Chemical Shift Trends Research Articles

125Te NMR chemical-shift trends in PbTe–GeTe and PbTe–SnTe alloys

Complex tellurides, such as doped PbTe, GeTe, and their alloys, are among the best thermoelectric materials. Knowledge of the change in 125Te NMR chemical shift due to bonding to dopant or “solute” atoms is useful for determination of phase composition, peak assignment, and analysis of local bonding. We have measured the 125Te NMR chemical shifts in PbTe-based alloys, Pb1−xGexTe and Pb1−xSnxTe, which have a rocksalt-like structure, and analyzed their trends. For low x, several peaks are resolved in the 22-kHz MAS 125Te NMR spectra. A simple linear trend in chemical shifts with the number of Pb neighbors is observed. No evidence of a proposed ferroelectric displacement of Ge atoms in a cubic PbTe matrix is detected at low Ge concentrations. The observed chemical shift trends are compared with the results of DFT calculations, which confirm the linear dependence on the composition of the first-neighbor shell. The data enable determination of the composition of various phases in multiphase telluride materials. They also provide estimates of the 125Te chemical shifts of GeTe and SnTe (+970 and +400±150ppm, respectively, from PbTe), which are otherwise difficult to access due to Knight shifts of many hundreds of ppm in neat GeTe and SnTe.

Cage-Like Local Structure of Ionic Liquids Revealed by a (129)Xe Chemical Shift.

The chemical shift of xenon (at natural abundance) dissolved in a variety of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) has been measured with (129)Xe NMR spectroscopy. The large chemical shift differences observed are mainly related to the type of anion; the strongest deshielding effect is observed ILs with I(-), Br(-), and Cl(-) anions, and the strongest shielding is found for the bis(trifluoromethanesulfonyl)imide ([Tf2N](-))-based IL. The measured (129)Xe chemical shift variations correlate well with the IL structure organization imposed by the anions and with the size of the empty voids due to charge alternation patterns. Descriptors taken from literature data on X-ray and neutron scattering, as well as single-crystal structures where available, support this interpretation. The proposed methodology adds a new investigating tool to the elucidation of the short-range order in ILs. The observed chemical shift trend provides information about how these solvents are organized.

A Detailed NMR- and DFT-Based Study of the Sakurai–Hosomi–Yamamoto Asymmetric Allylation Reaction

A Lewis acid complex between benzaldehyde and the silver catalyst was detected by (31)P NMR and shown to be the direct precursor to allylation within the Sakurai-Hosomi-Yamamoto reaction. Structural and thermochemical hybrid-DFT calculations indicated that benzaldehyde predominantly formed an η(1)-σ-complex with the catalyst; however, two other competing conformers involving different coordination modes were found, including an activated μ(2)-bound complex. The differences in (31)P NMR shifts upon complexation were calculated by the gauge-independent atomic orbital (GIAO-DFT) method for each conformer. The minimum energy conformer was found to correlate well with chemical shift trends observed experimentally, and an analysis of Mullikan charge populations revealed that the carbonyl carbon of the highest-energy conformer was the most electron-deficient. Furthermore, one minor and three major silicon intermediates were detected by (29)Si NMR and, with the aid of (1)H-(29)Si HSQC, were assigned by comparison with parent compounds and GIAO-DFT calculations. Finally, a tentative mechanism was proposed based on these findings.

Toward Reproducing Sequence Trends in Phosphorus Chemical Shifts for Nucleic Acids by MD/DFT Calculations.

This work addresses the question of the ability of the molecular dynamics-density functional theory (MD/DFT) approach to reproduce sequence trend in (31)P chemical shifts (δP) in the backbone of nucleic acids. δP for [d(CGCGAATTCGCG)]2, a canonical B-DNA, have been computed using density functional theory calculations on model compounds with geometries cut out of snapshots of classical molecular dynamics (MD) simulations. The values of (31)P chemical shifts for two distinct B-DNA subfamilies BI and BII, δP/BI and δP/BII, have been determined as averages over the BI and BII subparts of the MD trajectory. This has been done for various samplings of MD trajectory and for two sizes of both the model and the solvent embedding. For all of the combinations of trajectory sampling, model size, and embedding size, sequence dependence of δP/BI in the order of 0.4-0.5 ppm has been obtained. Weighted averages for individual (31)P nuclei in the studied DNA double-helix have been calculated from δP/BI and δP/BII using BI and BII percentages from free MD simulations as well as from approaches employing NMR structural restraints. A good qualitative agreement is found between experimental sequence trends in δP and theoretical δP employing short (24 ns) MD run and BI, BII percentages determined by Hartmann et al. or via MD with the inclusion of NMR structural restraints. Theoretical δP exhibit a systematic offset of ca. 11 ppm and overestimation of trends by a factor of ca. 1.7. When scaled accordingly, theoretical δP/BI and δP/BII can be used to determine the expected percentage of BII to match the experimental value of δP. As evidenced by the calculations on snapshots from Car-Parrinello molecular dynamics, the systematic offsets of the theoretical δP obtained by MD/DFT approach result primarily from the unrealistic bond lengths employed by classical MD. The findings made in this work provide structure-δP relationships for possible use as NMR restraints and suggest that NMR calculations on MD snapshots can be in the future employed for the validation of newly developed force fields.

