Fast 29Si magic-angle-spinning NMR acquisitions by RAPT-CP 27Al --> 29Si polarization transfer

Using enhancement of the 27Al central-transition magnetization by applying RAPT prior to 27Al --> 29Si cross-polarization, we demonstrate fast acquisition of 29Si one-dimensional MAS and two-dimensional 27Al-29Si HETCOR spectra on a new sialon phase Ba2Al3Si9N13O5.     

Characterization of silicon sites in monoclinic zeolite ZSM-5 using 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) and molecular modelling

An attempt has been made to correlate the experimentally observed 29Si MAS NMR chemical shifts of monoclinic phase of highly siliceous ZSM-5 with their electronic properties. In order to incorporate the influence of next neighbor atoms on the 29Si chemical shielding of central SiO4, a pentameric cluster model (H12Si5O16) has been chosen. Each of the 24 crystallographically distinct Si sites, of ZSM-5 framework has been modelled by such cluster models. Based on semi-empirical quantum chemical calculations, a multiple linear regression analysis of the various electronic properties with the 29Si chemical shifts has been attempted. The relative difference in 29Si chemical shifts for the Si sites in ZSM-5 is reasonably accounted, although quantitative prediction may require non-empirical quantum chemical calculations.     

Unambiguously distinguishing Si[3Si,1Al] and Si[3Si,1OH] stuctural units in zeolite by 1H/29Si/27Al triple resonance solid state NMR spectroscopy

We present an experimentally feasible triple-resonance NMR method that establishes the correlation among three different nuclei, avoiding the difficulty to directly explore the weak coupling between two NMR nuclei, such as (29)Si and (27)Al. Using this method, we are able to give an unambiguous assignment to the various peaks in (29)Si CP NMR spectrum of MCM-22 zeolite and discriminate (29)Si signals from SiOHAl and SiOH groups. In addition, in combination with (1)H/(27)Al double-resonance technique, the (1)H/(27)Al/(29)Si triple-resonance experiment suggests the presence of two different kinds of Br?nsted acid sites in H-MCM-22 zeolite.     

Multinuclear (27Al, 29Si, 47,49Ti) solid-state NMR of titanium substituted zeolite USY

Multinuclear solid-state NMR spectroscopy, employing 29Si MAS,27Al MAS/3Q-MAS and (47,49)Ti wide-line experiments, has been used for the structural characterization of titanium substituted ultra-stable zeolite Y (Ti-USY). 27Al MAS experiments show the presence of aluminum in four (Al(IV)), five (Al(V)), and six (Al(VI)) coordination, whereas the multiplicity within Al(IV) and Al(VI) is revealed by 27Al 3Q-MAS experiments. Two different tetrahedral and octahedral Al environments are resolved and their isotropic chemical shifts (delta(CS)) and second-order quadrupole interaction parameters (P(Q)) have been determined by a graphical analysis of the 3Q-MAS spectra. The emergence of signal with higher intensity at -101 ppm in the 29Si MAS spectrum of Ti-USY samples indicates the possible occurrence of Q4(3Si,1Ti) type silicon environments due to titanium substitution in the faujasite framework. High-field (11.74T) operation, using a probehead specially designed to handle a large sample volume, has enabled the acquisition of 47,49Ti static spectra and identification of the titanium environment in the zeolite. The chemical shielding and electric field gradient tensors for the titanium environment in the zeolite have been determined by a computer simulation of the quadrupolar broadened static 47,49Ti NMR spectra.     

29Si NMR in solid state with CPMG acquisition under MAS

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.     

129Xe,29Si and27Al NMR studies of dealuminated Y zeolite

The dealumination of a commercial Y zeolite by contact with SiCl₄ is monitored with NMR spectroscopy of²⁹Si,²⁷Al and¹²⁹Xe. The results are interpreted in terms of the changes in the material as silicon is incorporated in the framework and aluminum is deposited in the pores.     

煅烧高岭土的NMR研究

采用²⁹Si、²⁷Al MAS NMR谱对不同产地高岭在同一煅烧温度及同一产地高岭土在不同煅烧温度的煅烧产物进行了研究. 实验结果表明：不同产地高岭土在同一煅烧温度煅烧产物的²⁹Si MAS NMR相似,²⁷Al MAS NMR谱有所区别,煅烧温度对煅烧产物的²⁹Si、²⁷Al MAS NMR有一定影响.     

Spatially inhomogeneous development of antiferromagnetism in URu2Si2: evidence from 29Si NMR under pressure

From 29Si NMR study, we present evidence for spatially inhomogeneous development of antiferromagnetic (AF) ordering below T(o) = 17.5 K in URu2Si2. In the pressure range between 3.0 and 8.3 kbar, we have observed the 29Si NMR lines arising from the AF region as well as the previously observed 29Si NMR line which correspond to the nonmagnetic region in the sample. The AF volume fraction is enhanced by applied pressure, whereas the magnitude of internal field at the Si site remains constant (910 Oe) up to 8.3 kbar. In the AF region, the ordered moment is about an order of magnitude larger than 0.03 mu(B)/U.     

