Solid-state 33S MAS NMR of inorganic sulfates

33S NMR: Recent Advances and Applications

The purpose of this review is to present advances and applications of 33S NMR, which is an underutilized NMR spectroscopy. Experimental NMR aspects in solution, chemical shift tendencies, and quadrupolar relaxation parameters will be briefly summarized. Emphasis will be given to advances and applications in the emerging fields of solid-state and DFT computations of 33S NMR parameters. The majority of the examples were taken from the last twenty years and were selected on the basis of their importance to provide structural, electronic, and dynamic information that is difficult to obtain by other techniques.

Indirectly detected satellite-transition quadrupolar NMR via progressive saturation of the proton reservoir

Static satellite-transitions (ST) NMR line shapes from half-integer quadrupolar nuclei could be very informative: they can deliver insight about local motions over a wide range of timescales, and can report on small changes in the local electronic environments as reflected by variations in the quadrupolar parameters. Satellite transitions, however, are typically "invisible" for half-integer quadrupolar nuclei due to their sheer breadth, leading to low signal-to-noise ratio -especially for unreceptive low-gamma or dilute quadrupolar nuclei. Very recently we have introduced a method for enhancing the NMR sensitivity of unreceptive X nuclei in static solids dubbed PROgressive Saturation of the Proton Reservoir (PROSPR), which opens the possibility of magnifying the signals from such spins by repeatedly imprinting frequency-selective X-driven depolarizations on the much more sensitive ¹H NMR signal. Here, we show that PROSPR's efficacy is high enough for enabling the detection of static ST NMR for challenging species like ³⁵Cl, ³³S and even ¹⁷O -all at natural-abundance. The ensuing ST-PROSPR NMR experiment thus opens new approaches to probe ultra-wideline (6-8 MHz wide) spectra. These highly pronounced anisotropies can in turn deliver new vistas about dynamic changes in solids, as here illustrated by tracking ST line shapes as a function of temperature during thermally-driven events.

Recent progress in solid-state nuclear magnetic resonance of half-integer spin low-γ quadrupolar nuclei applied to inorganic materials

An overview is presented of recent progress in the solid-state nuclear magnetic resonance (NMR) observation of low-γ nuclei, with a focus on applications to inorganic materials. The technological and methodological advances in the last 20 years, which have underpinned the increased accessibility of low-γ nuclei for study by solid-state NMR techniques, are summarised, including improvements in hardware, pulse sequences and associated computational methods (e.g., first principles calculations and spectral simulation). Some of the key initial observations from inorganic materials of these nuclei are highlighted along with some recent (most within the last 10 years) illustrations of their application to such materials. A summary of other recent reviews of the study of low-γ nuclei by solid-state NMR is provided so that a comprehensive understanding of what has been achieved to date is available.

Natural abundance solid-state 33S NMR study of NbS3: applications for battery conversion electrodes

The first known solid-state ³³S NMR spectrum of disulfide (S₂²⁻) anions is reported, in the Li-ion battery conversion material NbS₃.

A high-resolution natural abundance 33S MAS NMR study of the cementitious mineral ettringite

Natural abundance ³³S STMAS NMR spectroscopy is used to determine the ³³S chemical shift and quadrupolar parameters in the cement-forming mineral ettringite.

Testing the sensitivity limits of ³³S NMR: an ultra-wideline study of elemental sulfur

Preliminary DFT investigations into the feasibility of using (33)S solid-state NMR to study organic and biological molecules suggest that very large (33)S quadrupolar coupling constants (>40MHz) are not uncommon. We have therefore investigated the possibility of using recently developed ultra-wideline techniques to record such (33)S powder patterns at a high magnetic field (21.1T). A WURST-echo sequence was used to record the spectrum from a>99.9% enriched sample of elemental sulfur, resulting in the largest (33)S quadrupolar coupling constant yet measured by solid-state NMR (C(Q)=43.3 MHz). Implications of this experiment are briefly discussed.

A natural abundance (77)Se solid-state NMR study of inorganic compounds

Various inorganic selenium-based compounds were analysed by (77)Se solid-state NMR, and a distinct difference in chemical shift ranges for compounds where selenium is present as selenide (Se(2-)) ionically and covalently bonded systems was observed. The selenides exhibit a shift range of approximately -700 to -100ppm, as opposed to 700 to 1600ppm for the compounds where there tends to be more direct covalent bonding to the selenium. The anisotropic hyperfine shift observed in NbSe(2) is shown to be axially symmetric, where the H(11) component is found to be normal to the Se3-trigonal plane.

Sensitivity enhancement in natural-abundance solid-state 33S MAS NMR spectroscopy employing adiabatic inversion pulses to the satellite transitions

The WURST (wideband uniform rate smooth truncation) and hyperbolic secant (HS) pulse elements have each been employed as pairs of inversion pulses to induce population transfer (PT) between the four energy levels in natural abundance solid-state (33)S (spin I=3/2) MAS NMR, thereby leading to a significant gain in intensity for the central transition (CT). The pair of inversion pulses are applied to the satellite transitions for a series of inorganic sulfates, the sulfate ions in the two cementitious materials ettringite and thaumasite, and the two tetrathiometallates (NH(4))(2)WS(4) and (NH(4))(2)MoS(4). These materials all exhibit (33)S quadrupole coupling constants (C(Q)) in the range 0.1-1.0 MHz, with precise C(Q) values being determined from analysis of the PT enhanced (33)S MAS NMR spectra. The enhancement factors for the WURST and HS elements are quite similar and are all in the range 1.74-2.25 for the studied samples, in excellent agreement with earlier reports on HS enhancement factors (1.6-2.4) observed for other spin I=3/2 nuclei with similar C(Q) values (0.3-1.2 MHz). Thus, a time saving in instrument time by a factor up to five has been achieved in natural abundance (33)S MAS NMR, a time saving which is extremely welcome for this important low-gamma nucleus.

Advancements in natural abundance solid-state33S MAS NMR: characterization of transition-metal MS bonds in ammonium tetrathiometallates

We report the first (33)S chemical shift anisotropy (CSA) data as obtained from a combined determination of (33)S CSA and quadrupole coupling parameters utilizing the observation of both the (33)S (I = 3/2) central and satellite transitions in a natural abundance (33)S MAS NMR study aimed at characterizing the two important tetrathiometallates (NH4)(2)MoS(4) and (NH4)(2)WS(4).

Satellite transitions in natural abundance solid-state (33)S MAS NMR of alums--sign change with zero-crossing of C(Q) in a variable temperature study

Experiences obtained from recent improvements in the performance of solid-state (14)N MAS NMR spectroscopy have been used in a natural abundance (33)S 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 (33)S MAS NMR as observed for the two alums XAl(SO(4))(2) x 12H(2)O with X=NH(4) and K. For the NH(4)-alum a variable temperature (33)S MAS NMR study, employing the satellite transitions, shows that the (33)S quadrupole coupling constant (C(Q)) exhibits a linear temperature dependence (in the range -35 degrees C to 70 degrees C) with a temperature gradient of 3.1 kHz/ degrees C and undergoes a sign change with zero-crossing for C(Q) at 4 degrees C (277 K). For the isostructural K-alum a quite similar increase in the magnitude of C(Q) with increasing temperature is observed, and with a temperature gradient of 2.3 kHz/ degrees 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.