Sulfur-33 NMR at natural abundance in solids

H. Eckert and J.P. Yesinowski

Abstract

The first detailed study of 33S NMR in the solid state has been carried out for a variety of metal sulfides and sulfates, including naturally occurring minerals. The difficulties inherent with 33S NMR at natural isotopic abundance can be overcome by using a very high magnetic field strength (11.74 T) in order to minimize second-order nuclear quadrupole coupling effects and by employing a RIDE (RIngDown Elimination) pulse sequence to eliminate the severe problem of acoustic ringing. Modification of this sequence by introducing composite pulses enables observation of a significantly larger chemical shift range. Metal sulfates, including the alums, show evidence of second-order quadrupolar broadening effects, from which nuclear electric quadrupolar coupling constants ranging from 0.5 to 2.2 MHz are calculated. The sulfate chemical shifts (330 ppm downfield from CS2) are nearly identical and are close to the value measured in solution. In contrast, a large chemical shift range (ca. 600 ppm) is observed for the metal sulfides. For ZnS and CdS, both of which occur in the wurtzite structure, Δσ values of 23 and 28 ppm, respectively, are determined. The explanation of the chemical shift trends in the metal sulfides involves more than simple electronegativity arguments. It is necessary to consider both crystalline ionicities, as measured from dielectric constants, and the effects of orbital overlap. The more covalently bonded post-transition-metal sulfides (ZnS, CdS, and PbS) are appropriately described in terms of the bond orbital model formulated by Harrison. The order of the chemical shifts found for these compounds agrees with that predicted from crystalline ionicities and bond polarity parameters, with the calculated values of the average excitation energy being in a reasonable range. For the alkali and alkaline earth sulfides an extended Kondo-Yamashita approach provides the appropriate description; as observed previously for the alkali halides the shift trends can be rationalized in terms of changes in the anion-cation and anion-anion overlap integrals. The temperature dependence of the chemical shift has been measured for a number of compounds and supports the above conclusions.