Spinning Sideband Patterns Research Articles

Effects of La3+ and Y3+ doping on spatial homogeneity of Ho3+ ions in high silica glass

In this work, we have investigated the influence of La3+ and Y3+ doping on the spatial homogeneity of Ho3+ ions in high silica glasses via structural and spectroscopic analyses. It is found that both of La3+ and Y3+ doping could lead to depolymerization of the glass network. Under the same doping concentration, Y3+ gives rise to a larger silicate connectivity than La3+. The spinning sideband patterns of the 27Al NMR spectra indicate that the population of Ho3+ ions in the vicinity of Al3+ decreases with the increase in the concentration of La3+ or Y3+. In comparison to La3+, the doping of Y3+ more favors Ho3+ ions to be populated around Al3+ in the glass network. The fluorescence centered at ~2 μm from Ho3+ ions exhibits an increase tendency when increasing the concentration of La3+ or Y3+. The results indicate that the doping of La3+ or Y3+ ions into high silica glasses can improve the spatial distribution homogeneity of Ho3+ ions and La3+ plays a more positive role than Y3+.

Terminal and bridging fluorine ligands in TiF4 as studied by 19F NMR in solids

To examine bonding nature of fluorine ligands in a metal coordinated system, 19F high-resolution solid-state NMR has been applied to TiF4, which bears both bridging and terminal fluorines. Observed 12 isotropic signals are assigned to 12 crystallographically different fluorines (6 terminal and 6 bridging fluorines) in TiF4 by referring to the calculated isotropic shifts using density functional theory (DFT). The isotropic chemical shift (δiso) for terminal F (FT) appears at high frequency (420–480 ppm from δ(CCl3F) = 0 ppm) with large shielding anisotropy Δσ ∼ 850 ppm. Whereas the δiso and Δσ values for bridging F (FB) are moderate; δiso ∼ 0–25 ppm and Δσ ∼ 250 ppm. The origin of the observed high-frequency shift for FT is ascribed to the second-order paramagnetic shift with increased covalency, shorter Ti–F bonds, and smaller energy difference between the occupied and vacant orbitals. Examination of the orientation of the shielding tensor relative to the molecular structure shows that the most deshielded component of the shielding tensor is oriented along the Ti–F bond. The characteristic orientation is consistent with a Ti–F σ bond formed by dYZ of Ti and pz of F. Further, we show that the selectively observed spinning sideband patterns and the theoretical patterns with the calculated Δσ and η (shielding asymmetry) values are not consistent with each other for FB, indicating deficiency of the present DFT calculation in evaluating Δσ.

Cross-term Splittings Due to the Orientational Inequivalence of Proton Magnetic Shielding Tensors: Do Water Molecules Trapped in Crystals Hop or Tunnel?

Water molecules trapped in crystals of barium chlorate monohydrate have been investigated by magic-angle spinning (MAS) proton NMR spectroscopy in the temperature range 110-300 K. At high temperatures, a single spinning sideband pattern is observed. Below 150 K, however, two interleaved spinning sideband manifolds appear, with distinct centerbands that do not coincide with the average isotropic chemical shift seen at high temperatures. This hitherto unknown "cross-term splitting" results from the interplay of the homonuclear dipole-dipole coupling and two anisotropic proton shielding tensors that have identical principal components but nonequivalent orientations. The resulting cross terms cannot be averaged out by rotation about the magic angle. The analysis of the exchange-induced broadening, coalescence, and narrowing of the cross-term splitting in MAS spectra allows one to estimate the rate of exchange of the two protons between 140 and 190 K. The experimental data is compared with 2H and 1H NMR studies of the same sample reported in the literature. Density functional theory methods are utilized to estimate the thermal activation energy for a 2-fold hopping process of proton exchange about the H-O-H bisector. The Bell-Limbach model allows one to take into account contributions due to incoherent quantum tunneling in the low-temperature regime.

Solid-State 1H NMR Study of Structures and Dynamics of Proton Sites in Group II Salts of 12-Tungstophosphoric Acid and Related Compounds

The group II salts of 12-tungstophophoric acid were studied in the limiting state of hydration using a suite of 1H solid-state NMR experiments, theoretical analysis, and density functional theory (DFT) energy calculations. Various nonspinning (NS) 1H NS NMR, magic-angle spinning (MAS) 1H MAS NMR, and rotational-echo double-resonance (REDOR) 1H{31P} REDOR NMR experiments were performed. Chemical shift and spinning sideband patterns and comparison of 1H MAS two-pulse sequence NMR results with predictions of average Hamiltonian theoretical calculations definitively show that the limiting hydrated form of the group II salts of 12-tungstophosphoric acid contain only H+-protons and H2O-protons. Density matrix methods were used to derive expressions for the 1H NMR signal for a two-pulse NMR experiment as a function of the second pulse length under both NS and MAS conditions for H+-protons and H2O-protons. NMR structural parameters were obtained from the fit of theoretical expressions to the 1H NS two-pulse seque...

