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

Abstract

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 ( ν r = 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 ReS 4 - in [(C 2H 5) 4N][ReS 4]. This strategy of different NMR experiments combined with spectral analysis/simulations has allowed determination of precise values for two sets of quadrupole coupling parameters ( C Q and η Q) assigned to the two different S sites for the four sulfur atoms in the ReS 4 - anion in the ratio S1:S2 = 1:3. These sets of C Q, η Q values for the S1 and S2 site are quite similar and the magnitudes of the quadrupole coupling constants ( C Q = 2.2–2.5 MHz) are a factor of about three larger than observed for other tetrathiometalates A 2MS 4 (A = NH 4, Cs, Rb and M = W, Mo). In addition, the spectral analysis also leads to a determination of the chemical shift anisotropy (CSA) parameters ( δ σ and η σ) for the S1 and S2 site, however, with much lower precisions (about 20% error margins) compared to those for C Q, η Q, because the magnitudes of the two CSAs (i.e., δ σ = 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 C Q and δ σ for the ReS 4 - anion, compared to those for the WS 4 2 - and MoS 4 2 - anions determined previously under identical experimental conditions, accounts for the increased complexity of the PT-enhanced 33S MAS spectra observed for the ReS 4 - anion in this study. This difference in C Q 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.