Understanding the NMR properties and conformational behavior of indole vs. azaindole group in protoberberines: NICS and NCS analysis

We report here the preparation and the structural investigation into a series of 8-(indol-1-yl)-7,8-dihydroprotoberberine derivatives derived from berberine, palmatine, and coptisine. Structures of these new compounds were characterized mainly by 2D NMR spectroscopy and the conformational behavior was investigated by using methods of density-functional theory (DFT). PBE0/6-311+G** calculated NMR chemical shifts for selected derivatives correlate excellently with the experimental NMR data and support the structural conclusions drawn from the NMR experiments. An interesting role of the nitrogen atom in position N7′ of the indole moiety in 8-(7-azaindol-1-yl)-7,8-dihydroprotoberberines as compared to other 8-indolyl derivatives is investigated in detail. The experimentally observed trends in NMR chemical shifts are rationalized by DFT calculations and analysis based on the nucleus-independent chemical shifts (NICS) and natural localized molecular orbitals (NLMOs).

C–C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the σ/π hyperconjugation in EtFC and the π-conjugation in VFC and EFC.

15N and 13C{14N} NMR investigation of the major nitrogen‐containing segment in an aquatic fulvic acid: Evidence for a hydantoin derivative

A nitrogen-rich segment in a fulvic acid (FA) from Pony Lake, a coastal pond in Antarctica, was investigated by (15)N and (13)C{(14)N} solid-state NMR techniques. As reported previously, the (13)C{(14)N} spectrum of C bonded to N exhibits a peak at 157 ppm that is assigned to an sp(2)-hybridized carbon bonded to at least two nitrogen atoms. This segment contains 48% of all N in the sample. (15)N NMR shows distinct signals, 20 ppm upfield and downfield from the typical peptide resonance; dipolar dephasing confirmed that they are due to protonated N. The well-resolved downfield peak, which accounts for 1/4 of the spectral area, cannot be assigned to aromatic heterocycles, such as purines, because the fraction of aromatic C bonded to N in this sample is very small. Analysis of (15)N chemical-shift trends and (15)N NMR of model compounds, such as arginine and its derivatives, excludes assignment to a guanidinium ion or to substituted guanidino groups. Similarly, ureido groups, -NH-CO-NH-, that are not bonded to a second C = O do not match the observed (15)N peaks in the FA, since both N resonate upfield from the peptide resonance. On the other hand, all chemical shifts are matched within the observed range by the -C(alkyl)-NH-CO-NH-CO-C structure found in two nonaromatic heterocycles, hydantoin and dihydrouracil. The five-membered hydantoin ring, which is found in the purine metabolite allantoin, provides a better match than the six-membered dihydrouracil ring. Regular uracil or thymine fails to produce adequate agreement with observed chemical shifts.

Shape-persistent macrocycles — Self-assembly reactions and characterization by hyperpolarized 129Xe NMR spectroscopy**In memory of Professor Michael M. Pollard.

The use of three shape-persistent, conjugated macrocycles 2a–2c as ligands for self-assembly reactions is described. The macrocycles have been characterized through a combination of spectroscopic analyses and, for compounds 2b and 2c, X-ray crystallographic analysis. Whereas the reaction of 2a with the cis-Pt(II) species 3 successfully provides the porous solid 4a, the analogous reaction of 2b and 2c with 3 leads only to mixtures of products. The application of continuous-flow hyperpolarized 129Xe NMR spectroscopy to investigate the solid-state pores of macrocycles 2b and 2c and the supramolecular complex 4a as a function of temperature is described. All three species show permanent porosity upon removal of co-crystallized solvent molecules. Using trends in the 129Xe chemical shifts and temperature-dependent dynamics of Xe atoms in the solids, information is obtained on the nature of the pores in these systems. Using the 129Xe NMR data for complex 4a, the effective heat of adsorption (ΔHads) was calculated to be ~29 kJ mol–1.