Probing Disorder in Al-ZSM-5 Zeolites by 14N NMR Spectroscopy

14N solid-state NMR spectroscopy is used to investigate and quantify the nanometer scale disorder promoted by Al/Si substitution in ZSM-5 zeolites. After a preliminary characterization by SEM, XRD, and multinuclear (1H, 13C, 19F, 27Al, 29Si) solid-state NMR, the 14N MAS NMR spectra of a series of as-synthesized ZSM-5 zeolites containing various amounts of Al are analyzed. The 14N spinning sideband patterns are shown to evolve with the Si/Al ratio. The modeling of the NMR spectra allows one to estimate the local disorder arising from the Al site distribution within the tetrahedral sites of the zeolites, the variations of F locations, and the presence of silanol defects. The influence of the zeolite framework modifications due to Al/Si substitution on 14N NMR parameters is discussed on the basis of the results obtained with the Density Functional Theory periodic quantum chemical calculations augmented with an empirical London dispersion term. Analysis of the results highlighted the influence of CNC angle va...

Mechanism of cluster dissolution of Yb-doped high-silica lanthanum aluminosilicate glass: Investigation by spectroscopic and structural characterization

To investigate clusters of Yb ions and the effectiveness of lanthanum co-doping in dispersing and isolating Yb ions in silica glass, Yb-doped high-silica lanthanum aluminosilicate (LAS) glass with compositions Yb2O3Al2O3La2O3SiO2 was prepared by the sol–gel method combined with high temperature sintering. The glass structure and spectroscopic properties of Yb-doped high-silica LAS glass were studied in detail to explore the mechanism of cluster dissolution. 27Al and 29Si solid-state nuclear magnetic resonance (SSNMR) spectra were introduced to study the spatial distribution of the Yb ions in the glass. The spinning sideband patterns of the 27Al and 29Si NMR spectra showed that part of the Yb3+ ions in the vicinity of aluminium atoms transferred to Si ions with increasing La2O3 concentration. The results of two-dimensional hyperfine sublevel correlation (2D-HYSCORE) electron paramagnetic resonance (EPR) spectroscopy led to the conclusion that the vicinity of Yb3+ ions are mostly aluminum atoms for glass doped with Al and aluminum and silicon atoms for glass co-doped with Al and La. It was also found that Yb clusters exist in the form of YbOYb bonds in high-concentration Yb-doped silica glass and that co-doping with La can replace these bonds with YbOLa linkages to reduce the number of Yb3+ pairs. The fluorescence intensity and fluorescence lifetime of Yb increase steadily with increasing La2O3 concentration, which also indicates that the addition of La in silica glass can dissolve the Yb clusters. The results show that co-doping high-silica LAS glasses with La3+ ions is an ideal way to increase Yb solubility and decrease clustering.

Influence of La/Al ratio on the structure and spectroscopy of Tm3+ doped Al2O3[sbnd]La2O3[sbnd]SiO2 glasses

Lanthanum aluminosilicate (LAS) glasses with compositions (Tm2O3)0.8-(Al2O3)12-x (La2O3)x-(SiO2)87.2 (x = 0, 1.2, 4.5 mol%; La/Al = 0, 1/9, 3/5) were prepared via a sol–gel method combined with high temperature sintering. The total amount of Al2O3 and La2O3 was 12 mol%. The effect of the La/Al ratio on the structure and spectroscopic properties of Tm3+ has been studied in detail. The structure of Tm3+ doped LAS glasses was studied via 27Al nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The spinning sideband patterns of the 27Al NMR spectra showed that the population of Tm3+ ions in the vicinity of Al3+ decreased with an increase in the La2O3 concentration. FTIR and Raman spectra suggest that glasses with high La/Al ratios have less interconnected glass networks. The spectroscopic properties of Tm3+ ions were investigated via the Judd–Ofelt method using the measurement of absorption and emission spectra. Both the 1.8 μm fluorescence intensity and the relative intensity of up-conversion emission of Tm3+ ions decrease with an increase in the La/Al ratio. This indicates reduced cross relaxation between Tm3+ ions in glass with a high La/Al ratio. We conclude that the spatial distribution homogeneity of Tm3+ ions in LAS glass improves when the La/Al ratio increases.

