigate homonuclear J coupling in isolated spin pairs

Possibilities and limitations of iterative lineshape fitting procedures of MAS NhlR spectra of isolated homonuclear spin pairs, aiming at determination of magnitudes and orientations of the various interaction tensors, are explored. Requirements regarding experimental MAS NMR spectra as well as simulation and fitting procedures are discussed. Our examples chosen are the isolated P-31 spin pairs in solid Na4P2O7. 10H(2)O, (1), and Cd(NO3)(2). 2PPh(3), (2). In both cases the two P-31 chemical shielding tensors in the molecular unit are related by C-2 symmetry, and determination of the orientations of these two tensors in the molecular frame is possible. In addition, aspects of homonuclear J coupling will be addressed. For 1, both magnitude and sign of (2)J(iso)(P-31, P-31) (J(iso) = -19.5 +/- 2.5 Hz) are obtained; for 2, (J(iso) = +139 +/- 3 Hz) anisotropy of J with an orientation of the J-coupling tensor collinear, or nearly collinear, with the dipolar coupling tensor can be excluded, while absence or presence of anisotropy of J with any other relative orientation of the J-coupling tensor cannot be determined. (C) 1998 Academic Press.