Structure investigations of solid organosilicon polymers by high resolution solid state 29Si NMR

The high resolution ²⁹Si NMR spectra of five solid silicon polymers of different structure have been studied and the ²⁹Si chemical shifts of characteristic structure units determined. ²⁹Si¹H cross-polarization in combination with high speed magic angle sample spinning and high power proton decoupling was used to achieve high resolution in the solid state spectra. Comparison of the latter with the results obtained in the liquid state clearly indicates that no special solid state effects on ²⁹Si chemical shifts arise and the relations between δ(Si) and the molecular structure, well known from investigations of liquids, can be used for interpretation of the solid state spectra. It is shown that high resolution solid state ²⁹Si NMR spectroscopy offers detailed information about the structural units of the siloxane resin framework, and this opens up new possibilities for structural determinations of solid organosilicon polymers.     

Hydrogen bonding and conformational effects on13 chemical shifts of hydroxybenzaldehydes in the solid state

In high resolution solid-state CP/MAS¹³C NMR spectra of several hydroxybenzaldehydes, the downfield shifts due to hydrogen bonding for the vary inversely with the O...O hydrgen-bonddistances. Conformations of the aldehyde groups in the solid state are determined by the chemical shifts of the ortho carbons.     

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.     

Synthesis and Characterization of Base‐catalyzed Phenyl Modified PDMS/PHMS Copolymers

A series of phenyl modified polydimethylsiloxane (PDMS) / polyhydrogenmethylsiloxane (PHMS) random copolymers containing both internal Si‐H and terminal SiH₂ and T (MeSiO_3/2) units was synthesized in one step through n‐BuLi‐catalyzed ring‐opening polymerization of cyclic comonomers and characterized by GPC, IR and ¹H and ²⁹Si NMR. Sequential microstructures of these copolymers were determined by ²⁹Si‐NMR spectroscopy. Epoxy‐modified polysiloxanes were prepared and used as comparable standards for the assignment of the NMR spectra. A hydride‐transfer mechanism has been proposed to account for the formation of terminal Si‐H and T group. Detailed sequential analyses and chemical shifts of ²⁹Si‐NMR for various siloxane units are reported for the first time.     

29Si-NMR-Untersuchungen zur Anionenstruktur von Kristallinen Tetramethylammonium-alumosilicaten und -alumosilicatlösungen

²⁹Si NMR Investigations on the Anion Structure of Crystalline Tetramethylammoniumaluminosilicates and -aluminosilicate Solutions The ²⁹Si NMR spectra of crystalline tetramethylammonium (TMA) aluminosilicates with different Si/Al ratios exhibit up to 4 sharp signals with characteristic chemical shifts which can be assigned to the central Si atom of OSi(OSi)_3?n(OAl)_n building units of double four-ring (DFR) aluminosilicate anions. The number and distributions of the Al atoms in the DFR framework can be derived from the signal intensities in connection with the results of the trimethylsilylation method [1]. A good agreement of the results of both methods has been found. The DFR can exist as monomeric unit or can be connected to polymeric structures by SiOAl bridges, but no information can be obtained about this question by the ²⁹Si NMR spectra. The investigation of the TMA aluminosilicate solutions by ²⁹Si NMR and TMS method [1] show that stable aluminosilicate anions exist in these solutions. The structure of these aluminosilicate anions is different from the structure of the crystalline TMA aluminosilicates obtained from the solutions.     

Solid‐state NMR characterization of oxygen sites in organically modified aluminosilicate xerogels

Solid‐state NMR characterization of hybrid aluminosilicate xerogels, by ¹⁷O magic angle spinning (MAS) and triple quantum magic angle spinning (MQMAS) techniques, evidences Si—O—Si and Si—O—Al oxygen sites, spectrally separated in MQMAS experiments. Inversion of the MQMAS spectra allows the measurement of quadrupolar parameters, isotropic chemical shifts, distribution of chemical shift and discussion of the mobility of the structural units. Copyright © 2003 John Wiley & Sons, Ltd.     

