High-field 95 Mo and 183 W static and MAS NMR study of polyoxometalates

The potential of high-field NMR to measure solid-state ⁹⁵ Mo and ¹⁸³ W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low-gamma NMR nuclei, due mainly to the low spectral resolution and poor signal-to-noise ratio, the observed global trends compare well with the solution-state NMR data. This would open an avenue for application of solid-state NMR to POMs, especially when liquid-state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.

High-precision analysis of 182W/184W and 183W/184W by negative thermal ionization mass spectrometry: Per-integration oxide corrections using measured 18O/16O

Here we describe a new analytical technique for the high-precision measurement of 182W/184W and 183W/184W using negative thermal ionization mass spectrometry (N-TIMS). We improve on the recently reported method of Trinquier et al. (2016), which described using Faraday cup collectors coupled with amplifiers utilizing 1013 Ω resistors to continuously monitor the 18O/16O of WO3 - and make per-integration oxide corrections. In our study, we report and utilize a newly measured oxygen mass fractionation line, as well as average 17O/16O and 18O/16O, which allow for more accurate per-integration oxide interference corrections. We also report a Faraday cup and amplifier configuration that allows 18O/16O to be continuously monitored for WO3 - and ReO3 -, both of which are ionized during analyses of W using Re ribbon. The long-term external precision of 182W/184W is 5.7 ppm and 3.7 ppm (2SD) when mass bias corrected using 186W/184W and 186W/183W, respectively. For 183W/184W mass bias is corrected using 186W/184W, yielding a long-term external precision of 6.6 ppm. An observed, correlated variation in 182W/184W and 183W/184W, when mass bias corrected using 186W/184W, is most likely the result of Faraday cup degradation over months-long intervals.