Mixed-Alkali Effects in Aluminophosphate Glasses: A Re-examination of the System [xNa2O(1 – x)Li2O]0.46[yAl2O3(1 – y)P2O5]0.54

Mixed-alkali effects in the system [xNa2O(1 – x)Li2O]0.46[yAl2O3(1 – y)P2O5]0.54 (0 ≤ x ≤ 1; y = 0, 0.08, 0.16) have been examined using dc-conductivity measurements, thermal analysis, Raman spectroscopy, and various complementary solid-state NMR approaches. In contrast to the results of an earlier study reporting results on mixed-alkali aluminophosphate glasses with very similar chemical compositions (Faivre, A.; Viviani, D.; Phalippou, J. Solid State Ionics2005, 176, 325), ionic conductivities and Tg values exhibit composition dependences typical of “normal” mixed-alkali effect behavior. The 31P MAS NMR spectra have been quantitatively analyzed in terms of Q(n)m units, where “n” denotes the number of P–O–P linkages and “m” is the number of Al atoms connected to P. The data suggest that the number of P–O–Al linkages is maximized, and that no Al–O–Al linkages occur. 23Na triple-quantum MAS NMR studies indicate a monotonic trend in isotropic chemical shifts, consistent with intimate Na/Li mixing. In agreem...

Structural trends in TeI2RR′: Crystal structures and NMR spectra of TeI2(CH2SiMe3)2, TeI2Th(CH2SiMe3), TeI2Ph(CH2SiMe3), and TeI2Th2 (Th = 2‐Thienyl, C4H3S)

AbstractTeI2(CH2SiMe3)2(1), TeI2Th(CH2SiMe3 (2), TeI2Ph(CH2SiMe3)(3), and TeI2Th2(4) (Th = 2‐thienyl) were synthesized in excellent yields from the corresponding tellanes and I2. The products were characterized by 125Te NMR spectroscopy and single crystal X‐ray crystallography. Each TeI2RR′ molecule shows a trigonal bipyramidal coordination around tellurium with the iodine atoms occupying axial positions, and the organic groups and the lone pair occupying equatorial positions. The TeI bonds and the ITeI angles in 1–4 span a range of 2.8407(9)–3.0194(10) Å and 171.85(2)–175.71(2)°, respectively. The TeC bonds and CTeC angles show respective ranges of 2.100(6)–2.136(7) Å and 95.1(3)–100.9(3)°. In the solid state, the molecules show a varying degree of secondary bonding interactions. With the exception of TeI2(CH2SiMe3)2 (1), the compounds show Te···I or I···I secondary bonding interactions leading to supramolecular assemblies. TeI2Ph(CH2SiMe3) crystallizes as two polymorphs. One (3a) is isomorphic with TeI2Th(CH2SiMe3) (2). The other (3b) shows a lattice of two alternating layers of molecules. In one layer, the molecules are linked into a two‐dimensional network by Te···I and I···I interactions whereas the second layer consists of discrete dimers. The strongest Te···I secondary bonding interactions are observed for TeI2Th2. The lattice is composed of tetramers that is reminiscent of those in γ‐ and δ‐TeI4 and also have precedent in TeI2RR′ species. The structures of TeI2Me2 and TeI2Ph2 have been considered for comparison. The trends in 125Te NMR chemical shifts in TeI2RR′ (R, R′ = Me, CH2SiMe3, Th, Ph) are also discussed. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:348–357, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20688

Formation of PbS materials from lead xanthate precursors

Six lead xanthate adducts Pb(S(2)COR)(2).L [R = Et, (n)Bu, L = bipy, TMEDA (tetramethylethylenediamine), PMDETA (pentamethyldiethylenetriamine)] have been synthesised and the structures of all, save Pb(S(2)COBu(n))(2).TMEDA (4) which is an oil, determined. Pb(S(2)COEt)(2).TMEDA (3) is seven-coordinate at lead through three chelating ligands and one weak intermolecular Pb‥S interaction. Both Pb(S(2)COR)(2).bipy [R = Et (1), (n)Bu (2)] are dimers in which one xanthate is terminal and the other μ(2) bridging at each sulphur, generating an eight-coordinate lead when the bipy donor is included. Both Pb(S(2)COR)(2).PMDETA [R = Et (5), (n)Bu (6)] are seven-coordinate at lead by virtue of two bidentate chelating xanthate ligands and a tridentate PMDETA; there are no intermolecular interactions. Trends in the (207)Pb NMR chemical shifts mirror the changes in the intramolecular coordination number across the series. Pb(S(2)COEt)(2).TMEDA (3) has been used to deposit PbS films on glass, Mo-coated glass and Si by AACVD. Pb(S(2)COEt)(2) also generated PbS nanocubes when decomposed under an autogenerated pressure.

