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1.
Ekaterina Leonova Jekabs Grins Mohammadreza Shariatgorji Leopold L. Ilag Mattias Edn 《Solid state nuclear magnetic resonance》2009,36(1):11-18
We compare 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra from the two modifications of silicon nitride, α-Si3N4 and β-Si3N4, with that of a fully (29Si, 15N)-enriched sample 29Si315N4, as well as 15N NMR spectra of Si315N4 (having 29Si at natural abundance) and 29Si315N4. We show that the 15N NMR peak-widths from the latter are dominated by J(29Si–15N) through-bond interactions, leading to significantly broader NMR signals compared to those of Si315N4. By fitting calculated 29Si NMR spectra to experimental ones, we obtained an estimated coupling constant J(29Si–15N) of 20 Hz. We provide 29Si spin-lattice (T1) relaxation data for the 29Si315N4 sample and chemical shift anisotropy results for the 29Si site of β-Si3N4. Various factors potentially contributing to the 29Si and 15N NMR peak-widths of the various silicon nitride specimens are discussed. We also provide powder X-ray diffraction (XRD) and mass spectrometry data of the samples. 相似文献
2.
V. A. Pestunovich S. N. Tandura M. G. Voronkov V. P. Baryshok G. I. Zelchan V. I. Glukhikh 《光谱学快报》2013,46(5):339-344
1H, 13C, 19F and 29Si NMR chemical shifts and coupling constants for Si-substituted silatranes, XSi(OCH2CH2)3N, and triethoxysilanes, XSi(OCH2CH3)3, where X = H, CH3, and F have been studied. Expansion of the coordination numbers of silicon and tin leads to similar changes in the NMR parameters. 相似文献
3.
Ab initio molecular orbital calculations (Hartree–Fock, HF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and α-quartz, in order to investigate the reliability of this method for predicting 29Si and 17O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the 29Si and 17O chemical shifts (δiSi and δiO), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δiSi and δiO have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δiSi values agree well (within ±1 ppm) with experimental data. The calculated 17O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δiO for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting 29Si and 17O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method. 相似文献
4.
27Al and 29Si Magic-Angle Spinning NMR results are reported for conventionally prepared glass of cordierite stoichiometry (2MgO · 2Al2O3 · 5SiO2), the metastable high-quartz solid solution (μ-cordierite) and the high-temperature polymorph of cordierite (α-cordierite). Both, 27Al two-dimensional (2D) quadrupole nutation experiments and 27Al satellite transition spectroscopy (SATRAS) have been applied to identify two different tetrahedrally-coordinated aluminium sites (AlO4). SATRAS has been used to extract the quadrupole interaction parameters and their distribution, the isotropic chemical shifts and the relative populations of the different Al sites. Both, the 27Al and 29Si NMR results, lead to the conclusion that a perfect Si/Al disorder does not exist in these investigated cordierite samples. 相似文献
5.
Denis Schneider Helge Toufar Ago Samoson Dieter Freude 《Solid state nuclear magnetic resonance》2009,35(2):87-92
We demonstrate complementary 1H, 17O, 27Al and 29Si measurements for basic low-silica-X zeolites, which were unloaded and pyrrole and formic acid-loaded. It was found that the acid–base-system is not stabile, if the loading exceeds one pyrrole molecule or two formic acid molecules per supercage.17O DOR NMR spectra exhibit at least four lines, which are broadened by a distribution of chemical shifts in a similar extend as the 29Si MAS NMR spectra are broadened by distribution of Si–O–Al angles. A strong cation influence upon 17O shifts was observed. But there was no strong influence of the acid molecules on the mean value of the 17O shift of the spectra. 相似文献
6.
Karol Jackowski Micha? Jaszuński W?odzimierz Makulski Juha Vaara 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,135(2):444-453
The nuclear shielding constants in OCS are studied using ab initio theoretical methods and gas-phase NMR measurements. The shielding surfaces are calculated and the rovibrational effects and the resulting temperature dependence are analyzed. The temperature dependence of13C shielding in the gas phase is determined experimentally in the range 278–373 K.13C is the single nucleus for which the experimental data for the temperature dependence can be converted to a reference-independent scale, and good agreement of the measured and calculated ab initio results is observed. For33S, we discuss a new, more accurate absolute shielding scale. 相似文献
7.
