Pure quadrupole resonance of oxygen-17 and sodium-23 in NaAuCl4 · 2H2O

The double resonance pure quadrupole spectrum of NaAuCl₂ · 2H₂O displays four frequency bands: around 1.33 MHz. due to ²³Na, and around 1.89, 2.11, and 4.00 MHz due to ¹⁷O. The higher frequency band displays fine structure that is intepreted in terms of the intramolecular dipolar interactions within the water molecule.     

Short- and medium-range order in sodium aluminophosphate glasses: new insights from high-resolution dipolar solid-state NMR spectroscopy

The structures of sodium aluminophosphate glasses prepared by both sol-gel as well as melt-cooling routes have been extensively characterized by high-resolution solid-state 23Na, 27Al, and 31P single and double-resonance NMR techniques, including quantitative connectivity studies by 27Al <--> 31P and 23Na <--> 31P rotational echo double-resonance (REDOR) methods. Studies along four compositional lines, I: (AlPO4)x -(NaPO3)1-x, II: (Na2O)x -(AlPO4)1-x, III: (NaAlO2)x -(NaPO3)1-x, and IV: (Al2O3)x (NaPO3)1-x, reveal that the network structures of those glasses that are accessible by either preparation method are essentially identical. However, the significantly extended glass-forming ranges available by the sol-gel route facilitate exploration of the structure/composition relationships in more detail, revealing a number of interesting universal features throughout the whole glass system. Both short- and medium-range order appear to be controlled strongly by the O/P ratio of the glasses studied: Up to an O/P ratio of 3.5 (pyrophosphate composition), aluminum is predominantly six-coordinated and fully connected to phosphorus (Al(OP)6 sites). In the region 3.5 < or = O/P < or = 4.0, a dramatic structural transformation takes place, leading to the appearance of additional four- and five-coordinated aluminum species whose second coordination spheres are also entirely dominated by phosphorus. The structure of glasses with an O/P ratio of precisely 4.0 (orthophosphate) is dominated by Al(OP)4 units. As the O/P ratio increases beyond 4.0, the average extent of Al-O-P connectivity is decreased significantly. Here, new types of five- and six-coordinated aluminum units, which are only weakly connected to phosphorus, are formed, while the network modifier is attracted mainly by the phosphate units.     

Atomistic origin of germanate anomaly in GeO2 and Na-germanate glasses: insights from two-dimensional 17O NMR and quantum chemical calculations

The prominent problem in archetypal germanate glasses is the germanate anomaly where the density exhibits maxima at 15-20 mol % of the alkali oxide content. Here we report (17)O two-dimensional NMR spectra for GeO(2) and Na-germanate glasses where the presence of both bridging oxygen linking ([4])Ge and highly coordinated Ge (([5,6])Ge-O-([4])Ge) and nonbridging oxygen, and an increase in topological disorder are demonstrated at the density maximum, manifesting atomic origins of the anomaly. These densification mechanisms in germanate glasses with Na content are remarkably similar to densification in v-B(2)O(3) with pressure.     

Structural role of fluoride in aluminophosphate sol-gel glasses: high-resolution double-resonance NMR studies

The local structure of Na-Al-P-O-F glasses, prepared by a novel sol-gel route, was extensively investigated by advanced solid-state NMR techniques. 27Al{19F} rotational echo double resonance (REDOR) results indicate that the F incorporated into aluminophosphate glass is preferentially bonded to octahedral Al units and results in a significant increase in the concentration of six-coordinated aluminum. The extent of Al-F and Al-O-P connectivities are quantified consistently by analyzing 27Al{31P} and 27Al{19F} REDOR NMR data. Two distinct types of fluorine species were identified and characterized by various 19F{27Al}, 19F{23Na}, and 19F{31P} double resonance experiments, which were able to support peak assignments to bridging (Al-F-Al, -140 ppm) and terminal (Al-F, -170 ppm) units. On the basis of the detailed quantitative dipole-dipole coupling information obtained, a comprehensive structural model for these glasses is presented, detailing the structural speciation as a function of composition.     

Oxidation state and coordination environment of uranium in sodium iron aluminophosphate glasses

An analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of uranium determined the oxidation state and coordination environment of uranium atoms in glasses containing 40 mol % Na₂O, 10 mol % Al₂O₃, 10 mol % Fe₂O₃, and 40 mol % P₂O₅ to which uranium oxides were added to a concentration of 50 wt % (above 100%). If the added amount of UO₂ was small, uranium occurred as U(IV) in a near-octahedral oxygen environment with an average U–O distance in the first coordination sphere of 2.25 Å. At higher concentrations of uranium oxides introduced both as UO₂ and as UO₃, uranium occurred as U(V) and U(VI); the first coordination sphere is split; shorter (~1.7–1.8 Å) and longer (2.2–2.3 Å) distances were observed, which corresponded to the axial and equatorial U–O bonds in uranyl ions, respectively; and the redox equilibrium shifted toward U(VI). The glass with the maximal (~33 wt %) UO₃ concentration contained mainly U(VI). The existence of low-valence uranium species can be related to the presence of Fe(II) in glasses. The second coordination sphere of uranium manifests itself only at high concentrations of uranium oxides.     