Structural models for yttrium aluminium borate laser glasses: NMR and EPR studies of the system (Y2O3)0.2–(Al2O3)x–(B2O3)0.8−x

The structure of laser glasses in the system (Y(2)O(3))(0.2){(Al(2)O(3))(x))(B(2)O(3))(0.8-x)} (0.15 ≤ x ≤ 0.40) has been investigated by means of (11)B, (27)Al, and (89)Y solid state NMR as well as electron spin echo envelope modulation (ESEEM) of Yb-doped samples. The latter technique has been applied for the first time to an aluminoborate glass system. (11)B magic-angle spinning (MAS)-NMR spectra reveal that, while the majority of the boron atoms are three-coordinated over the entire composition region, the fraction of three-coordinated boron atoms increases significantly with increasing x. Charge balance considerations as well as (11)B NMR lineshape analyses suggest that the dominant borate species are predominantly singly charged metaborate (BO(2/2)O(-)), doubly charged pyroborate (BO(1/2)(O(-))(2)), and (at x = 0.40) triply charged orthoborate groups. As x increases along this series, the average anionic charge per trigonal borate group increases from 1.38 to 2.91. (27)Al MAS-NMR spectra show that the alumina species are present in the coordination states four, five and six, and the fraction of four-coordinated Al increases markedly with increasing x. All of the Al coordination states are in intimate contact with both the three- and the four-coordinate boron species and vice versa, as indicated by (11)B/(27)Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, non-segregated glass structure. (89)Y solid state NMR spectra show a significant chemical shift trend, reflecting that the second coordination sphere becomes increasingly "aluminate-like" with increasing x. This conclusion is supported by electron spin echo envelope modulation (ESEEM) data of Yb-doped glasses, which indicate that both borate and aluminate species participate in the medium range structure of the rare-earth ions, consistent with a random spatial distribution of the glass components.

Aggregation of Perfluoroctanoate Salts Studied by 19F NMR and DFT Calculations: Counterion Complexation, Poly(ethylene glycol) Addition, and Conformational Effects

The aggregation of perfluoroctanoate salts in H(2)O is studied by (19)F NMR on solutions of LiPFO, NaPFO, and CsPFO, without and with the addition of two poly(ethylene glycol) (PEG) oligomers of molecular weight 1500 and 3400 Da, respectively, and with the addition of suitable crown ethers. The (19)F chemical shift (cs) trends are monitored, at 25 °C, in a concentration range including the critical micellar concentration (cmc) or, in the presence of PEG, the critical aggregation concentration (cac). The cac values in the samples with PEG are lower than the cmc values of the corresponding samples without PEG; moreover, the (19)F cs trends above the cac and above the polymer saturation concentration reveal and help to explain some peculiarities of the aggregation process of PEG on PFO micelles, which, in the first step, seems to occur while the surfactant concentration in water is still increasing. Also in LiPFO/H(2)O or NaPFO/H(2)O solutions containing 12-crown-4 or 15-crown-5 ethers, suitable to complex Li(+) or Na(+) ions, respectively, the cmc decreases. On the other hand, the micellization process in the presence of crown ethers does not show other peculiarities. The prevailing conformations of the PFO chain are discussed on the basis of quantum-mechanical calculations. The theoretical chemical shifts were computed at the DFT level of theory, taking into account the effects of the environment by means of the IEF-PCM method. The helical structure is the most stable one, but anti conformations are easily accessible, in both the aqueous and fluorinated environment. The comparison between computed and experimental chemical shifts indicates that anti conformations are more important in the micelles than in water and in CsPFO micelles than in LiPFO or NaPFO ones.

