The Experts below are selected from a list of 147 Experts worldwide ranked by ideXlab platform
Piotr Tekely - One of the best experts on this subject based on the ideXlab platform.
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Exploiting 1H→29Si Cross-Polarization Features for Structural Characterization of Inorganic Materials
ChemInform, 2008Co-Authors: Piotr TekelyAbstract:High-resolution solid state 29Si NMR spectroscopy was frequently used during the last 25 years in structural studies of inorganic materials including zeolites [1–3], minerals [1–3], glasses [3], and cement-based systems [4]. Its important place in structure determination of these materials relies on the fact that 29Si NMR spectra permit a precise determination of the 29Si isotropic chemical shift in different silicon environments of powdered samples. Indeed, for materials with silicon in tetrahedral coordination, the isotropic position of resonance signal provides immediately the degree of condensation of SiO4 terahedra (Q(n), with n = 0, 1, 2, 3, 4) and informs about Si–O–Si bond angles and Si–O bond lengths [1–3]. Although the easiest way to record the quantitative 29Si NMR Spectrum is a direct excitation by a single pulse, this cannot be reasonably applied in most cases due to extremely long 29Si longitudinal relaxation times. To avoid this inconvenience, one can take advantage of magnetization transfer from protons to 29Si spins. In spite of the fact, that when using cross-polarization (CP) procedure, the quantitative proportions of chemically or crystallographically inequivalent sites cannot be reached as with single pulse excitation, an extremely valuable structural and dynamic information can be obtained in this manner. However, to access such information, some basic precautions have to be taken. Indeed, when recording the CP spectra under MAS at spinning frequencies of the same order of magnitude as the dipolar coupling interaction involved in polarization transfer, two complications immediately arise. First, the Hartmann–Hahn (H–H) matching condition is split into several sidebandmatching positions (Figure 1a–d). Consequently, 1H→29Si CP occurs effectively only when the generalized H–H matching condition ω1H = nωr + ω1Si with n = 1, 2, . . . is satisfied. As it is difficult to achieve stable and reproducible results using mismatched H–H CP, because even very small (
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Determining the geometry of strongly hydrogen-bonded silanols in a layered hydrous silicate by solid-state nuclear magnetic resonance.
Journal of Chemical Physics, 2005Co-Authors: Carole Gardiennet, Florea Marica, Colin A. Fyfe, Piotr TekelyAbstract:High-resolution solid-state NMR spectroscopy is exploited to obtain structural constraints involving strongly hydrogen-bonded silanols in octosilicate, a prominent member of the layered hydrous sodium silicates. Proton-silicon cross-polarization dynamics reveals that octosilicate contains two types of Q3 silicons present in hydrogen-bonded –Si–O–H⋯O–Si– and –Si–O−–type sites which can only be distinguished by their different abilities to cross polarize and the different mobilities of neighboring hydrous species. The theoretical analysis of the oscillating components of the polarization transfer buildup curves suggests that the model of heteronuclear pairs is an adequate description of the quantum spin system within hydrogen-bonded –Si–O–H⋯O–Si– fragments. We also show that dipolar modulated, slow speed magic-angle 29Si NMR Spectrum provides unique geometric information on strongly hydrogen-bonded silanols. The dipolar modulated spinning sidebands contain all the information necessary to determine the inte...
Chaohui Ye - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and characterization of styrene-butadiene block copolymer/silicate hybrid materials via the sol-gel process I. Four-arm star styrene-butadiene block copolymer/silica hybrids
Polymer Journal, 1997Co-Authors: Weibin Chen, Hanqiao Feng, Chaohui YeAbstract:Novel hybrid materials incorporating four-arm star styrene-butadiene block copolymer (SB-4A) with tetraethoxysilane (TEOS) have been successfully prepared by a sol-gel process. The properties, microstructure, and dynamics of the hybrid materials were investigated in detail by means of dynamic mechanical analysis (DMA), thermogravimetry (TG), and NMR techniques. The results indicate that the properties of the hybrids strongly depend on the TOES content and the silica networks prefer to form in the polystyrene (PS) domains. There exists an optimum range of TEOS content, in this work 30–50% by weight, within which the properties of the hybrids obtained were well improved: storage modulus increases, thermal stability raises, and the high-temperature Tg shifts to higher temperature. 29Si NMR Spectrum shows that in the hybrids with well improved properties silica networks formed with Q4 in the greatest abundance.
Weibin Chen - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and characterization of styrene-butadiene block copolymer/silicate hybrid materials via the sol-gel process I. Four-arm star styrene-butadiene block copolymer/silica hybrids
Polymer Journal, 1997Co-Authors: Weibin Chen, Hanqiao Feng, Chaohui YeAbstract:Novel hybrid materials incorporating four-arm star styrene-butadiene block copolymer (SB-4A) with tetraethoxysilane (TEOS) have been successfully prepared by a sol-gel process. The properties, microstructure, and dynamics of the hybrid materials were investigated in detail by means of dynamic mechanical analysis (DMA), thermogravimetry (TG), and NMR techniques. The results indicate that the properties of the hybrids strongly depend on the TOES content and the silica networks prefer to form in the polystyrene (PS) domains. There exists an optimum range of TEOS content, in this work 30–50% by weight, within which the properties of the hybrids obtained were well improved: storage modulus increases, thermal stability raises, and the high-temperature Tg shifts to higher temperature. 29Si NMR Spectrum shows that in the hybrids with well improved properties silica networks formed with Q4 in the greatest abundance.
V V Lobanov - One of the best experts on this subject based on the ideXlab platform.
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quantum chemical calculation of 29Si NMR Spectrum of silicon dioxide fullerene like molecules
Himia Fizika ta Tehnologia Poverhni, 2015Co-Authors: O V Filonenko, V S Kuts, M I Terebinska, V V LobanovAbstract:29 Si NMR spectra of fullerene-like hollow oligomers of silicic acid were calculated by the density functional theory method using hybrid exchange-correlation functional B3LYP, 6-311+G(2d, p) basis set and GIAO procedure. It is shown that fullerene-like molecule (SiO 2 ) 20 (H 2 O) 10 gives an intensive signal in the range from –102 to –105 ppm.
Carole Gardiennet - One of the best experts on this subject based on the ideXlab platform.
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Determining the geometry of strongly hydrogen-bonded silanols in a layered hydrous silicate by solid-state nuclear magnetic resonance.
Journal of Chemical Physics, 2005Co-Authors: Carole Gardiennet, Florea Marica, Colin A. Fyfe, Piotr TekelyAbstract:High-resolution solid-state NMR spectroscopy is exploited to obtain structural constraints involving strongly hydrogen-bonded silanols in octosilicate, a prominent member of the layered hydrous sodium silicates. Proton-silicon cross-polarization dynamics reveals that octosilicate contains two types of Q3 silicons present in hydrogen-bonded –Si–O–H⋯O–Si– and –Si–O−–type sites which can only be distinguished by their different abilities to cross polarize and the different mobilities of neighboring hydrous species. The theoretical analysis of the oscillating components of the polarization transfer buildup curves suggests that the model of heteronuclear pairs is an adequate description of the quantum spin system within hydrogen-bonded –Si–O–H⋯O–Si– fragments. We also show that dipolar modulated, slow speed magic-angle 29Si NMR Spectrum provides unique geometric information on strongly hydrogen-bonded silanols. The dipolar modulated spinning sidebands contain all the information necessary to determine the inte...
