Analysis of the anisotropic dimension in the RIACT (II) multiple quantum MAS NMR experiment for I = 3/2 nuclei.

Numerical simulations of the anisotropic dimension of the Rotation-Induced Adiabatic Coherence Transfer (RIACT) MQ experiment have been performed as a function of the asymmetry parameter, eta, for different spacings between the triple quantum (3Q) excitation and 3Q to single quantum (1Q) reconversion pulses. Large distortions of the spectra are observed, in comparison to the spectra obtained with 1D MAS NMR methods. The method is very sensitive to the relative orientation of the quadrupolar tensor and rotor axis, and signal can only be obtained from a maximum of 60% of the powder. The intensity varies with the spacing between the two pulses, t1, reaching a minimum of 30% when t1 is either a multiple of a full rotor period (n tau(r)) or for (n + 1/2) rotor periods, for pulse lengths (tau1SL) of a quarter of a rotor period. The maximum of 60% is obtained when t1 = n tau(r) - tau1SL. Experimental spectra were acquired for anhydrous Na2HPO4. Good fits were obtained between the experimental and simulated spectra, even for the non-rotor synchronized experiment, by choosing fixed values of t1. The simulations allowed the quadrupole coupling constants and asymmetry parameters to be extracted from the experimental data.