Accurate Measurement of Methyl 13C Chemical Shifts by Solid-State NMR for the Determination of Protein Side Chain Conformation: The Influenza A M2 Transmembrane Peptide as an Example

The use of side chain methyl (13)C chemical shifts for the determination of the rotameric conformation of Val and Leu residues in proteins by solid-state NMR spectroscopy is described. Examination of the solution NMR stereospecifically assigned methyl groups shows significant correlation between the difference in the two methyl carbons' chemical shifts and the side chain conformation. It is found that alpha-helical and beta-sheet backbones cause different side chain methyl chemical shift trends. In alpha-helical Leu's, a relatively large absolute methyl (13)C shift difference of 2.89 ppm is found for the most populated mt rotamer (chi(1) = -60 degrees, chi(2) = 180 degrees), while a much smaller value of 0.73 ppm is found for the next populated tp rotamer (chi(1) = 180 degrees, chi(2) = 60 degrees). For alpha-helical Val residues, the dominant t rotamer (chi(1) = 180 degrees) has more downfield Cgamma2 chemical shifts than Cgamma1 by 1.71 ppm, while the next populated m rotamer (chi(1) = -60 degrees) shows the opposite trend of more downfield Cgamma1 chemical shift by 1.23 ppm. These significantly different methyl (13)C chemical shifts exist despite the likelihood of partial rotameric averaging at ambient temperature. We show that these conformation-dependent methyl (13)C chemical shifts can be utilized for side chain structure determination once the methyl (13)C resonances are accurately measured by double-quantum (DQ) filtered 2D correlation experiments, most notably the dipolar DQ to single-quantum (SQ) correlation technique. The advantage of the DQ-SQ correlation experiment over simple 2D SQ-SQ correlation experiments is demonstrated on the transmembrane peptide of the influenza A M2 proton channel. The methyl chemical shifts led to predictions of the side chain rotameric states for several Val and Leu residues in this tetrameric helical bundle. The predicted Val rotamers were further verified by dipolar correlation experiments that directly measure the chi(1) torsion angles. It was found that the chemical-shift-predicted side chain conformations are fully consistent with the direct torsion angle results; moreover, the methyl (13)C chemical shifts are sensitive to approximately 5 degrees changes in the chi(1) torsion angle due to drug binding.

Sol-gel synthesis of sodium-modified AlPO4–SiO2 glasses and structural characterization by solid state NMR

Sodium aluminophosphosilicate gels and glasses along the composition line (Na2O)x–[(AlPO4)0.5(SiO2)0.5]1−x were synthesized via the sol-gel process using sodium acetate, aluminium lactate, phosphoric acid and tetraethyl orthosilicate (TEOS) as precursors. The structural evolution from solution to gel and to the final glass was monitored by 27Al, 31P, 29Si and 23Na magic-angle spinning (MAS) NMR. In the solution state aluminium forms a mixture of octahedral coordinated complexes coordinated to lactate, water and phosphate units with varying degrees of polymerization. With increasing temperature up to about 150 °C the lactate ligands are successively replaced by phosphate, resulting in an increased extent of Al–O–P linking. Annealing the xerogel at 400 °C results in a glassy network containing mostly four-coordinated Al species (Al(IV)), in addition to small concentrations of Al(V) and Al(VI). While the distribution of Al coordination numbers remains constant, the compositional trend of the 27Al chemical shift indicates a gradual replacement of Al–O–P linkages by Al–O–Si linkages with increasing sodium content. This conclusion is confirmed by 27Al{31P} Rotational Echo Double Resonance (REDOR) experiments, which reveal a reduction in the number of Al–O–P linkages. Concomitantly, the interaction of Na with the phosphate species is successively increased as indicated by 31P chemical shift trends and 31P{23Na} REDOR results. The results illustrate that the network segregation found in pure AlPO4–SiO2 sol-gel glasses can be overcome, at least partially, by introduction of the network modifier Na2O.

Crystalline Aluminum Hydroxy Fluorides: Structural Insights Obtained by High Field Solid State NMR and Trend Analyses

A series of crystalline aluminum hydroxy fluorides in cubic pyrochlore structure AlFx(OH)3−x·H2O with variable F-content x were investigated by solid-state NMR by applying different magnetic fields up to 21.1 T. Distinguishable octahedral species AlFx(OH)6−x (x = 1−6) were identified in the crystalline aluminum hydroxy fluorides. The subsequent analysis of the highfield 27Al MAS NMR data allows the derivation of the trend analysis graphs giving correlations between the 27Al chemical shifts and the quadrupolar frequencies and the F-content x in AlFx(OH)6−x. Clear trends were obtained for both, which are, along with the 19F MAS chemical-shift trend analysis presented earlier, valuable tools for the interpretation of MAS NMR spectra of amorphous AlFx(OX)3−x compounds (X = H, alkyl). Following the dehydration of the pyrochlores by solid-state NMR eventually reveals a remarkable influence of the incorporated solvent molecules (H2O) on the 19F chemical shift. On that basis, a new chemical-shift trend analysis for 19F chemical shifts in correlation with x in AlFx(OH)6−x units for proton-poor substances (in the Al, F, O, H system) was determined. By using this correlation, high-surface AlF3 has a mean bulk Al:F ratio similar to that found for ACF, namely, AlF2.8(O/OH)0.2.