Detection of Fluorite-Structured MgD2/TiD2: Deuterium NMR

Deuterium magic-angle spinning NMR spectra of MgD2/TiD2 composites are presented. After high-energy, high-pressure ball-milling of an 80:20 Mg:Ti hydride, the material was H → D exchanged using high-pressure D2 gas. This material displayed the deuterium NMR spinning sideband pattern of the MgD2 component that is typical of rutile-structured MgD2. After heat treatment at 450 °C under high D2 pressure, the sidebands became much weaker, indicating that most D atoms were now in sites of local cubic symmetry. This is in agreement with a previous proposal of a nearly fully Mg–Ti segregated, lattice coherent fluorite (fcc) structure of the composite. Because H(D) diffusion is expected to be more rapid in the fluorite structure, compared to the rutile form, this may be important to the kinetics of hydrogen storage and release from MgH2/TiH2 composites.

1H Solid State NMR Study of Poly(methacrylic acid) Hydrogen-Bonded Complexes

The hydrogen bond structure of a series of poly(methacrylic acid) (PMAA) complexes was studied by solid-state NMR. 13C and 2H labeled PMAA samples were complexed with poly(ethylene oxide) (PEO), poly(vinyl methyl ether) (PVME), poly(acrylamide) (PAAM), poly(vinyl caprolactam) (PVCL) and poly(vinylpyrrolidone) (PVPon). The presence and relative strengths of PMAA’s hydrogen bonds with itself versus those with the complementary polymer was assessed by combining 13C CP-MAS NMR, 1H–13C HETCOR, 1D and 2D DQ 1H MAS NMR experiments. Analyses of 1H DQ spinning sideband patterns gave estimates of the proton–proton distances. Only the polyether–PMAA complexes, PEO and PVME, show resolved 13C and 1H resonances. This spectral resolution is proposed to be due to the selective disruption and stabilization of PMAA’s open and cyclic dimers, respectively. Residual PMAA dimers are detected by 1H NMR for the polylactam complexes, PVCL and PVPon, but both types dimers are weakened, reflecting the greater amount of interpolyme...

High-Resolution Solid-State2H NMR Spectroscopy of Polymorphs of Glycine

High-resolution solid-state (2)H MAS NMR studies of the α and γ polymorphs of fully deuterated glycine (glycine-d(5)) are reported. Analysis of spinning sideband patterns is used to determine the (2)H quadrupole interaction parameters, and is shown to yield good agreement with the corresponding parameters determined from single-crystal (2)H NMR measurements (the maximum deviation in quadrupole coupling constants determined from these two approaches is only 1%). From analysis of simulated (2)H MAS NMR sideband patterns as a function of reorientational jump frequency (κ) for the -N(+)D(3) group in glycine-d(5), the experimentally observed differences in the (2)H MAS NMR spectrum for the -N(+)D(3) deutrons in the α and γ polymorphs is attributed to differences in the rate of reorientation of the -N(+)D(3) group. These simulations show severe broadening of the (2)H MAS NMR signal in the intermediate motion regime, suggesting that deuterons undergoing reorientational motions at rates in the range κ ≈ 10(4)-10(6) s(-1) are likely to be undetectable in (2)H MAS NMR measurements for materials with natural isotopic abundances. The (1)H NMR chemical shifts for the α and γ polymorphs of glycine have been determined from the (2)H MAS NMR results, taking into account the known second-order shift. Further quantum mechanical calculations of (2)H quadrupole interaction parameters and (1)H chemical shifts reveal the structural dependence of these parameters in the two polymorphs and suggest that the existence of two short intermolecular C-H···O contacts for one of the H atoms of the >CH(2) group in the α polymorph have a significant influence on the (2)H quadrupole coupling and (1)H chemical shift for this site.

Li2FeSiO4 Polymorphs Probed by 6Li MAS NMR and 57Fe Mössbauer Spectroscopy

Several samples of pure Li2FeSiO4 polymorphs, prepared by hydrothermal synthesis and then quenched from either 400 °C (Pmn21), 700 °C (P21/n), or 900 °C (Pmnb), were thoroughly investigated by Mossbauer and NMR spectroscopy. 57Fe Mossbauer spectroscopy showed that (i) iron in all the polymorphs existed in the divalent oxidation state, and (ii) chemical shifts and quadrupolar splittings correlated well with Fe–O bond lengths. 6Li MAS NMR spectra of the three different polymorphs exhibit substantially different spinning-sideband patterns and slightly different isotropic shifts. The sideband patterns stemming from the anisotropic electron–nucleus dipole–dipole interaction reflect the arrangement of paramagnetic iron ions around lithium nuclei and allowed unambiguous assignments of 6Li MAS NMR signals to crystallographically nonequivalent Li sites. Analysis of isotropic NMR shifts of 6Li nuclei revealed that they comprised two contributions of comparable magnitude, a hyperfine (contact) shift and a pseudo-con...

