Probing crystal structures and transformation reactions of ammonium molybdates by 14N MAS NMR spectroscopy

The unique high-resolution feature offered by 14N magic-angle spinning (MAS) NMR spectroscopy of ammonium ions has been used to characterize the crystal structures of various ammonium molybdates by their 14N quadrupole coupling parameters, i.e., CQ, the quadrupole coupling constant, and etaQ, the asymmetry parameter. Two polymorphs of diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction (XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two sets of characteristic 14N CQ and etaQ values have been obtained. Similarly, the well-characterized ammonium polymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24.4H2O, and (NH4)6Mo8O27.4H2O also give rise to distinct and characteristic 14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24.4H2O spectrum requires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (nu(r) = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity. These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessment of the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experiments performed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 and mP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.     

One-dimensional polymers based on [{CpMo(CO)2}2(mu,eta2-P2)]: solid-state conformation analysis by NMR spectroscopy and DFT calculations

Reaction of the complex [{CpMo(CO)2}2(mu,eta2-P2)] (1) with CuI halides leads to the quantitative formation of the novel one-dimensional linear polymers [CuX{Cp2Mo2(CO)4(mu,eta2:eta1:eta1-P2)}](infinity) (X=Cl (4), Br (5), I (6)). The same products 4 and 5 were obtained when 1 was treated with CuCl2 and CuBr2, respectively. The solid-state structures are compared and their remarkable influence on the respective (31)P magic angle spinning (MAS) NMR spectra is interpreted with the help of density functional theory (DFT) calculations on the model compounds [{(CuX)2{Cp2Cr2(CO)4(mu,eta(2):eta1:eta1-P2)}2}3] (X=Cl (4 a), Br (5 a)) in which the molybdenum atoms are replaced by their lighter homologue chromium.     

Refinement of borate structures from 11B MAS NMR spectroscopy and density functional theory calculations of 11B electric field gradients

The refinement of borate structures using DFT calculations combined with experimental (11)B quadrupole coupling parameters from solid-state NMR spectroscopy is presented. The (11)B electric field gradient (EFG) tensors, calculated using the WIEN2k software for trigonal and tetrahedral boron sites in a series of model compounds, exhibit a convincing linear correlation with the quadrupole coupling tensor elements, determined from (11)B MAS NMR spectra of the central or satellite transitions. The model compounds include Li(2)B(4)O(7), Mg(2)B(2)O(5), Mg(3)B(2)O(6), NH(4)B(C(6)H(5))(4), and colemanite (CaB(3)O(4)(OH)(3).H(2)O). The (11)B quadrupole moment, Q = 0.0409 +/- 0.0002 barn, derived from the linear correlation, is in excellent agreement with the accepted value for Q((11)B). This demonstrates that DFT (WIEN2k) calculations can provide precise (11)B quadrupole coupling parameters on an absolute scale. On the other hand, DFT calculations based on the reported crystal structures for datolite (CaBSiO(4)(OH)) and danburite (CaB(2)Si(2)O(8)) cannot reproduce the experimental (11)B quadrupole coupling parameters to the same high precision. However, optimization of these structures by minimization of the forces between the atoms (obtained by DFT) results in a significant improvement between the calculated and experimental (11)B quadrupole coupling parameters, which indicates that reliable refinements of the borate structures are obtained by this method. Finally, the DFT calculations also provide important structural information about the sign and orientation of the EFG tensor elements in the crystal frame, a kind of information that cannot be achieved from (11)B NMR experiments on powdered samples.     

Characterization of divalent metal metavanadates by 51V magic-angle spinning NMR spectroscopy of the central and satellite transitions

51V quadrupole coupling and chemical shielding tensors have been determined from 51V magic-angle spinning (MAS) NMR spectra at a magnetic field of 14.1 T for nine divalent metal metavanadates: Mg(VO3)2, Ca(VO3)2, Ca(VO3)(2).4H2O, alpha-Sr(VO3)2, Zn(VO3)2, alpha- and beta-Cd(VO3)2. The manifold of spinning sidebands (ssbs) from the central and satellite transitions, observed in the 15V MAS NMR spectra, have been analyzed using least-squares fitting and numerical error analysis. This has led to a precise determination of the eight NMR parameters characterizing the magnitudes and relative orientations of the quadrupole coupling and chemical shielding tensors. The optimized data show strong similarities between the NMR parameters for the isostructural groups of divalent metal metavanadates. This demonstrates that different types of metavanadates can easily be distinguished by their anisotropic NMR parameters. The brannerite type of divalent metal metavanadates exhibits very strong 51V quadrupole couplings (i.e., CQ = 6.46-7.50 MHz), which reflect the highly distorted octahedral environments for the V5+ ion in these phases. Linear correlations between the principal tensor elements for the 51V quadrupole coupling tensors and electric field gradient tensor elements, estimated from point-monopole calculations, are reported for the divalent metal metavanadates. These correlations are used in the assignment of the NMR parameters for the different crystallographic 51V sites of Ca(VO3)(2).4H2O, Pb(VO3)2, and Ba(VO3)2. For alpha-Sr(VO3)2, with an unknown crystal structure, the 51V NMR data strongly suggest that this metavanadate is isostructural with Ba(VO3)2, for which the crystal structure has been reported. Finally, the chemical shielding parameters for orthovanadates and mono- and divalent metal metavanadates are compared.     