Careful NMR measurements on a very lightly-doped reference silicon sample provide a convenient highly precise and accurate secondary chemical shift reference standard for 29Si MAS-NMR applicable over a wide temperature range. The linear temperature-dependence of the 29Si chemical shift measured in this sample is used to refine an earlier presentation of the paramagnetic (high-frequency) 29Si resonance shifts in heavily-doped n-type silicon samples near the metal–nonmetal transition. The data show systematic decreases of the local magnetic fields with increasing temperature in the range 100–470 K for all samples in the carrier concentration range from 2×1018 cm−3 to 8×1019 cm−3. This trend is qualitatively similar to that previously observed for the two-orders of magnitude larger 31P impurity NMR resonance shifts in the same temperature and concentration ranges. The 29Si and 31P resonance shifts are not related by a simple scaling factor, however, indicating that impurity and host nuclei are affected by different subsets of partially-localized extrinsic electrons at all temperatures. 相似文献
8.
Isotropic 13C chemical shifts of the ribose sugar in model RNA nucleosides are calculated using SCF and DFT-GIAO ab initio methods for different combinations of ribose sugar pucker, exocyclic torsion angle, and glycosidic torsion angle. Idealized conformations were obtained using structures that were fully optimized by ab initio DFT methods starting with averaged parameters from a collection of crystallographic data. Solid-state coordinates of accurate crystal or neutron diffraction structures were also examined directly without optimization. The resulting 13C chemical shifts for the two sets of calculations are then compared. The GIAO-DFT method overestimates the shifts by an average of 5 ppm while the GIAO-SCF underestimates the shifts by the same amount. However, in the majority of cases the errors appear to be systematic, as the slope of a plot of calculated vs experimental shifts is very close to unity, with minimal scatter. The values of the 13C NMR shifts of the ribose sugar are therefore sufficiently precise to allow for statistical separation of sugar puckering modes and exocyclic torsion angle conformers, based on the canonical equation model formulated in a previous paper. 相似文献
9.
David W. Boykin 《光谱学快报》2013,46(8):1199-1205
Natural abundance 17O NMR data for 14 substituted methyl N-arylcarbamates obtained in acetonitrile solution at 75°C are reported. The 17O NMR chemical shifts of hindered ortho N-arylcarbamates are shielded relative to unhindered ones; a quantitative relationship is observed between the carbonyl 17O NMR chemical shifts and molecular mechanics (MM2) predicted torsion angles. The carbonyl 17O NMR chemical shifts of meta and para substituted N-arylcarbamates are correlated with Hammett sigma constants. 相似文献
10.
Evaluation by empirically derived equations for the Substituent effect (α, β, γ, δ) on the 13C NMR chemical shifts for C-3, C-4. C-5 and halomethyl-substituent carbon (C-6) in isoxazoles 1-5 [where C-3 substituent (R1) = H, alkyl or phenyl, C-4 Substituent (R2) = H, alkyl, and C-5 substituent (R3) = di-or trihalomethyl, methyl and H], taking as reference the compound la, is reported. From the calculated values for the α, β, γ, δ effects for each substituent it was possible to estimate the chemical shift of each carbon of the compounds 1–5. The 13 C chemical shifts of the C-3, C-4, C-5, C-6 of these compounds, can be estimated with good precision: 94% of the calculated chemical shifts are found to be within ±1.0ppm, and 100% are found to be within ±1.5ppm. 相似文献
11.
β-NMR experiments on hyperpolarized 8Li adsorbed on the Si(111)-(7×7) and Ru(001) surfaces yield detailed information on the electronic properties of these adsorbate-surface
systems. For Li adsorption on Ru(001) they are compared to ab initio all electron calculations.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
12.
《光谱学快报》2013,46(5-6):477-485
A concentration dependence of the 1H‐NMR chemical shifts of the aromatic protons in sampangine derivatives with a fused imidazole ring is observed. This variation is probably ascribable to self‐association of the molecules through an intermolecular π‐stacking interaction of the aromatic rings. The quantitative variation is correlated with the calculated electrostatic potential for these derivatives. The concentration variation appears to be independent of the nature of the substitution in the imidazole ring. 相似文献
13.