Solid‐state 13C, 15N and 29Si NMR characterization of block copolymers with CO2 capture properties

Natural abundance solid‐state multinuclear (¹³C, ¹⁵N and ²⁹Si) cross‐polarization magic‐angle‐spinning NMR was used to study structures of three block copolymers based on polyamide and dimethylsiloxane and two polyamides, one of which including ferrocene in its structure. Assignment of most of the resonance lines in ¹³C, ¹⁵N and ²⁹Si cross‐polarization magic‐angle‐spinning NMR spectra were suggested. A comparative analysis of ¹³C isotropic chemical shifts of polyamides with and without ferrocene has revealed a systematic shift towards higher δ ‐values (de‐shielding) explained as the incorporation of paramagnetic ferrocene into the polyamide backbone. In addition, the ¹³C NMR resonance lines for ferrocene‐based polyamide were significantly broadened, because of paramagnetic effects from ferrocene incorporated in the structure of this polyamide polymer. Single resonance lines with chemical shifts ranging from 88.1 to 91.5 ppm were observed for ¹⁵N sites in all of studied polyamide samples. ²⁹Si chemical shifts were found to be around ?22.4 ppm in polydimethylsiloxane samples that falls in the range of chemical shifts for alkylsiloxane compounds. The CO₂ capture performance of polyamide‐dimethylsiloxane‐based block copolymers was measured as a function of temperature and pressure. The data revealed that these polymeric materials have potential to uptake CO₂ (up to 9.6 cm³ g^?1) at ambient pressures and in the temperature interval 30–40 °C. Copyright © 2016 John Wiley & Sons, Ltd.     

Charged Si9 Clusters in Neat Solids and the Detection of [H2Si9]2− in Solution: A Combined NMR,Raman, Mass Spectrometric,and Quantum Chemical Investigation

Polyanionic silicon clusters are provided by the Zintl phases K₄Si₄, comprising [Si₄]⁴⁻ units, and K₁₂Si₁₇, consisting of [Si₄]⁴⁻ and [Si₉]⁴⁻ clusters. A combination of solid‐state MAS‐NMR, solution NMR, and Raman spectroscopy, electrospray ionization mass spectrometry, and quantum‐chemical investigations was used to investigate four‐ and nine‐atomic silicon Zintl clusters in neat solids and solution. The results were compared to ²⁹Si isotope‐enriched samples. ²⁹Si‐MAS NMR and Raman shifts of the phase‐pure solids K₄Si₄ and K₁₂Si₁₇ were interpreted by quantum‐chemical calculations. Extraction of [Si₉]⁴⁻ clusters from K₁₂Si₁₇ with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si₉ clusters. This compound was characterized by elemental and EDX analyses and ²⁹Si‐MAS NMR and Raman spectroscopy. Charged Si₉ clusters were detected by ²⁹Si NMR in solution. ²⁹Si and ¹H NMR spectra reveal the presence of the [H₂Si₉]²⁻ cluster anion in solution.     

Charged Si9 Clusters in Neat Solids and the Detection of [H2Si9]2− in Solution: A Combined NMR,Raman, Mass Spectrometric,and Quantum Chemical Investigation

Polyanionic silicon clusters are provided by the Zintl phases K₄Si₄, comprising [Si₄]⁴⁻ units, and K₁₂Si₁₇, consisting of [Si₄]⁴⁻ and [Si₉]⁴⁻ clusters. A combination of solid‐state MAS‐NMR, solution NMR, and Raman spectroscopy, electrospray ionization mass spectrometry, and quantum‐chemical investigations was used to investigate four‐ and nine‐atomic silicon Zintl clusters in neat solids and solution. The results were compared to ²⁹Si isotope‐enriched samples. ²⁹Si‐MAS NMR and Raman shifts of the phase‐pure solids K₄Si₄ and K₁₂Si₁₇ were interpreted by quantum‐chemical calculations. Extraction of [Si₉]⁴⁻ clusters from K₁₂Si₁₇ with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si₉ clusters. This compound was characterized by elemental and EDX analyses and ²⁹Si‐MAS NMR and Raman spectroscopy. Charged Si₉ clusters were detected by ²⁹Si NMR in solution. ²⁹Si and ¹H NMR spectra reveal the presence of the [H₂Si₉]²⁻ cluster anion in solution.     