95Mo nuclear magnetic resonance parameters of molybdenum hexacarbonyl from density functional theory: appraisal of computational and geometrical parameters

Solid-state (95)Mo nuclear magnetic resonance (NMR) properties of molybdenum hexacarbonyl have been computed using density functional theory (DFT) based methods. Both quadrupolar coupling and chemical shift parameters were evaluated and compared with parameters of high precision determined using single-crystal (95)Mo NMR experiments. Within a molecular approach, the effects of major computational parameters, i.e. basis set, exchange-correlation functional, treatment of relativity, have been evaluated. Except for the isotropic parameter of both chemical shift and chemical shielding, computed NMR parameters are more sensitive to geometrical variations than computational details. Relativistic effects do not play a crucial part in the calculations of such parameters for the 4d transition metal, in particular isotropic chemical shift. Periodic DFT calculations were tackled to measure the influence of neighbouring molecules on the crystal structure. These effects have to be taken into account to compute accurate solid-state (95)Mo NMR parameters even for such an inorganic molecular compound.     

Characterization of a new hexasodium diphosphopentamolybdate hydrate, Na6[P2Mo5O23]x7H2O, by 23Na MQMAS NMR spectroscopy and X-ray powder diffraction

A novel hexasodium disphosphopentamolybdate hydrate, Na6[P2Mo5O23]x7H2O, has been identified using X-ray powder diffraction, 1H, 23Na, and 31P magic-angle spinning (MAS) NMR, and 23Na multiple-quantum (MQ) MAS NMR. Powder XRD reveals that the hydrate belongs to the triclinic spacegroup P1 with cell dimensions a = 10.090(3) A, b = 15.448(5) A, c = 8.460(4) A, alpha = 101.45(6) degrees, beta = 104.09(2) degrees, gamma = 90.71(5) degrees, and Z = 2. The number of water molecules of crystallization has been determined on the basis of a quantitative evaluation of the 1H MAS NMR spectrum, the crystallographic unit cell volume, and a hydrogen content analysis. The 23Na MQMAS NMR spectra of Na6[P2Mo5O23]x7H2O, obtained at three different magnetic fields, clearly resolve resonances from six different sodium sites and allow a determination of the second-order quadrupolar effect parameters and isotropic chemical shifts for the individual resonances. These data are used to determine the quadrupole coupling parameters (CQ and eta Q) from simulations of the complex line shapes of the central transitions, observed in 23Na MAS NMR spectra at the three magnetic fields. This analysis illustrates the advantages of combining MQMAS and MAS NMR at moderate and high magnetic fields for a precise determination of quadrupole coupling parameters and isotropic chemical shifts for multiple sodium sites in inorganic systems. 31P MAS NMR demonstrates the presence of two distinct P sites in the asymmetric unit of Na6[P2Mo5O23].7H2O while the 31P chemical shielding anisotropy parameters, determined for this hydrate and for Na6[P2Mo5O23]x13H2O, show that these two hydrates can easily be distinguished using 31P MAS NMR.     

Aluminum orthovanadate (AlVO4): synthesis and characterization by (27)Al and 51V MAS and MQMAS NMR spectroscopy