TOP-PASS: A processing algorithm to reduce 2D PASS acquisition time

A slow speed MAS spectrum contains a pattern of spinning sideband resonances separated by integer multiples of the rotor frequency and centered about an isotropic frequency. The 2D signal acquired in a two-dimensional Phase Adjusted Spinning Sideband (PASS) experiment correlates this slow speed MAS spectrum, obtained in the direct dimension, to an indirect dimension spectrum containing the same pattern of spinning sideband resonances centered about a frequency of zero. An affine transformation is used to convert the acquired 2D PASS signal into a 2D signal that correlates a spectrum of pure isotropic frequencies to a spectrum of spinning sideband resonances with no isotropic frequency contributions. The conventional affine transform applied to 2D PASS consists of an active shear of the signal parallel to the indirect time domain coordinate followed by a passive scaling of the indirect time domain coordinate. Here we show that an alternative affine transform, previously employed in the Two-dimensional One Pulse (TOP) experiment, can be employed to create the same 2D signal correlation with an enhanced spectral width in the anisotropic (spinning sideband) dimension. This enhancement can provide a significant reduction in the minimum experiment time required for a 2D PASS experiment, particularly for spectra where the individual spinning sideband patterns are dispersed over a wider spectral range than the isotropic resonance frequencies. The TOP processing consists of an active shear of the signal parallel to the direct time domain, followed by an active shear of the signal parallel to the new indirect time domain coordinate followed by a passive scaling of the new direct time domain coordinate. A theoretical description of the affine transformation in the context of 2D PASS is given along with illustrative examples of 29Si in Clinoenstatite and 13C in l-Histidine.

Broadband inversion for MAS NMR with single-sideband-selective adiabatic pulses

We explain how and under which conditions it is possible to obtain an efficient inversion of an entire sideband family of several hundred kHz using low-power, sideband-selective adiabatic pulses, and we illustrate with some experimental results how this framework opens new avenues in solid-state NMR for manipulating spin systems with wide spinning-sideband (SSB) manifolds. This is achieved through the definition of the criteria of phase and amplitude modulation for designing an adiabatic inversion pulse for rotating solids. In turn, this is based on a framework for representing the Hamiltonian of the spin system in an NMR experiment under magic angle spinning (MAS). Following earlier ideas from Caravatti et al. [J. Magn. Reson. 55, 88 (1983)], the so-called "jolting frame" is used, which is the interaction frame of the anisotropic interaction giving rise to the SSB manifold. In the jolting frame, the shift modulation affecting the nuclear spin is removed, while the Hamiltonian corresponding to the RF field is frequency modulated and acquires a spinning-sideband pattern, specific for each crystallite orientation.

Nuclear magnetic resonance spectroscopic investigations of phase biaxiality in the nematic glass of a shape-persistent V-shaped mesogen

Deuterium and carbon-13 NMR spectroscopy were used to study both the high temperature uniaxial nematic and the low temperature biaxial nematic glass of a shape-persistent V-shaped mesogen. It was found that biaxial ordering determined in the domains of the latter has symmetry lower than D(2h) and is compatible with C(2h) symmetry or lower. In particular, elements of the ordering matrix including biaxial phase order parameters were determined from (2)H NMR at two temperatures, one just below the glass transition, and the other deep inside the biaxial glass, which allowed for the characterization of the dominant molecular motions at these temperatures. (13)C NMR magic angle spinning sideband patterns, collected both in the high temperature nematic phase and in the nematic glass, clearly show the difference between them in terms of the phase symmetry.

6Li MAS NMR spectroscopy and first-principles calculations as a combined tool for the investigation of Li2MnSiO4 polymorphs

Polymorphism of Li(2)MnSiO(4) was inspected by (6)Li MAS NMR spectroscopy. The detected isotropic shifts and spinning-sideband patterns were successfully reproduced by first-principles calculations and offered an insight into structural differences among the polymorphs. The approach for predicting isotropic shifts was also tested on several other Li-containing paramagnetic structures.

