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

A sulfur-33 nuclear quadrupole resonance study of 33 S2 -labeled L-cystine

The sulfur electric-field-gradient tensor for a disulfide bond in ³³ S₂ -labeled L-cystine has been investigated by ³³ S nuclear quadrupole resonance (NQR). ³³ S₂ -labeled L-cystine is synthesized by introduction of disulfide ions prepared from elemental ³³ S-sulfur into an amino acid derivative, the side chain of which is iodinated. In its NQR spectrum, sharp single peaks are observed at between 24.63 and 24.90 MHz in the temperature range from 80 to 298 K. The two-dimensional nutation echo ³³ S NQR experiment is carried out at 160 K, and the quadrupole coupling constant, C_Q , and the asymmetric parameter, η_Q , are obtained to be 46.9(9) MHz and 0.6(1), respectively. The calculated ³³ S electric-field-gradient tensor components with respect to the molecular frame is briefly discussed.

Anion Redox in an Amorphous Titanium Polysulfide

Amorphous transition-metal polysulfides are promising positive electrode materials for next-generation rechargeable lithium-ion batteries because of their high theoretical capacities. In this study, sulfur anion redox during lithiation of amorphous TiS4 (a-TiS4) was investigated by using experimental and theoretical methods. It was found that a-TiS4 has a variety of sulfur valence states such as S2-, S-, and Sδ-. The S2- species became the main component in the Li4TiS4 composition, indicating that sulfur is a redox-active element up to this composition. The simulated a-TiS4 structure changed gradually by lithium accommodation to form a-Li4TiS4: S-S bonds in the disulfide units and polysulfide chains were broken. Bader charge analysis suggested that the average S valency decreased drastically. Moreover, deep lithiation of a-TiS4 provided a conversion reaction to metallic Ti and Li2S, with a high practical capacity of ∼1000 mAh g-1 when a lower cutoff voltage was applied.

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.

Sulfur Kβ X-ray emission spectroscopy: comparison with sulfur K-edge X-ray absorption spectroscopy for speciation of organosulfur compounds

Until recently, sulfur was known as a "spectroscopically silent" element because of a paucity of convenient spectroscopic probes suitable for in situ chemical speciation. In recent years the technique of sulfur K-edge X-ray absorption spectroscopy (XAS) has been used extensively in sulfur speciation in a variety of different fields. With an initial focus on reduced forms of organic sulfur, we have explored a complementary X-ray based spectroscopy - sulfur Kβ X-ray emission spectroscopy (XES) - as a potential analytical tool for sulfur speciation in complex samples. We compare and contrast the sensitivity of sulfur Kβ XES with that of sulfur K-edge XAS, and find differing sensitivities for the two techniques. In some cases an approach involving both sulfur K-edge XAS and sulfur Kβ XES may be a powerful combination for deducing sulfur speciation in samples containing complex mixtures.

77Se NMR Probes the Protein Environment of Selenomethionine

Sulfur is critical for the correct structure and proper function of proteins. Yet, lacking a sensitive enough isotope, nuclear magnetic resonance (NMR) experiments are unable to deliver for sulfur in proteins the usual wealth of chemical, dynamic, and structural information. This limitation can be circumvented by substituting sulfur with selenium, which has similar physicochemical properties and minimal impact on protein structures but possesses an NMR compatible isotope (⁷⁷Se). Here we exploit the sensitivity of ⁷⁷Se NMR to the nucleus' chemical milieu and use selenomethionine as a probe for its proteinaceous environment. However, such selenium NMR spectra of proteins currently resist a reliable interpretation because systematic connections between variations of system variables and changes in ⁷⁷Se NMR parameters are still lacking. To start narrowing this knowledge gap, we report here on a biological ⁷⁷Se magnetic resonance data bank based on a systematically designed library of GB1 variants in which a single selenomethionine was introduced at different locations within the protein. We recorded the resulting isotropic ⁷⁷Se chemical shifts and relaxation times for six GB1 variants by solution-state ⁷⁷Se NMR. For four of the GB1 variants we were also able to determine the chemical shift anisotropy tensor of SeM by solid-state ⁷⁷Se NMR. To enable interpretation of the NMR data, the structures of five of the GB1 variants were solved by X-ray crystallography to a resolution of 1.2 Å, allowing us to unambiguously determine the conformation of the selenomethionine. Finally, we combine our solution- and solid-state NMR data with the structural information to arrive at general insights regarding the execution and interpretation of ⁷⁷Se NMR experiments that exploit selenomethionine to probe proteins.

Direct investigation of chalcogen bonds by multinuclear solid-state magnetic resonance and vibrational spectroscopy

We report a multifaceted experimental and computational study of three self-complementary chalcogen-bond donors as well as a series of seven chalcogen bonded cocrystals.

