High-resolution Solid-state 29Si NMR Research Articles

Elemental and structural analysis of silicon forms in herbal drugs using silicon-29 MAS NMR and WD-XRF spectroscopic methods

The objective of this work was to study concentration of silicon and its structural forms present in herbal drugs. Equisetum arvense and Urtica dioica L. from teapot bags, dietary supplements (tablets and capsules) containing those herbs, dry extract obtained from a teapot bag of E. arvense, and samples of the latter herb harvested in wild habitat over four months were studied using wavelength dispersive X-ray spectroscopy (WD-XRF) and high-resolution solid-state 29Si NMR. The highest concentration of Si, ca. 27 mg/g, was found in the herbal material from the teapot bags containing E. arvense. The Si content in natural E. arvense (whole plants) increased from May to August by ca. 7 mg/g, reaching value 26 mg/g. Three different silicon forms were detected in the studied herbal samples: Si(OSi )4 (Q 4), Si(OH)(OSi )3 (Q 3) and Si(OH)2(OSi )2 (Q 2). Those sites were populated in E. arvense in the following order: Q 4 ≫ Q 3 > Q 2. A dramatic, ca. 50-fold decrease of the Si concentration during the infusion process was observed. The infusion process and the subsequent drying procedure augmented population of the Q 4 sites at the cost of the Q 2 sites. The WD-XRF and 29Si NMR methods occurred useful and complementary in the study of herbal materials.

Silicon-29 NMR Study of Alkali-Exchanged NaY Zeolites

High-resolution solid-state 29Si NMR has been applied to the study of partially exchanged Li, K, and Cs NaY zeolites. The order of the 29Si chemical shifts of dehydrated samples is Li, Na-Y < Na-Y < K, Na-Y. The correlation between the 29Si chemical shift and the Li or K loading on Li, Na-Y or K, Na-Y was rationalized in terms of the interaction between the framework and the cations inside the small cages. Because of the restrictive migration of large Cs+ ions from the supercages to the small cages, the 29Si chemical shift of Cs, Na-Y was found to be similar to that of Na-Y.

The combination of synchrotron powder diffraction and high-resolution solid-state NMR experiments

Structural investigations on three zeolites, ZSM-11, ZSM-12 and ZSM-5, have been carried out by a combination of high-resolution solid-state 29Si NMR experiments and Rietveld refinement of synchrotron X-ray powder data. These techniques, being sensitive to short- and long-range order, respectively, gave a most complete picture of the structure of the solid state. Variable temperature NMR experiments of ZSM-11 revealed a displacive phase transition with a change in symmetry at about 300 K. A Rietveld refinement of a data set recorded at 373 K confirmed the proposed structure model. The structure of ZSM-12 was refined from room temperature synchrotron powder data. Space group ambiguities because of psuedo-symmetry of the silica framework of the zeolite were resolved with the aid of 29Si NMR experiments leading to C2/ c as the true space group symmetry. 29Si NMR and X-ray studies of ZSM-5 showed that the symmetry of the crystal structure depends on the temperature, the concentration of the guest molecule, and on the number of Al atoms in the framework. A full pattern refinement of silica-ZSM-5 loaded with two molecules p-dichlorobenzene per unit cell showed that the site occupied by the sorbed molecule is not at the intersection of the channels but shifted by 1 4 along b 0.

Theoretical interpretation of 29Si NMR chemical shifts of aluminosilicates—III. semiempirical determination of the paramagnetic shielding constant

CNDO/2 calculations of the relative paramagnetic shielding constant of the silicon nucleus have been performed for the molecular models (HO) 3SiOT(OH) 3, T = Si or Al −, using different bond angles at the bridging oxygen atoms. The calculated quantity describes the high-field shift of the 29Si NMR signal observed by high-resolution solid-state 29Si NMR spectroscopy of aluminosilicates with increasing SiOT bond angle. Factors of influence are the silicon net charge, the silicon 3 p electron distribution and the average excitation energy. These influences compensate each other in part. The “imbalance term” which takes into account the distribution of the 3 p electrons at the resonance nucleus is shown to be a simple measure for the trend of the 29Si NMR chemical shift of the investigated material.

Charge differences between silicon atoms in aluminosilicates and their relation to 29Si NMR chemical shifts. A quantum-chemical study

High-resolution solid-state 29Si NMR spectroscopy can distinguish between five different building units of aluminosilicates. For the latter the models Si(OSiH 3) 4−n(OAlH 3) n n− (with n = 0, 1, 2, 3, 4) were adopted and the atomic charges were calculated by the SCF method (STO-3G basis set). The results for the central silicon atom correlate well with the observed 29Si chemical shifts. The calculations were repeated by semiempirical methods (CNDO/2, INDO with Gordon's refined parameters, EHT and IEHT) and the influences of size and termination of the models were studied.