Calcium-43 Chemical Shift Tensors as Probes of Calcium Binding Environments. Insight into the Structure of the Vaterite CaCO3 Polymorph by 43Ca Solid-State NMR Spectroscopy

Natural-abundance (43)Ca solid-state NMR spectroscopy at 21.1 T and gauge-including projector-augmented-wave (GIPAW) DFT calculations are developed as tools to provide insight into calcium binding environments, with special emphasis on the calcium chemical shift (CS) tensor. The first complete analysis of a (43)Ca solid-state NMR spectrum, including the relative orientation of the CS and electric field gradient (EFG) tensors, is reported for calcite. GIPAW calculations of the (43)Ca CS and EFG tensors for a series of small molecules are shown to reproduce experimental trends; for example, the trend in available solid-state chemical shifts is reproduced with a correlation coefficient of 0.983. The results strongly suggest the utility of the calcium CS tensor as a novel probe of calcium binding environments in a range of calcium-containing materials. For example, for three polymorphs of CaCO3 the CS tensor span ranges from 8 to 70 ppm and the symmetry around calcium is manifested differently in the CS tensor as compared with the EFG tensor. The advantages of characterizing the CS tensor are particularly evident in very high magnetic fields where the effect of calcium CS anisotropy is augmented in hertz while the effect of second-order quadrupolar broadening is often obscured for (43)Ca because of its small quadrupole moment. Finally, as an application of the combined experimental-theoretical approach, the solid-state structure of the vaterite polymorph of calcium carbonate is probed and we conclude that the hexagonal P6(3)/mmc space group provides a better representation of the structure than does the orthorhombic Pbnm space group, thereby demonstrating the utility of (43)Ca solid-state NMR as a complementary tool to X-ray crystallographic methods.

Crystalline aluminium hydroxy fluorides—Suitable reference compounds for 19F chemical shift trend analysis of related amorphous solids

On the basis of MAS NMR-data for crystalline AlF x (OH) 3− x ·H 2O samples in the pyrochlore structure, 19F chemical shifts correlate with the average chemical composition of the octahedral environment, given by AlF x O 6− x in these compounds. The attribution of local structures in sol–gel derived amorphous AlF x (OX) 3− x ·XOH (X = H, R (alkyl)) compounds is of special interest as these or consecutively prepared solids exhibit remarkable features, for example, a high surface (HS) area accompanied by a high Lewis acidity. By transferring this scale of a 19F chemical shift trend analysis to such compounds a prediction of the chemical nature of the average Al coordination becomes possible. A new synthetic approach to crystalline aluminium hydroxy fluorides involving a sol gel fluorination as the first reaction step and an aluminium alkoxide as precursor compound is presented. Varying the amount of HF leads to different F–OH-ratios in the AlF x (OH) 3− x compounds.

195Pt NMR Chemical Shift Trend Analysis as a Method to Assign New Pt(IV)—Halohydroxo Complexes.

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Nitrogen-rich La–Si–Al–O–N oxynitride glass structures probed by solid state NMR

We investigate the local structures of oxynitride La–Si–(Al)–O–N glasses by 29Si and 27Al magic-angle spinning (MAS) solid state Nuclear Magnetic Resonance (NMR). The glasses studied span an unprecedented range of compositions, up to >50 at.% lanthanum and nitrogen out of the cations and anions, respectively, and achievable through a recently introduced glass preparation route. Transmission as well as scanning electron microscopy verified homogeneous samples over length-scales down to 20 nm. As the nitrogen content of the glasses increased, 29Si NMR evidenced a progressive formation of Si–N bonds, with SiO 2N 2 tetrahedra dominating in the nitrogen-rich glass networks. In the oxygen-rich end of the series, aluminum is predominantly present in tetrahedral coordination as AlO 4, whereas the glasses with highest nitrogen contents have a major fraction of AlO 3N structural units. Trends in isotropic 29Si and 27Al chemical shifts and 27Al quadrupolar couplings are compared with results of La–Si–Al–O glasses and are discussed in relation to the glass compositions and their proposed structures.