Polycrystalline samples of AlVO(4) have been prepared by two methods of synthesis and characterized by (27)Al and (51)V MAS NMR spectroscopy at 14.1 T. The MAS NMR spectra clearly reveal that essentially pure samples with minor impurities of V(2)O(5) and alumina have been obtained. From these samples, (27)Al quadrupole coupling parameters and isotropic chemical shifts as well as the magnitudes and relative orientations of the (51)V quadrupole coupling and chemical shift tensors have been determined with high precision for AlVO(4). These data have been obtained from a combined analysis of multiple-quantum (MQ) MAS NMR spectra and MAS NMR spectra of the central and satellite transitions. The (27)Al and (51)V NMR data show that the asymmetric unit for AlVO(4) contains three isolated VO(4) tetrahedra, one pentacoordinated Al site, and two AlO(6) octahedra. This is in agreement with the supposition that AlVO(4) is isostructural with FeVO(4) and with a recent structure refinement for AlVO(4) based on powder X-ray diffraction (XRD) data. The favorable agreement between the refined crystal structure from powder XRD and the NMR parameters is apparent from a convincing correlation between experimental (51)V quadrupole tensor elements and calculated (51)V electric field gradient tensor elements obtained by the point-monopole approach. An assignment of the (27)Al NMR data is obtained from similar calculations of the (27)Al electric field gradients and by estimation of the distortion of the AlO(6) octahedra.     

Solid-state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside

Three flavonoids of pharmaceutical importance-baicalein, baicalin, and wogonoside-were isolated from a Chinese medicinal plant Scutellaria baicalensis Georgi and studied by 13C NMR in solution and solid state. Two-dimensional (2D) NMR spectroscopy in the liquid phase and dipolar dephasing (DD) experiments in magic-angle spinning (MAS) spectra enabled the assignment of 13C resonances. The cross-polarization (CP) time constants T(CH) and relaxation times T(H) (1rho) were obtained from the variable-contact time experiments. The principal elements of the 13C chemical shift tensor were determined in the spectra recorded under slow sample spinning (2 kHz) using phase-adjusted spinning sideband (PASS)-2D NMR technique, and were verified by density functional theory gauge-independent atomic orbital (DFT GIAO) calculations of shielding constants. Analysis of the 13C delta(ii) and comparison with shielding parameters calculated for different conformers of compounds 1-3 enabled the selection of the most reliable geometry in the solid phase. In all three compounds, an intramolecular hydrogen bond C5--OH...=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable.     

High‐field 95Mo and 183W static and MAS NMR study of polyoxometalates

The potential of high‐field NMR to measure solid‐state ⁹⁵Mo and ¹⁸³W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low‐gamma NMR nuclei, due mainly to the low spectral resolution and poor signal‐to‐noise ratio, the observed global trends compare well with the solution‐state NMR data. This would open an avenue for application of solid‐state NMR to POMs, especially when liquid‐state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.     

The use of variable temperature 13C solid-state MAS NMR and GIPAW DFT calculations to explore the dynamics of diethylcarbamazine citrate

Experimental ¹³C solid-state magic-angle spinning (MAS) Nuclear Magnetic Resonance (NMR) as well as Density-Functional Theory (DFT) gauge-including projector augmented wave (GIPAW) calculations were used to probe disorder and local mobility in diethylcarbamazine citrate, (DEC)⁺(citrate)⁻. This compound has been used as the first option drug for the treatment of filariasis, a disease endemic in tropical countries and caused by adult worms of Wuchereria bancrofti, which is transmitted by mosquitoes. We firstly present 2D ¹³C─¹H dipolar-coupling-mediated heteronuclear correlation spectra recorded at moderate spinning frequency, to explore the intermolecular interaction between DEC and citrate molecules. Secondly, we investigate the dynamic behavior of (DEC)⁺(citrate)⁻ by varying the temperature and correlating the experimental MAS NMR results with DFT GIPAW calculations that consider two (DEC)⁺ conformers (in a 70:30 ratio) for crystal structures determined at 293 and 235 K. Solid-state NMR provides insights on slow exchange dynamics revealing conformational changes involving particularly the DEC ethyl groups.     

Ion-binding study by 17O solid-state NMR spectroscopy in the model peptide Gly-Gly-Gly at 19.6 T

Li(+) and Ca(2+) binding to the carbonyl oxygen sites of a model peptide system has been studied by (17)O solid-state NMR spectroscopy. (17)O chemical shift (CS) and quadrupole coupling (QC) tensors are determined in four Gly-(Gly-(17)O)-Gly polymorphs by a combination of stationary and fast magic-angle spinning (MAS) methods at high magnetic field, 19.6 T. In the crystal lattice, the carbonyl oxygen of the central glycyl residue in two gly-gly-gly polymorphs form intermolecular hydrogen bonds with amides, whereas the corresponding carbonyl oxygens of the other two polymorphs form interactions with Li(+) and Ca(2+) ions. This permits a comparison of perturbations on (17)O NMR properties by ion binding and intermolecular hydrogen bonding. High quality spectra are augmented by density functional theory (DFT) calculations on large molecular clusters to gain additional theoretical insights and to aid in the spectral simulations. Ion binding significantly decreases the two (17)O chemical shift tensor components in the peptide plane, delta(11) and delta(22), and, thus, a substantial change in the isotropic chemical shift. In addition, quadrupole coupling constants are decreased by up to 1 MHz. The effects of ion binding are found to be almost an order of magnitude greater than those induced by hydrogen bonding.     