Trimethylsilylation - Aid in NMR Analysis of Oligosaccharides. Assigment of 29Si and 13C NMR Spectra of Trimethylsilylated Methyl β-D-Xylobiosides by 2D NMR

The ¹H, ¹³C and ²⁹Si NMR spectra of methyl β-D-xylopyranoside and three methyl β-D-xylopyranosyl-β-D-xylopyranosides have been measured and assigned by two-dimensional NMR spectroscopy. According to the determined proton-proton coupling constants, the ring conformer ratio is essential ly the same in the studied compounds. The assigned chemical shifts provide correct substituent chemical shifts for assignments in the spectra of higher trimethylsilylated xylooligosaccharides. Heteronuclear chemical shift correlated 2D NMR spectroscopy is proven to be a usable experimental method for ²⁹Si NMR line assignment in carbohydrates. The assigned silicon shifts identify the site of glycosidation.     

High-Resolution Solid-State 13C-NMR Spectroscopy of Polymers [New Analytical Methods (37)]

Until a few years ago, solid-state nuclear resonance yielded spectra containing broad lines only. Meanwhile, CP/MAS-NMR spectroscopy has provided a method which gives narrow nuclear resonance lines from a solid-state specimen as well. Using this technique, it is now possible to produce spectra of “rare” nuclei (¹³C, ²⁹Si, ¹⁵N etc.) which are resolved in terms of chemical structure. The analytical capabilities of NMR spectroscopy can be applied to the solid state: it may be that it is necessary to identify compounds in the solid state because, for example, a solvent would alter the coordination sphere, or that it is desired to monitor chemical reactions in the solid state, for example the baking of an enamel. Where a substance in the solid state is concerned, high-resolution ¹³C-NMR spectroscopy provides not only information about the chemical structure, but also about the solid state itself. To mention just a few examples, information on the conformation, crystal structure and molecular dynamics, as well as molecular miscibility is given. This opens up a broad spectrum of applications, from a statement concerning the crystal modification of an active substance in ready-to-use pharmaceutical preparations, e.g. tablets, to the question of whether two polymers are miscible with one another at a molecular level.     

29Si-NMR-Untersuchungen zur Verteilung der Silicium-und Aluminiumatome im Alumosilicatgitter von Zeolithen mit Faujasit-Struktur

²⁹SiNMR Investigation of Silicon-Aluminum Ordering in the Aluminosilicate Framework of Faujasite-Type Zeolites The high resolution magic angle spinning ²⁹Si NMR spectra of a series of NaX and NaY zeolites with Si/Al ratios of 1.18 to 67 exhibit up to five sharp signals which could be assigned to the central silicon atoms of Si(OSi)_4–n(OAl)_n building units (n = 0–4) of the aluminosilicate framework. From the signal intensities the quantitative distribution of the building units and the Si/Al ratio of the aluminosilicate lattice have been estimated. By comparison of the building units obtained from the ²⁹Si NMR spectra with those from theoretical model structures detailed information on silicon-aluminum ordering of the zeolite framework has been derived. Except for NaX of Si/Al = 1.4 a centrosymmetrical distribution of Si and Al atoms within a double-cubooctahedra unit has been found which agrees well with the Si/Al ordering scheme proposed by Dempsey.     

29Si and 17O NMR investigations on Si−O−Ti bonds in solutions of diphenylsilanediol and titanium-tetra-isopropoxide

