A strategy for acquisition and analysis of complex natural abundance 33S solid-state NMR spectra of a disordered tetrathio transition-metal anion

A strategy, involving (i) sensitivity enhancement for the central transition (CT) by population transfer (PT) employing WURST inversion pulses to the satellite transitions (STs) in natural abundance 33S MAS NMR for two different MAS frequencies (νr = 5.0 and 10.0 kHz) at 14.1 T and (ii) a 33S static QCPMG experiment at 19.6 T, has allowed acquisition and analysis of very complex solid-state 33S CT NMR spectra for the disordered tetrathioperrhenate anion ReS4- in [(C2H5)4N][ReS4]. This strategy of different NMR experiments combined with spectral analysis/simulations has allowed determination of precise values for two sets of quadrupole coupling parameters (CQ and ηQ) assigned to the two different S sites for the four sulfur atoms in the ReS4- anion in the ratio S1:S2 = 1:3. These sets of CQ, ηQ values for the S1 and S2 site are quite similar and the magnitudes of the quadrupole coupling constants (CQ = 2.2-2.5 MHz) are a factor of about three larger than observed for other tetrathiometalates A2MS4 (A = NH4, Cs, Rb and M = W, Mo). In addition, the spectral analysis also leads to a determination of the chemical shift anisotropy (CSA) parameters (δσ and ησ) for the S1 and S2 site, however, with much lower precisions (about 20% error margins) compared to those for CQ, ηQ, because the magnitudes of the two CSAs (i.e., δσ = 60-90 ppm) are about a factor of six smaller than observed for the other tetrathiometalates mentioned above. This large difference in the magnitudes of the anisotropic parameters CQ and δσ for the ReS4- anion, compared to those for the WS42- and MoS42- anions determined previously under identical experimental conditions, accounts for the increased complexity of the PT-enhanced 33S MAS spectra observed for the ReS4- anion in this study. This difference in CQ also contributes significantly to the intensity distortions observed in the outer wings of the CTs when employing PT from the STs under conditions of slow-speed MAS.