33S nuclear quadrupole resonance study of dibenzyl disulfide toward understanding of cross-linked structures in rubber

Experimental and theoretical investigations of a sulfur-33 electric-field-gradient (EFG) tensor of disulfide bonds in ³³S-labled dibenzyl disulfide have been presented. Temperature dependence of quadrupolar frequencies, ν_Q, is observed in the temperature range between 80 and 280 K, in which single peaks appear in all the ³³S nuclear quadrupole resonance (NQR) spectra. Analysis of nutation echo ³³S NQR spectra at 200 K yields the quadrupolar coupling constant, C_Q value, of 46.8(6) MHz and the asymmetry parameter, η_Q, of 0.48(7). The orientation of the ³³S EFG tensor of the disulfide is obtained by quantum chemical calculations: the largest EFG tensor component, V_ZZ, is approximately perpendicular to the molecular plane (C-S-S), and the smallest component, V_XX, is approximately 41° off the C-S bond. Extensive quantum chemical calculations are systematically performed to investigate dependences of ³³S EFG tensors on changes of torsion angles in disulfide and trisulfide bonds, indicating that analysis of ν_Q and C_Q values potentially makes it possible to assign the secondary structures of cross-linking in a rubber.

Sulfur-33 NQR investigation of the electric-field-gradient tensor in an organosulfur compound

33S nuclear quadrupole resonance (NQR) frequencies of 33S-enriched S-4-phenyl 4-toluenethiosulfonate were observed in the range of 22.96–23.31 MHz at temperatures between 110 and 300 K. A single sharp signal was observed at all the temperatures. The two-dimensional (2D) nutation echo method was applied at 150 K, providing the 33S electric field gradient (EFG) tensor information, the quadrupolar coupling constant, C Q, of 42.3 MHz and the asymmetry parameter, ηQ, of 0.80(9). Quantum chemical calculations were performed to obtain the 33S EFG tensor orientations with respect to the molecular frame.

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.

Recent advances in solid-state nuclear magnetic resonance spectroscopy of exotic nuclei

We present a review of recent advances in solid-state nuclear magnetic resonance (SSNMR) studies of exotic nuclei. Exotic nuclei may be spin-1/2 or quadrupolar, and typically have low gyromagnetic ratios, low natural abundances, large quadrupole moments (when I > 1/2), or some combination of these properties, generally resulting in low receptivities and/or prohibitively broad line widths. Some nuclides are little studied for other reasons, also rendering them somewhat exotic. We first discuss some of the recent progress in pulse sequences and hardware development which continues to enable researchers to study new kinds of materials as well as previously unfeasible nuclei. This is followed by a survey of applications to a wide range of exotic nuclei (including e.g., ⁹Be, ²⁵Mg, ³³S, ³⁹K, ⁴³Ca, ^47/49Ti, ⁵³Cr, ⁵⁹Co, ⁶¹Ni, ⁶⁷Zn, ⁷³Ge, ⁷⁵As, ⁸⁷Sr, ¹¹⁵In, ¹¹⁹Sn, ^121/123Sb, ^135/137Ba, ^185/187Re, ²⁰⁹Bi), most of them quadrupolar. The scope of the review is the past ten years, i.e., 2007-2017.

Structural and Crystallographic Information from 61Ni Solid-State NMR Spectroscopy: Diamagnetic Nickel Compounds

Despite the significance of nickel compounds, NMR spectroscopy of the active nickel isotope ⁶¹Ni remains a largely unexplored field. While nickel(0) compounds have been studied by ⁶¹Ni NMR in solution, solid-state experiments have been limited to Knight shift studies of nickel metal and nickel intermetallics. In conjunction with an NMR study of their ligands and ⁶¹Ni relativistic computations, the first ⁶¹Ni solid-state NMR (SSNMR) spectra of diamagnetic compounds are reported here. Specifically, bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)₂], tetrakis(triphenylphosphite)nickel(0) [Ni[P(OPh)₃]₄], and tetrakis(triphenylphosphine)nickel(0) [Ni(PPh₃)₄] were studied. ⁶¹Ni SSNMR spectra of Ni(cod)₂ were used to determine its isotropic chemical shift (δ_iso = 965 ± 10 ppm), span (Ω = 1700 ± 50 ppm), skew (κ = -0.15 ± 0.05), quadrupolar coupling constant (C_Q = 2.0 ± 0.3 MHz), quadrupolar asymmetry parameter (η = 0.5 ± 0.2), and the relative orientation of the chemical shift and electric field gradient tensors. A solution study of Ni(cod)₂ in C₆D₆ yielded a narrow ⁶¹Ni signal, and the temperature dependence of δ_iso(⁶¹Ni) was assessed (δ_iso being 936.5 ppm at 295 K). The solution is proposed as a secondary chemical shift reference for ⁶¹Ni NMR in lieu of the extremely toxic Ni(CO)₄ primary reference. For Ni[P(OPh)₃]₄, ⁶¹Ni SSNMR was used to infer the presence of two distinct crystallographic sites and establish ranges for δ_iso in the solid state, as well as an upper bound for C_Q (3.5 MHz for both sites). For Ni(PPh₃)₄, line shape fitting provided a δ_iso value of 515 ± 10 ppm, Ω of 50 ± 50 ppm, κ of 0.5 ± 0.5, C_Q of 0.05 ± 0.01 MHz, and η of 0.0 ± 0.2. The study of Ni(PPh₃)₄, in particular, demonstrates the utility of ⁶¹Ni SSNMR given the lack of a previously reported crystal structure and transient nature of Ni(PPh₃)₄ in solution.