Lithium-7, sodium-23, and cesium-133 NMR and far infrared study of alkali complexes with C222-dilactam in various solvents

Nuclear magnetic resonances of alkali nuclei,⁷Li,²³Na, and¹³³Cs, as well as far infrared measurements are used to study alkali complexes of a bicyclic diazapolyoxa ligand—the dilactam of cryptand C222. Measurements were carried out in pyridine, tetrahydrofuran, acetonitrile, nitromethane, dimethylformamide, and aqueous solutions. The complexing ability of the dilactam is similar to, but weaker than, that of the cryptand C222. The limiting chemical shifts of the complexed cations were solvent-dependent, indicating incomplete enclosure of the cation by the ligand. Formation constants of Li⁺ and Cs⁺ complexes were calculated from the chemical-shift dependence on the ligand/metal ion mole ratio.     

Spectroscopic studies of ionic solvation. XIX. Fluorine-19 and sodium-23 NMR studies of ionic interactions in nonaqueous solutions of hexafluorophosphate salts

Fluorine-19 and sodium-23 NMR measurements were carried out on sodium hexafluorophosphate solutions in a number of solvents. In solvents of medium polarity and donicity (e.g., propylene carbonate, acetone, acetonitrile) the ¹⁹ F chemical shift moved upfield with increasing concentration of the salt. This behavior is indicative of anion-cation interactions which may be of long-range type, i.e., formation of solvent-separated ion pairs; the possibility of contact ion pair formation, however, cannot be excluded. In solvents of low polarity and donicity (acetic acid, tetrahydrofuran), the salt is essentially completely associated in the 0.1–1.0M concentration range. On the other hand, in solvating solvents with high dielectric constants, such as dimethyl-formamide, dimethylsulfoxide, and formamide, there is very little ionic association in the same concentration range. The above conclusions are supported by ²³ Na chemical shift measurements. Potassium hexafluorophosphate solutions do not show any concentration dependence of the ¹⁹ F chemical shifts, while for tetra-n-butylammonium solutions the ¹⁹ F resonance moves downfield with increasing concentration of the salt.To whom correspondence should be addressed.     

Two-quanta progressive saturation: pulsed NMR of 23Na in single crystal sodium nitrate

Progressive saturation effects due to induction of two-quanta transitions in the quadrupolar-split levels of a spit, 3/2, (²³Na) nucleus were studied by monitoring changes occuring in the 1/2 → ? 1/2 magnetization after applying trains of identical pulses at various radiofrequencies and repetition rates. The variety of single- and double-quantum (DO) interconnected population effects observed in the experiments is explained by incorporating solutions of the ordinary two-level Bloch equations with those of the two-quanta, three-level Bloch-like equations (including off-resonance conditions) derived recently by Gold and Hahn. Small displacements of nuclear levels under DQ irradiation (analogous to the optical ac Stark shift) the cause rf power-dependent DQ frequency shifts are predicted for all spins I> 1, and explicit expressions are derived. Additional effects due to state mixing and to population transfer by satellite excitation are presented, and the relevance of DQ saturation to the recently developed double-resonance interferometric spectroscopy is discussed.     

In situ monitoring of solid-state polymerization reactions in sodium chloroacetate and sodium bromoacetate by 23Na and 13C solid-state NMR spectroscopy

The thermally induced solidstate polymerization reactions in sodium chloroacetate and sodium bromoacetate, leading to poly(hydroxyacetic acid) (polyglycolide) and NaCl and NaBr, respectively, were studied by isothermal in situ solid-state NMR spectroscopy at 120, 130 and 140 degrees C with a time resolution of the order of 5 to 25 min. The nuclei probed were 23Na and 13C, allowing the parent compounds (sodium halogenoacetates) and both reaction products (polymer and alkali halide) to be monitored. For sodium chloroacetate, there is no evidence for the involvement of intermediate phases during the reaction whereas this cannot be excluded for sodium bromoacetate. The crystal structure of sodium bromoacetate was determined directly from powder diffraction data by the Monte Carlo method, and was found to be isostructural with sodium chloroacetate. The topochemical reaction mechanism proposed previously for sodium chloroacetate is thus also applicable for the polymerization reaction in sodium bromoacetate. The mechanistic and kinetic information obtained from our in situ solid-state NMR investigations is compared and contrasted with information obtained from other in situ probes of the polymerization reactions in these materials.     