The fine structure of high-resolution solid state 29Si NMR spectra of zeolites X and Y

Examination of 29Si magic-angle-spinning NMR spectra of zeolites with the faujasite structure reveals that the characteristic pattern of linewidths and chemical shifts is due to effects in the first coordination shell of silicon.

High-resolution solid-state 29Si NMR of polymorphs of Ca 2SiO 4

High-resolution solid-state 29Si NMR spectra of four ( γ, β, α L 1 and α) polymorphs of Ca 2SiO 4 have been measured. The isotropic silicon 29Si chemical shifts lie between −73.5 and −70.1 ppm, and increasing coordination number of calcium leads to low-field 29Si shifts. The 29Si shifts in combination with the linewidth information allow to identify the C 2S phases by 29Si NMR.

STUDIES OF 29Si SPIN-LATTICE RELAXATION TIMES AND PARAMAGNETIC IMPURITIES IN CLAY MINERALS BY MAGIC-ANGLE SPINNING 29Si-NMR AND EPR

Abstract High-resolution solid state 29Si-NMR spectra and EPR spectra have been examined on twelve natural philosilicates. It has been found that presence of paramagnetic impurities is markedly effective in shortening the 29Si spin-lattice relaxation times (10 to 1000 ms), and in broadening the linewidths and is favorable for the use of solid state high-resolution 29Si NMR as a tool for structural studies of inorganic materials.

Detection of imogolite in soils using solid state 29Si NMR

High resolution solid state 29Si-NMR has recently been used to examine silicates, and synthetic and natural zeolites1–4. This has been possible through the use of dipolar decoupling, magicangle spinning and cross-polarization techniques which have enabled line narrowing and also considerable signal enhancement for 29Si nuclei in close proximity to protons. It has been found that isotropic 29Si chemical shifts in solids and solutions are generally similar and depend mainly on the degree of condensation of silicon–oxygen tetrahedra1 For silicates, increasing condensation from single (Q0, −66 to −72 p.p.m.) to double tetrahedra (Q1, −78 to −82 p.p.m.), to chains (Q2, −86 to −88 p.p.m.) and cyclic layered structures (Q3, −107 to −109 p.p.m.) leads to increasing diamagnetic shielding. In solid aluminosilicates, the 29Si chemical shift is sensitive to the substitution of aluminium. A further important point was that the 29Si linewidth in aluminosilicates is sensitive to the regularity of aluminium distribution in the lattice. Thus 29Si-NMR has obvious potential for the elucidation of the coordination of silicon in soils and mineral components. We report here the first 29Si-NMR study of the clay mineral imogolite, and show that 29Si-NMR is an excellent analytical technique for the identification of imogolite (or imogolite like materials, that is proto-imogolite) in clay fractions. The observed chemical shift of imogolite (−78 p.p.m.) is consistent with the proposal that imogolite is a tubular hydrated aluminosilicate in which silicon tetrahedra are isolated by coordination through oxygen with three aluminium atoms and one proton.

Ordering of aluminium and silicon in synthetic faujasites

The distribution of aluminium and silicon atoms on the tetrahedral sites in synthetic faujasites (zeolites X and Y) has been debated for many years. Although the framework structure of the zeolite is known1, the detailed distribution of Si and Al in the lattice cannot be established by conventional methods, partly because of the very similar scattering powers of silicon and aluminium for X rays and partly because of the difficulties in obtaining crystals of adequate size and perfection. Some inferences have, however, been made on the basis of four indirect approaches: variation of the (cubic) lattice parameter as a function of aluminium content2,3; electrostatic calculations4; change of zeolitic acidity on successive extraction of aluminium from the framework5–7; and the kinetics of crystallization8. All these approaches have assumed the validity of ‘Loewenstein's rule’ which forbids Al atoms from occupying neighbouring tetrahedral sites. High-resolution solid-state 29Si NMR (with magic angle spinning) can determine the detailed Si, Al ordering in zeolites9–16 . NMR coupled with electron diffraction studies has revealed that Loewenstein's rule, generally treated as axiomatic, is broken in Linde A10,13,14 and in at least three other zeolites where Si/Al is nominally unity15. In view of its importance as an archetypal zeolite, particularly commercially, we have re-examined the question of framework ordering in faujasite. We propose new ordering schemes and explain discontinuities in the unit cell constant as a function of composition.