Structural studies of NaPO3-MoO3 glasses by solid-state nuclear magnetic resonance and Raman spectroscopy

Vitreous samples were prepared in the (100 - x)% NaPO(3)-x% MoO(3) (0 相似文献   

Experimental and theoretical studies of 45Sc NMR interactions in solids

Solid-state 45Sc NMR spectroscopy, ab initio calculations, and X-ray crystallography are applied to examine the relationships between 45Sc NMR interactions and molecular structure and symmetry. Solid-state 45Sc (I = 7/2) magic-angle spinning (MAS) and static NMR spectra of powdered samples of Sc(acac)3, Sc(TMHD)3, Sc(NO3)3.5H2O, Sc(OAc)3, ScCl3.6H2O, ScCl3.3THF, and ScCp3 have been acquired. These systems provide a variety of scandium coordination environments yielding an array of distinct 45Sc chemical shielding (CS) and electric field gradient (EFG) tensor parameters. Acquisition of spectra at two distinct magnetic fields allows for the first observations of scandium chemical shielding anisotropy (CSA). 45Sc quadrupolar coupling constants (CQ) range from 3.9 to 13.1 MHz and correlate directly with the symmetry of the scandium coordination environment. Single-crystal X-ray structures were determined for Sc(TMHD)3, ScCl3.6H2O, and Sc(NO3)3.5H2O to establish the hitherto unknown scandium coordination environments. A comprehensive series of ab initio calculations of EFG and CS tensor parameters are in excellent agreement with the observed parameters. Theoretically determined orientations of the NMR interaction tensors allow for correlations between NMR tensor characteristics and scandium environments. Solid-state 45Sc, 13C, and 19F NMR experiments are also applied to characterize the structures of the microcrystalline Lewis acid catalyst Sc(OTf)3 (for which the crystal structure is unknown) and a noncrystalline, microencapsulated, polystyrene-supported form of the compound.     

C CP MAS NMR and DFT study of vascular-selective drugs felodipine and amlodipine

Amlodipine besylate (AM) and felodipine (FL) have been studied in solid by ¹³C CP MAS NMR and DFT. The spectra have been successfully reproduced from the theoretical calculations of the nuclear magnetic shielding tensors, using the GIAO method, after the SCF partial geometry optimisation. A very good correlation between the ¹³C chemical shifts and quadrupole coupling constants for chlorobenzene, AM and FL was obtained. ¹³C NMR as well as ³⁵Cl NQR do not differentiate between S and R enantiomers of AM and FL due to the symmetry of the 4-aryl ring comprising one (AM) or two (FL) chlorine atoms oriented in a perpendicular fashion over the 1,4-DHP ring.     

Natural abundance solid-state 67Zn NMR characterization of microporous zinc phosphites and zinc phosphates at ultrahigh magnetic field

Zinc-phosphite and -phosphate based microporous materials are crystalline open framework materials with potential industrial applications. Although (31)P MAS NMR has been used for characterization of these materials, the local environments around zinc centres have never been directly probed by solid-state NMR due to the many unfavourable NMR characteristics of (67)Zn. In this work, we have characterized the local structure around the Zn centres in several representative microporous zinc phosphites and zinc phosphates by acquiring natural abundance (67)Zn solid-state NMR spectra at ultrahigh magnetic field of 21.1 T. The observed line-shapes are mainly determined by the second order quadrupolar interaction. The NMR tensor parameters were extracted from the spectra and are related to the local geometry around the Zn centre. Computational study of the electric field gradient (EFG) tensor at Zn was performed using hybrid density functional theory (DFT) calculations at B3LYP level of theory on model clusters. The calculations using Projector Augmented-Wave (PAW) method were also carried out with the CASTEP code wherever it was possible. The work has shown that it is possible to study Zn environments in porous materials which often have very low Zn concentration by natural abundance (67)Zn SSNMR at very high magnetic fields.     