Indirectly Detected DNP-Enhanced 17O NMR Spectroscopy: Observation of Non-Protonated Near-Surface Oxygen at Naturally Abundant Silica and Silica-Alumina

Recent studies have shown that dynamic nuclear polarization (DNP) can be used to detect ¹⁷O solid-state NMR spectra of naturally abundant samples within a reasonable experimental time. Observations using indirect DNP, which relies on ¹H mediation in transferring electron hyperpolarization to ¹⁷O, are currently limited mostly to hydroxyls. Direct DNP schemes can hyperpolarize non-protonated oxygen near the radicals; however, they generally offer much lower signal enhancements. In this study, we demonstrate the detection of signals from non-protonated ¹⁷O in materials containing silicon. The sensitivity boost that made the experiment possible originates from three sources: indirect DNP excitation of ²⁹Si via protons, indirect detection of ¹⁷O through ²⁹Si nuclei using two-dimensional ²⁹Si{¹⁷O} D-HMQC, and Carr-Purcell-Meiboom-Gill refocusing of ²⁹Si magnetization during acquisition. This ²⁹Si-detected scheme enabled, for the first time, 2D ¹⁷O−²⁹Si heteronuclear correlation spectroscopy in mesoporous silica and silica-alumina surfaces at natural abundance. In contrast to the silanols showing motion-averaged ¹⁷O signals, the framework oxygens exhibit unperturbed powder patterns as unambiguous fingerprints of surface sites. Along with hydroxyl oxygens, detection of these moieties will help in gaining more atomistic-scale insights into surface chemistry.     

New limits for solid-state 17O NMR spectroscopy: complete resolution of multiple oxygen sites in a simple biomolecule

A solid-state 17O NMR 1H-decoupled double angle rotation (DOR) study of monosodium l-glutamate monohydrate (l-MSG) is reported. It is shown that all eight inequivalent sites can be resolved with DOR line widths ( approximately 65 Hz) approximately 120 times narrower than those in the MAS spectrum. The lines are tentatively assigned on the basis of their behavior under proton decoupling and the isotropic chemical shift and the quadrupole interaction parameter for each extracted by a combination of DOR and 3Q MAS at variable magnetic fields. With a shift range of approximately 45 ppm for these similar oxygen sites and spectral resolution under DOR comparable to that for spin-1/2 nuclei, solid-state 17O NMR should have tremendous potential in the study of biomolecules.     

NMR spectroscopy in coordination supramolecular chemistry: A unique and powerful methodology

Metal-mediated self-assembly is emerging as a very important strategy for the synthesis of supramolecular species. Still, a major challenge in coordination supramolecular chemistry continues to be the characterization of the self-assembled complexes and the investigation of their dynamic behaviour in solution. In this context, NMR spectroscopy appears as a unique and powerful methodology. This practical-oriented review describes the rich variety of NMR techniques which are applied to the investigation of different aspects of the structure and behaviour of supramolecular complexes. “Classic” 1D NMR spectra reflect characteristic chemical shifts due to metal–ligand interactions or encapsulation phenomena, as well as symmetry and chiral properties of host–guest assemblies. Mainstream ¹H, ¹³C, ¹⁹F and ³¹P spectra are eventually complemented by the use of NMR-active metal nuclides. Homo- and heteronuclear 2D correlation experiments are ubiquitous in the literature, providing through-bond and through-space connectivities. Increasingly, diffusion measurements are also gaining popularity in this field, being used to gain information about molecular size, intermolecular interactions and even association constants of supramolecular complexes. Knowledge about the thermodynamic properties and the dynamic behaviour of coordination supramolecular assemblies is essential for the development of their practical applications. The most frequently used NMR methodologies for the calculation of association constants (simple signal integration, NMR titration and diffusion measurements) and for the investigation of dynamic supramolecular equilibria (lineshape analysis, selective inversion recovery experiments and 2D EXSY spectra) are described, together with the use of variable-temperature investigations for the determination of the thermodynamic and activation parameters of self-assembly and encapsulation processes.     

Entanglement and network formation in polystyrene: Viscoelastic behaviour from pulsed NMR

A series of NMR proton spin-spin relaxation measurements in three monodisperse polystyrene samples over a range of temperatures shows the conditions required for the existence of a dynamic network in polystyrene due to entanglements. The density of entangled units, N_e, i.e. their number per unit mass, can be determined by this NMR technique and is found to decrease with increasing temperature, in a manner which indicates its dependence on free volume, as in viscosity. The T₂₁ relaxation time for low molecular weight-polystyrene and T_2S for high molecular weight polystyrene increase with increasing temperature approximately as does (viscosity)^−0.5. The NMR data provide a quantitative value for the elastic modulus, and its dependence on temperature, both being in agreement with the experimental data previously obtained from viscoelasticity. The model can be extended to analyse creep and subsequent recovery. The transition in viscosity from a M¹ to a M^3,4 can also be deduced from the NMR results.     

Water binding sites in 2-para- and 2-ortho-fluorophenylethanol: a high-resolution UV experiment and ab initio calculations

The singly hydrated complexes of the flexible prototype molecules 2-para-fluorophenylethanol and 2-ortho-fluorophenylethanol have been investigated by combination of high-resolution resonance-enhanced two-photon ionization spectroscopy in a cold supersonic beam and quantum chemistry ab initio calculations. We have identified the conformational structures of the above complexes, which correspond to water binding to the most stable gauche monomer's conformers in both cases. No structural changes of the host molecules upon the attachment of a single water molecule have been found. For the 2-ortho-fluorophenylethanol-water complex we have observed an additional structure with one of the higher-in-energy gauche conformers of the monomer. This corroborates the assumption that the complexation with water stabilizes the higher-energy conformer of the monomer, precluding it from relaxation to the lowest-energy geometry.