Dipolar coupling information in multispin systems: Application of a compensated REDOR NMR approach to inorganic phosphates

An experimental strategy has been developed for measuring multiple dipole-dipole interactions in inorganic compounds using the technique of rotational echo double resonance (REDOR) NMR. Geometry-independent information about the dipole couplings between the observe nuclear species S (arbitrary quantum number) and the heteronuclear species I (spin-1/2) can be conveniently obtained from the experimental curve of DeltaS/S-0 versus dipolar evolution time by limiting the analysis to the initial data range 0 < <Delta>S/S-0 < 0.30. Numerical simulations have been carried out on a three-spin system of type SI2 in order to assess the effect of the I-I homonuclear dipole-dipole coupling and the influence of experimental imperfections such as finite pulse length and misadjustments of the 180<degrees> pulses applied to the I-spin species. The simulations show further that within the initial data range the effects of such misadjustments can be internally compensated by a modified sequence having an additional 180 degrees pulse on the I channel in the middle of the dipolar evolution periods. Experimental Al-27{P-31} REDOR results on the multispin systems Al(PO3)(3), AlPO4, [AlPO4](12)(C3H7)(4)NF, and Na3PO4 confirm the general utility of this approach. Thus, for applications to unknown systems the compensation strategy obviates calibration procedures with model compounds. (C) 2000 Academic Press.