Satellite transitions in natural abundance solid-state 33S MAS NMR of alums-Sign change with zero-crossing of CQ in a variable temperature study

Experiences obtained from recent improvements in the performance of solid-state 14N MAS NMR spectroscopy have been used in a natural abundance 33S MAS NMR investigation of the satellite transitions for this interesting spin I = 3/2 isotope. This study reports the first observation of manifolds of spinning sidebands for these transitions in 33S MAS NMR as observed for the two alums XAl(SO4)2·12H2O with X = NH4 and K. For the NH4-alum a variable temperature 33S MAS NMR study, employing the satellite transitions, shows that the 33S quadrupole coupling constant (CQ) exhibits a linear temperature dependence (in the range −35 °C to 70 °C) with a temperature gradient of 3.1 kHz/°C and undergoes a sign change with zero-crossing for CQ at 4 °C (277 K). For the isostructural K-alum a quite similar increase in the magnitude of CQ with increasing temperature is observed, and with a temperature gradient of 2.3 kHz/°C. Finally, for optimization purposes, a study on the effect of the applied pulse widths at constant rf field strength on the intensity and variation in second-order quadrupolar lineshape for the central (1/2 ↔ −1/2) transition of the K-alum has been performed.