Calcium oxalates are naturally occurring minerals, with the monohydrate form CaC2O4·H2O (COM) being the most stable. COM contains two crystallographically different water sites. We investigate the details of water internal dynamics in the high and low temperature phases of COM using 17O central transition solid-state NMR line shapes, as well as laboratory and rotating frame relaxation rates. The measurements were performed either under static or magic angle spinning conditions and in a wide temperature range from 343 to 180 K. The combination of all measurements allows for precise constraints on motional mechanisms, rate constants, and amplitudes of motions. The high temperature phase is dominated by large-angle fluctuations with an amplitude of about 100°, identical in both sites. During the phase transition between 323 and 300 K, these large-angle jumps freeze out in one of the water sites, while remaining active in the other. In the low temperature phase from 280 to 180 K, small-angle fluctuations of 2-8° in amplitude dominate the relaxation. Transverse relaxation rates also point to the existence of a very slow collective “rocking” motion down to about 220-200 K.
In this review, we present recent applications of nuclear magnetic resonance (NMR) for probing, at the microscopic level, new quantum states of condensed matter that emerge in strong static and pulsed magnetic fields. We focus on experiments performed in resistive magnets (up to 34 T) and hybrid magnets (max 45 T), further extended to fields beyond 60 T using pulsed magnets, which together offer broad access to these quantum states and the quantum phase transitions between them. After a brief overview of NMR observables, we discuss several topics: quantum spin systems (including spin chains, spin ladders, spin-nematic phases), and their exotic phenomena (magnetization plateaux, Bose-Einstein condensation of triplet excitations), as well as aspects of novel multiferroic systems.
Ultra-high magnetic field NMR spectrometers operating at proton frequencies approaching and exceeding 1.2 GHz are opening new opportunities for biomolecular solid-state NMR. While higher magnetic fields are expected to improve spectral resolution through increased chemical-shift dispersion, the extent to which they continue to improve proton linewidths under fast magic-angle spinning (MAS) conditions has remained unclear. Addressing this question is particularly important as increasingly fast MAS and ever higher magnetic fields become available. Here, we systematically investigate proton linewidths and coherence lifetimes across proton Larmor frequencies from 500 to 1200 MHz under fast MAS conditions. Using microcrystalline GB1 as a benchmark system, we combine bulk and site-specific analyses of proton linewidths and transverse coherence lifetimes (T2') to characterize linewidths as a function of magnetic field strength. Increasing field strengths systematically improve proton spectral resolution and prolong proton coherence lifetimes, yielding gains that exceed those expected from increased chemical-shift dispersion alone. Linewidth analysis reveals a progressive reduction of non-refocusable broadening at high fields, consistent with suppression of coherent proton spin dynamics. The resulting benefits are particularly pronounced for aliphatic side-chain resonances and remain clearly observable in membrane proteins reconstituted in lipid bilayers, where spectral crowding often limits analysis. These results provide a systematic view of linewidth evolution in proton-detected solid-state NMR and support continuing developments toward higher magnetic fields and faster MAS frequencies for challenging biomolecular systems.
In this study, we have established an empirical 25Mg chemical shift scale for silicates based on experimentally determined isotropic shift δiso values for Mg sites in crystalline compounds spanning coordination numbers from four to eight. These data reveal a robust linear correlation between δiso and the average Mg-O bond distance in the coordination polyhedra, defining approximate δiso ranges of 50 to 30, 27 to 17, 17 to -5, and ≤ -15 ppm for MgIV, MgV, MgVI, and MgVIII environments, respectively. This scale is applied to a comprehensive slice-by-slice analysis of previously reported ultra-high field (35.2 T) 25Mg 3QMAS NMR spectra of three glasses of composition CaMgSi2O6, Na2MgSi3O8, and K2MgSi5O12, revealing a positive correlation between the quadrupolar coupling constant CQ and δiso within two structurally distinct Mg environments assignable to MgVI and MgIV. Quantitative analysis indicates that ∼70-75% of Mg is present as MgIV in all three glasses, yielding an average Mg-O coordination number of ∼4.6 for CaMgSi2O6 glass, in close agreement with the value of ∼4.4 reported in a recent isotope-substituted neutron diffraction study. The finding of the lack of any significant dependence of the Mg-O coordination number on the field strength of the second network modifying cation in these glasses is in sharp contrast with previous reports of a preference of Mg for fourfold coordination in silicate glasses containing low field strength alkali cations.
The use of a nucleus other than the nucleus of interest to establish the chemical shift scale (i.e., indirect referencing) in solid-state nuclear magnetic resonance (SSNMR) spectroscopy is discussed. While the technique is routinely applied in solution NMR studies, it is not applied as widely within the SSNMR community. The benefits of the indirect referencing approach in many SSNMR applications are discussed. In particular, the methods by which indirect methods improve efficiency, reproducibility and help avoid misassignments of chemical shifts are shown. Measurements via direct and indirect methods of a representative sampling of the NMR periodic table are presented to demonstrate the viability of the indirect approach. While any sample with a known chemical shift may be used for indirect referencing, adamantane is demonstrated to be a particularly well-suited solid sample.
