Double-quantum filtration under rotational-resonance conditions: Numerical simulations and experimental results

The dependence of the performance of a recently introduced pulse sequence to achieve double-quantum excitation under the n = 1 rotational-resonance condition (T. Karlsson, M. Eden, H. Luthman, and M. H. Levitt, 2000, J. Magn, Reson. 145, 95-107) on different spin-system properties is investigated by means of numerical simulations and C-13 MAS NMR experiments. For spin systems where chemical shielding anisotropies amount to only an insignificant fraction of the isotropic chemical shielding difference, high efficiencies are found for large and small dipolar coupling interactions. In the presence of significant chemical shielding anisotropies the overall efficiencies decrease and become strongly dependent on the duration of the excitation period. It is demonstrated that those spin-system parameters which are sensitively encoded in the lineshapes of a conventional n = 1 rotational-resonance spectrum are similarly sensitively encoded in the corresponding rotational-resonance double-quantum-filtered lineshapes and may be quantitatively recovered by iterative lineshape-fitting approaches. In certain favorable circumstances, the in-built selectivity of the rotational-resonance double-quantum-filtration approach permits successful application of the experiment on spin systems with more than two spins. (C) 2000 Academic Press.