Evaluation by empirically derived equations for the substituent effect (α,β,γ,δ) on the 13C NMR chemical shifts for C-1, C-2, C-3 and C-4 in β-aryl-β-methoxyvinylhalomethylketones 1a-g to 2a-g [R3C(O)-CH=C(Ar)-OMe, where R3 = CCl3, CF3 and Ar = p-YC6H4 (Y = H, Me, MeO, F, Cl, Br, NO2)], taking as reference the β-ethoxyvinyltrichloromethylketone (3), is reported. From the calculated values for the α,β,γ,δ effects for each substituent it was possible to estimate the chemical shift of each carbon of the compounds 1,2. The 13C chemical shifts of the C-1, C-2, C-3, C-4 of these compounds, can be estimated with good to rasoable precision: 84% of the calculated chemical shifts are found to be within ±1.0ppm, and 100% are found to be within ±1.5ppm. The Y-Effects on C-3 and C-4 are compared with carbon charge densities (qr). 相似文献
14.
Silicon-29 NMR spectroscopy was used to characterize aqueous and alcoholic alkaline solutions of tri-butylmethyl ammonium (TBMA) silicates. The effect of TBMA cation on the equilibrium of silicate oligomers in aqueous alkaline silicate solutions was investigated using 29Si NMR spectra. It was found that TBMA cation has a structure directing role and directs the silicate species to form minor amounts of silicate anion in the presence of high concentration of silicon. Silicon-29 NMR spectra of TBMA silicate solutions indicate that considerable changes occurred by changing the Si/TBMA ratio. The distribution of silicate species was affected by the presence of the alcohols, specifically methanol. 相似文献
15.
Brouwer DH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,194(1):136-146
A strategy for performing crystal structure refinements with NMR chemical shift tensors is described in detail and implemented for the zeolite silica-ZSM-12 (framework type code MTW). The 29Si chemical shift tensors were determined from a slow magic-angle spinning spectrum obtained at an ultrahigh magnetic field of 21.1T. The Si and O atomic coordinate parameters were optimized to give the best agreement between experimentally measured and ab initio calculated principal components of the 29Si chemical shift tensors, with the closest Si-O, O-O, and Si-Si distances restrained to correspond with the distributions of the distances found in a set of single-crystal X-ray diffraction (XRD) structures of high-silica zeolites. An improved structure for the silica-ZSM-12 zeolite, compared to a prior structure derived from powder XRD data, is obtained in which the agreement between the experimental and calculated 29Si chemical shift tensors is dramatically improved, the Si-O, O-O, and Si-Si distances correspond to the expected distributions, while the calculated powder XRD pattern remains in good agreement with the experimental powder XRD data. It is anticipated that this "NMR crystallography" structure refinement strategy will be an important tool for the accurate structure determination of materials that are difficult to fully characterize by traditional diffraction methods. 相似文献
16.
I. Skwirczyńska E. Kozik A. Budzanowski J. Płoskonka A. Strzałkowski 《Nuclear Physics A》1981,371(2):288-300
The 27Al(α, d)27Si reaction has been studied at three bombarding energies: 26.2, 26.45 and 26.7 MeV. The energy-averaged angular distributions for states populated in 29Si have been compared with the microscopic and macroscopic DWBA calculations and Hauser-Feshbach theory. Spectroscopic amplitudes were calculated using the available shell-model wave functions. 相似文献
17.
In this paper, the equilibrium geometry, harmonic frequency and dissociation energy of S2^- and S3^- have been calculated at QCISD/6-311++G(3d2f) and B3P86/6-311++G(3d2f) level. The S2^- ground state is of 2IIg, the S3^- ground state is of 2B1 and S3^- has a bent (C2v) structure with an angle of 115.65° The results are in good agreement with these reported in other literature. For S3^- ion, the vibration frequencies and the force constants have also been calculated. Base on the general principles of microscopic reversibility, the dissociation limits has been deduced. The Murrell-Sorbie potential energy function for S2^- has been derived according to the ab initio data through the least- squares fitting. The force constants and spectroscopic data for S2^- have been calculated, then compared with other theoretical data. The analytical potential energy function of S3^- have been obtained based on the many-body expansion theory. The structure and energy can correctly reappear on the potential surface. 相似文献
18.