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.

Determination of nuclear quadrupolar parameters using singularities in field-swept NMR patterns

We propose a simple data-analysis scheme to determine the coupling constant and the asymmetry parameter of nuclear quadrupolar interactions in field-swept nuclear magnetic resonance (NMR) for static powder samples. This approach correlates the quadrupolar parameters to the positions of the singularities, which can readily be found out as sharp peaks in the field-swept pattern. Moreover, the parameters can be determined without quantitative acquisition and elaborate calculation of the overall profile of the pattern. Since both experimental and computational efforts are significantly reduced, the approach presented in this work will enhance the power of the field-swept NMR for yet unexplored quadrupolar nuclei. We demonstrate this approach in ³³S in α-S₈ and ³⁵Cl in chloranil. The accuracy of the obtained quadrupolar parameters is also discussed.

The WURST kind of pulses in solid-state NMR

WURST pulses (wideband, uniform rate, smooth truncation) were first introduced two decades ago by Kupče and Freeman as a means of achieving broadband adiabatic inversion of magnetisation for solution-state (13)C decoupling at high magnetic field strengths. In more recent years these pulses have found use in an increasingly diverse range of applications in solid-state NMR. This article reviews a number of recent developments that take advantage of WURST pulses, including broadband excitation, refocusing and cross polarisation for the acquisition of ultra-wideline powder patterns, signal enhancement for half-integer and integer spin quadrupolar nuclei, spectral editing, direct and indirectly observed (14)N overtone MAS, and symmetry-based homonuclear recoupling. Simple mathematical descriptions of WURST pulses and some brief theory behind their operation in the adiabatic and non-adiabatic regimes are provided, and various practical considerations for their use are also discussed.

Ultra-wideline solid-state NMR spectroscopy

Although solid-state NMR (SSNMR) provides rich information about molecular structure and dynamics, the small spin population differences between pairs of spin states that give rise to NMR transitions make it an inherently insensitive spectroscopic technique in terms of signal acquisition. Scientists have continuously addressed this issue via improvements in NMR hardware and probes, increases in the strength of the magnetic field, and the development of innovative pulse sequences and acquisition methodologies. As a result, researchers can now study NMR-active nuclides previously thought to be unobservable or too unreceptive for routine examination via SSNMR. Several factors can make it extremely challenging to detect signal or acquire spectra using SSNMR: (i) low gyromagnetic ratios (i.e., low Larmor frequencies), (ii) low natural abundances or dilution of the nuclide of interest (e.g., metal nuclides in proteins or in organometallic catalysts supported on silica), (iii) inconvenient relaxation characteristics (e.g., very long longitudinal or very short transverse relaxation times), and/or (iv) extremely broad powder patterns arising from large anisotropic NMR interactions. Our research group has been particularly interested in efficient acquisition of broad NMR powder patterns for a variety of spin-1/2 and quadrupolar (spin > 1/2) nuclides. Traditionally, researchers have used the term "wideline" NMR to refer to experiments yielding broad (1)H and (2)H SSNMR spectra ranging from tens of kHz to ∼250 kHz in breadth. With modern FT NMR hardware, uniform excitation in these spectral ranges is relatively easy, allowing for the acquisition of high quality spectra. However, spectra that range in breadth from ca. 250 kHz to tens of MHz cannot be uniformly excited with conventional, high-power rectangular pulses. Rather, researchers must apply special methodologies to acquire such spectra, which have inherently low S/N because the signal intensity is spread across such large spectral breadths. We have suggested the term ultra-wideline NMR (UWNMR) spectroscopy to describe this set of methodologies. This Account describes recent developments in pulse sequences and strategies for the efficient acquisition of UWNMR spectra. After an introduction to anisotropically broadened NMR patterns, we give a brief history of methods used to acquire UWNMR spectra. We then discuss new acquisition methodologies, including the acquisition of CPMG echo trains and the application of pulses capable of broadband excitation and refocusing. Finally, we present several applications of UWNMR methods that use these broadband pulses.