Chemical shift and quadrupole coupling of the 27Al NMR spectra of LiAIO2 polymorphs

²⁷Al chemical shift data of polycrystalline LiAlO₂ polymorphs, obtained from high field measurements in combination with magic-angle spinning techniques, have been used for the determination of the Al coordination in the α-, β-, and γ-phase. The β-phase was shown to have only tetrahedrally coordinated Al atoms. In addition quadrupole coupling data have been derived from analysis of low field NMR spectra of 27A1 in order to characterize the symmetry of the Al sites. The NMR results are discussed with regard to the crystal structure of the phases.     

The complete 51V MAS NMR spectrum of surface vanadia nanoparticles on anatase (TiO2): vanadia surface structure of a DeNOx catalyst

The first observations of the complete manifold of spinning sidebands (ssbs) including both the central and satellite transitions in (51)V MAS NMR spectra of surface vanadia nanoparticles on titania in DeNO(x) catalysts are presented. (51)V quadrupole coupling and chemical shift anisotropy parameters for the dominating vanadia structure are determined from (51)V MAS NMR spectra recorded at 9.4 and 14.1 T. Based on correlations previously established between (51)V NMR parameters and crystal structure data for inorganic vanadates, the NMR data are consistent with vanadium in a distorted octahedral oxygen coordination environment for the so-called strongly bonded vanadia species on the surface. The investigation includes two vanadia-titania model catalysts and six industrial-type DeNO(x) catalysts.     

A solid-state 95Mo NMR and computational investigation of dodecahedral and square antiprismatic octacyanomolybdate(IV) anions: is the point-charge approximation an accurate probe of local symmetry?

Solid-state 95Mo NMR spectroscopy is shown to be an efficient and effective tool for analyzing the diamagnetic octacyanomolybdate(IV) anions, Mo(CN)(8)4-, of approximate dodecahedral, D(2d), and square antiprismatic, D(4d), symmetry. The sensitivity of the Mo magnetic shielding (sigma) and electric field gradient (EFG) tensors to small changes in the local structure of these anions allows the approximate D(2d) and D(4d) Mo(CN)(8)4- anions to be readily distinguished. The use of high applied magnetic fields, 11.75, 17.63 and 21.1 T, amplifies the overall sensitivity of the NMR experiment and enables more accurate characterization of the Mo sigma and EFG tensors. Although the magnitudes of the Mo sigma and EFG interactions are comparable for the D(2d) and D(4d) Mo(CN)(8)4- anions, the relative values and orientations of the principal components of the Mo sigma and EFG tensors give rise to 95Mo NMR line shapes that are significantly different at the fields utilized here. Quantum chemical calculations of the Mo sigma and EFG tensors, using zeroth-order regular approximation density functional theory (ZORA DFT) and restricted Hartree-Fock (RHF) methods, have also been carried out and are in good agreement with experiment. The most significant and surprising result from the DFT and RHF calculations is a significant EFG at Mo for an isolated Mo(CN)(8)4- anion possessing an ideal square antiprismatic structure; this is contrary to the point-charge approximation, PCA, which predicts a zero EFG at Mo for this structure.     

An NMR crystallography investigation of furosemide

This paper presents an NMR crystallography study of three polymorphs of furosemide. Experimental magic-angle spinning (MAS) solid-state NMR spectra are reported for form I of furosemide, and these are assigned using density-functional theory (DFT)-based gauge-including projector augmented wave (GIPAW) calculations. Focusing on the three known polymorphs, we examine the changes to the NMR parameters due to crystal packing effects. We use a recently developed formalism to visualise which regions are responsible for the chemical shielding of particular sites and hence understand the variation in NMR parameters between the three polymorphs.     

(39)K NMR of solid potassium salts at 21 T: effect of quadrupolar and chemical shift tensors

39K Solid State NMR spectra (static and magic angle spinning (MAS)) on a set of potassium salts measured at 21.14 T show that the chemical shift range for K(+) ions in diamagnetic salts is well in excess of 100 ppm contrary to previous assumptions that it was quite small. Inequivalent potassium sites in crystals can be resolved through differences in chemical shifts, with chemically similar sites showing differences of over 10 ppm. The quadrupolar coupling constants obtained from MAS and solid echo experiments on powders cover the range from zero for potassium in cubic environments in halides to over 3 MHz for the highly asymmetric sites in K2CO3. Although the quadrupolar effects generally dominate the 39K spectra, in several instances, we have observed subtle but significant contributions of chemical shift anisotropy with values up to 45 ppm, a first such observation. Careful analysis of static and MAS spectra allows the observation of the various chemical shift and quadrupole coupling tensor components as well as their relative orientations, thereby demonstrating that high-field 39K NMR spectroscopy in the solid state has a substantial sensitivity to the local environment with parameters that will be of considerable value in materials characterization and electronic structure studies.