Ch.V.S. Ramachandra Rao 《Journal of Molecular Spectroscopy》1981,89(1):197-213
Self-Consistent Field (SCF) and Configuration Interaction (CI) studies are performed on the bending mode of the water molecule using a double zeta plus polarization basis set. The ab initio points are fitted to a three-parameter double minimum potential consisting of a quadratic plus Lorentzian terms. The vibration-rotation energies are then evaluated using the large amplitude Hamiltonian developed by P. R. Bunker and co-workers at various levels of approximations. It is found that the calculated frequencies improve significantly as one proceeds from approximate Hb00(ρ) to rigid bender Hb0(ρ) [P. R. Bunker and J. M. R. Stone, J. Mol. Spectrosc.41, 310–332 (1972)] to semirigid bender Hb0(r, ρ) [P. R. Bunker and P. M. Landsberg, J. Mol. Spectrosc.67, 374–385 (1977)] Hamiltonian. With Hb0(r, ρ), the ab initio calculated bending frequency ν2 differs from the observed value (1595 cm?1) by 30 cm?1 and the barrier height is 12 229 cm?1. It is also shown that ν2 and its first four overtones are better calculated by 45–98 cm?1 when the ab initio potential is used directly instead of the three-parameter analytic potential fitted to ab initio data. Finally, rotation bending energy levels are calculated for v2 ≤ 3 and J ≤ 10 on the basis of a nonrigid bender Hamiltonian of A. R. Hoy and P. R. Bunker [J. Mol. Spectrosc.74, 1–8 (1979)], using the ab initio quadratic force field of P. Hennig, W. P. Kraemer, G. H. F. Diercksen, and G. Strey, [Theor. Chim. Acta47, 233–248 (1978)]. These results show that the accuracy of calculated force constants and frequencies is critically dependent not only on the size of the basis set but also on the number and spacing of the ab initio points used to derive the force field. 相似文献
19.
Wiench JW Lin VS Pruski M 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,193(2):233-242
A remarkable enhancement of sensitivity can be often achieved in 29Si solid-state NMR by applying the well-known Carr–Purcell–Meiboom–Gill (CPMG) train of rotor-synchronized π pulses during the detection of silicon magnetization. Here, several one- and two-dimensional (1D and 2D) techniques are used to demonstrate the capabilities of this approach. Examples include 1D 29Si{X} CPMAS spectra and 2D 29Si{X} HETCOR spectra of mesoporous silicas, zeolites and minerals, where X = 1H or 27Al. Data processing methods, experimental strategies and sensitivity limits are discussed and illustrated by experiments. The mechanisms of transverse dephasing of 29Si nuclei in solids are analyzed. Fast magic angle spinning, at rates between 25 and 40 kHz, is instrumental in achieving the highest sensitivity gain in some of these experiments. In the case of 29Si–29Si double-quantum techniques, CPMG detection can be exploited to measure homonuclear J-couplings. 相似文献
20.
In this contribution, we have explored the potential and strength of one-dimensional (1D) 29Si and two-dimensional (2D) 29S–29Si and 29Si–17O NMR as invariants of non-oriented graph for fingerprinting zeolite frameworks. 1D and 2D 29Si NMR can indeed provide indications on the graph vertices, edges and allow the construction of the adjacency matrix, i.e. the set of connections between the graph vertices. From the structural data, hypothetical 1D 29Si and 2D 29Si–29Si NMR signatures for 193 of the zeolite frameworks reported in the Atlas of Zeolite Structures have been generated. Comparison between all signatures shows that thanks to the 1D 29Si NMR data only, almost 20% of the known zeolite frameworks could be distinguished. Further NMR signatures were generated by taking into account 2D 29Si–29Si and 29Si–17O correlations. By sorting and comparison of all the NMR data, up to 80% of the listed zeolites could be unambiguously discriminated. This work indicates that (i) solid-state NMR data indeed represent a rather strong graph invariant for zeolite framework, (ii) despite their difficulties and costs (isotopic labeling is often required, the NMR measurements can be long), 29Si and 17O NMR measurements are worth being investigated in the frame of zeolites structure resolution. This approach could also be generalized to other zeolite-related materials containing NMR-measurable nuclides. 相似文献