高岭土与针铁矿胶结样中27Al、29Si NMR谱与其原子电荷关系

由NMR谱实验得高岭土与针铁矿胶结样中²⁷Al、²⁹Si的化学位移较单一高岭土分别正移了2.7ppm和2.33ppm.由CNDO/2方法计算得到胶结样中Al、Si原子电行比高岭土分别减少了0.0787和0.0286.胶结样中Al、Si原子电荷的减少与²⁷Al、²⁹Si NMR谱化学位移相吻合,可以认为在高岭土与外铁矿的界面上形成了氢键而呈现胶结态.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

Comparison of one and two-neutron transfer near the coulomb barrier for the27Al(18O,16O)29Al,27Al(180,17O)28Al and27Al(13C,12C)28Al reactions

Total reaction cross sections for the transfer reactions²⁷Al(¹⁸O,¹⁶O)²⁹Al,²⁷Al(¹⁸O,¹⁷O)²⁸Al and²⁷Al(¹³C,¹²C)²⁸Al are reported for center-of-mass energies between 13 and 20 MeV for¹⁸O projectiles and between 11 and 17.5 MeV for¹³C projectiles. The reaction products,²⁹Al and²⁸Al, beta decay to²⁹Si and²⁸Si, respectively, and the subsequentγ decays of²⁹Si and²⁸Si were measured. Due to the relatively long beta decay half lives, data were taken in a beam-off mode, resulting in very clean spectra. Total cross sections were calculated and compared with a theoretical model for barrier penetration proposed by C.Y. Wong. Differences between¹⁸O induced one and two-neutron total transfer reaction cross sections are discussed.     

On the mechanism of the 27Al(α, d)29Si reaction

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.     

Study of the28Si(d,p)29Si reaction at 3·22 MeV

The differential cross section of the²⁸Si(d,p)²⁹Si reaction at deuteron energy 3·22 MeV has been measured and analyzed including transitions to final states in²⁹Si up to the 5·809 MeV level. Angular distributions are compared with the DWBA and Hauser-Feshbach calculations. Satisfactory agreement is obtained for most of the levels. An attempt is made to explain the angular distribution leading to the excitation of the 4·078 MeV level as a two-step process.     

Photoneutron cross section of 29Si

The photoneutron cross section of ²⁹Si has been measured over the energy range 8 to 28 MeV using the bremsstrahlung yield curve technique. The giant dipole resonance is observed to be centered at 21.2 MeV and is 5.5 MeV wide. In addition, a pygmy resonance containing much fine structure is observed below 17 MeV. An attempt is made to interpret these observations in terms of the weak coupling of the extra neutron to a ²⁸Si core. However, detailed comparisons of the structure and relative strengths of the ²⁸Si and ²⁹Si cross sections reveal that the coupling of the extra-core nucleon is large and significantly modifies the core wave function.     

The study of the width for intermediate resonances in the 27Al(d,p)28Al, 27Al(d, α)25Mg, 31P(d,p)32P and 31(d, α)29Si reactions

The protons and α-particles from the reactions ²⁷Al(d, p)²⁸Al(d, α)²⁵Mg, ³¹P(d, p)³²P and ³¹P(d, α)²⁹Si were measured and analyzed with the channel cross correlation function and auto-correlation function to determine the correlating numbers N_d and average width 〈Γ_μ〉. With these values, the theoretical intermediate widths were calculated to be 119 ± 30 keV in the ²⁹Si nucleus and 249 ± 46 keV in the ³³S nucleus, which were in good agreement, within the errors, with the present experimental results of 185 ± 37 keV in ²⁹Si and 204 ± 24 keV in ³³S.     

Temperature-dependent 29Si NMR resonance shifts in lightly- and heavily-doped Si:P

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.     

Full profile analysis of the 29Si NMR spectra of LTL-framework zeolites

Full profile analyses of the digitized ²⁹Si NMR spectra of a potassium zeolite L and a gallosilicate analog are described. Both spectra can be fitted reasonably based on the assumption that they comprise only a single set of SinAl(Ga); n=0−4 peaks, reflecting that the mean T-O-T angles for the two T-sites (T=tetrahedral species, Si, Al or Ga) are similar.     

A structure refinement strategy for NMR crystallography: an improved crystal structure of silica-ZSM-12 zeolite from 29Si chemical shift tensors

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.     

变生锆石的29Si MAS NMR与红外光谱研究

关于锆石变生过程中SiO₄四面体行为的信息对于了解变生态结构及变生过程机理有重要意义。虽然有人推测,在变生过程中,伴随着ZrO₈多面体上氧空位的产生可能同时发生SiO₄四面体的聚合^[7],但迄今并无直接的实验证据证明这一假设。本文的²⁹Si MASNMR研究表明,在锆石变生的最初阶段,Si的局部结构环境没有明显的变化,但在中等变生程度的样品中,探测到Q³和Q⁴Si环境,这是一个有力的证据,支持随着变生程度增加SiO₄四面体发生逐步聚合的假设。红外光谱的结果也显示,变生锆石中可能有无定形的氧化硅畴存在。     

19F/29Si rotational-echo double-resonance and heteronuclear spin counting under fast magic-angle spinning in fluoride-containing octadecasil

19F/29Si rotational-echo double-resonance (REDOR) and theta-REDOR NMR techniques have been applied under fast magic-angle spinning to a powder sample of fluoride-containing octadecasil. Efficient dipolar recoupling was observed and the effect of finite pulse lengths was found to be negligible using standard radiofrequency field strengths. Moreover, the determined internuclear distance of the 19F-29Si spin pairs formed by the silicons in the D4R units (T-1 site) and the fluoride anions is in very good agreement with previous REDOR and Hartmann-Hahn cross-polarization measurements. Numerical simulation of the REDOR dephasing curves at both the T-1 and T-2 sites considering all fluoride anions in the infinite solid lattice clearly confirm the X-ray crystal structure of octadecasil. Heteronuclear spin-counting theta-REDOR experiments are found to be very useful to obtain direct insight into the local network of dipolar interactions. Indeed, while 19F-29Si pair-like behavior is confirmed at the T-1 site, multiple dipolar interactions are clearly evidenced at the T-2 site.