Shift Anisotropy Tensors in Amorphous Natural-Abundance Solids: High-Resolution 29Si Chemical Shift Anisotropy Distributions under Very Slow Sample Rotation

The magic-angle turning technique is applied to amorphous natural-abundance silicate materials and high-resolution silicon-29 correlations between the isotropic and anisotropic chemical shifts are obtained. Very narrow tilted spinning sideband patterns are resolved in the two-dimensional spectra whose line width corresponds to the natural line width of the silicon nuclei. Various numerical approaches are implemented to extract the distribution of the chemical shift anisotropy tensors in these materials and their results are compared.

Molecular Dynamics in Paramagnetic Materials as Studied by Magic-Angle Spinning 2H NMR Spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)<or=k<or=10(8) s(-1) can be obtained for the paramagnetic compounds from the analysis of 2H MAS NMR spectra when these paramagnetic effects are taken into account.

Double Quantum 1H MAS NMR Studies of Hydrogen-Bonded Protons and Water Dynamics in Materials

Two-dimensional double quantum (DQ) 1H MAS NMR was used to investigate different proton environments in a series of alkali (Na, K, Rb, Cs) [Nb6O19]8- Lindqvist salts, with the water and hydrogen-bound intercluster protons being clearly resolved. Through the analysis of the DQ 1H NMR spinning sideband pattern, it is possible to extract both the mean and distribution of the motionally averaged intramolecular homonuclear 1H-1H dipolar coupling for the different water environments and the intercluster protons. Motional order parameters for the water environments were then calculated from the averaged dipolar couplings. The influence of additional intermolecular dipolar couplings due to multispin interactions were simulated and discussed.

Solid-State NMR and Computational Chemistry Study of Mononucleotides Adsorbed to Alumina

Solid-state NMR spectroscopy and ab initio computational chemistry are used to determine the structure of the complex formed upon adsorption of the mononucleotide 2'-deoxyadenosine 5'-monophosphate (dAMP) to the surface of a mesoporous alumina. In this multi-technique approach, rotational-echo double-resonance NMR results reveal that the phosphate group of dAMP interacts predominantly with octahedrally coordinated aluminum species at the surface, and therefore, adsorption is modeled with both mono- and bidentate sorption of the nucleotide phosphate group with octahedral aluminum. 31P chemical shielding tensors are calculated from the structure of the lowest energy conformations, and these results are compared to tensor values extracted from analysis of spinning-sideband patterns in the experimental 31P cross-polarization magic-angle-spinning NMR spectrum. The chemical shift anisotropy and asymmetry parameter indicate that the binding is via a monodentate, inner-sphere complex.

Bayesian and information theory analysis of MAS sideband patterns in spin 1/2 systems

Bayesian statistics and information theory are used to analyze the reliability of extracting chemical shift parameters from spinning sideband patterns of spin 1/2 systems. Efficient code has been written to calculate the two-dimensional posterior probability as a function of the chemical shift anisotropy, δ, and the asymmetry parameter, η, given the sideband intensities and the signal-to-noise ratio. This method has the advantage of assuming only that the noise in the sideband intensities is distributed as a Gaussian. It assumes nothing about the distribution of the values of parameters δ and η, which are shown in some cases to be highly non-Gaussian. The utility of Bayesian analysis is demonstrated on 1D slow-spinning MAS spectra and on sideband patterns extracted from 2D PASS spectra. Previous investigations have shown that there is an optimal range of spinning frequencies for determining δ. In this study, information theory is used to determine the signal-to-noise ratio dependence of the entropy in δ, η, and total entropy in spinning sideband spectra. The entropy is a measure of the information content of a probability distribution. When the entropy is zero, there is perfect information on a system, while if it is infinite, there is no information on the system. It is found that for all values of η and for signal-to-noise ratios in the range 50–1000, an entropy minimum in ν δ / ν r occurs for values 1.5 ⩽ ν δ / ν r ⩽ 3. In the same range of signal-to-noise ratios, the entropy in η is a monotonically decreasing function of ν δ / ν r . The global information content of a spinning sideband pattern (i.e., the total entropy) is dependent on the signal-to-noise ratio and has an optimal value at ν δ / ν r ≈ 2 at a signal-to-noise ratio of 50 and increasing to ≈2.5 for signal-to-noise ratios of 1000. Finally, the increase of information in a sideband pattern as a function of the number of sidebands used in the analysis is examined. Most of the information about δ and η is contained in the five central sidebands; i.e., sidebands −2 to 2.