The structural units in diphenylsilanediol/titanium-isopropoxide solutions with molar ratio Si:Ti between 1:0.1 and 1:5 were examined by means of ²⁹Si and ¹⁷O NMR. The main component in solutions with molar ratio Si/Ti=1:0.1 is the chain-like octaphenyltetrasiloxanediol. With increasing Ti-isopropoxide content (1:0.25–1:05) Si–O–Ti units of the spirocyclic titanosiloxane Ti[O₅Si₄(C₆H₅)₈]₂ prevail in the solutions accompanied by the chain-like tetrasiloxanediol. The ²⁹Si NMR spectra of 1:1 solutions indicate a lot of different Si containing building units with chemical shifts mainly between-40 and-46 ppm. The signals with a chemical shift between-40 and-46 ppm are probably caused by Si atoms which are connected via oxygen bridges directly (Si–O–Ti) or indirectly (Si–O–Si–O–Ti) with titanium. Contrary to the 1:1 solutions only one or two different species with Si–O–Ti units are present in high Ti-alkoxide containing solutions (1:5). ²⁹Si and ¹⁷O NMR results reveal a quick hydrolysis of the Ti–O–Si bonds to titanium-oxo-hydroxo-polymers and phenylsiloxanediols or their isopropyl esters after the addition of water to the solutions. This separation into species only containing either Ti–O–Ti or Si–O–Si bonds can entail a decreased homogeneity of the reaction products on a molecular level.     

Atomic Contributions from Spin‐Orbit Coupling to 29Si NMR Chemical Shifts in Metallasilatrane Complexes

New members of a novel class of metallasilatrane complexes [X‐Si‐(μ‐mt)₄‐M‐Y], with M=Ni, Pd, Pt, X=F, Cl, Y=Cl, Br, I, and mt=2‐mercapto‐1‐methylimidazolide, have been synthesized and characterized structurally by X‐ray diffraction and by ²⁹Si solid‐state NMR. Spin‐orbit (SO) effects on the ²⁹Si chemical shifts induced by the metal, by the sulfur atoms in the ligand, and by heavy halide ligands Y=Cl, Br, I were investigated with the help of relativistic density functional calculations. Operators used in the calculations were constructed such that SO coupling can selectively be switched off for certain atoms. The unexpectedly large SO effects on the ²⁹Si shielding in the Ni complex with X=Y=Cl reported recently originate directly from the Ni atom, not from other moderately heavy atoms in the complex. With respect to Pd, SO effects are amplified for Ni owing to its smaller ligand‐field splitting, despite the smaller nuclear charge. In the X=Cl, Y=Cl, Br, I series of complexes the Y ligand strongly modulates the ²⁹Si shift by amplifying or suppressing the metal SO effects. The pronounced delocalization of the partially covalent M←Y bond plays an important role in modulating the ²⁹Si shielding. We also demonstrate an influence from the X ligand on the ²⁹Si SO shielding contributions originating at Y. The NMR spectra for [X‐Si‐(μ‐mt)₄‐M‐Y] must be interpreted mainly based on electronic and relativistic effects, rather than structural differences between the complexes. The results highlight the sometimes unintuitive role of SO coupling in NMR spectra of complexes containing heavy atoms.     

29Si NMR spectra of methylphenylcyclotri-and -tetrasilazanes

²⁹Si NMR spectra of various methylphenyl-substituted cyclotri- and -tetrasilazanes, composed of different combinations of the units d = Me₂SiNH, d^ph = MePhSiNH and d = Ph₂SiNH, have been investigated. For most of the compounds having more than one asymmetric centre, the spectra of both the individual isomers and their mixtures have been studied, and the signals of the individual isomers have been assigned. The effect of a different arrangement of phenyl groups in the rings, and the influence of the stereochemistry of the isomers on the ²⁹Si chemical shifts in cyclotri- and -tetrasilazanes have been studied. The ²⁹Si chemical shifts in cyclotrisilazanes are additive and can be represented as the sum of increments corresponding to the number and positions of the phenyl groups in the rings. In these compounds, the spatial structure of the isomers does not exert any noticeable effect on the spectra. On the contrary, chemical shifts are not simply additive in cyclotetrasilazanes: a pronounced stereochemical dependence is apparent.     

High magnetic field solid‐state NMR analyses by combining MAS,MQ‐MAS,homo‐nuclear and hetero‐nuclear correlation experiments

A general strategy of structural analysis of alumina silicate by combining various solid‐state NMR measurements such as single pulse, multi‐quantum magic angle spinning, double‐quantum homo‐nuclear correlation under magic angle spinning (DQ‐MAS), and cross‐polarization hetero‐nuclear correlation (CP‐HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for ¹H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of ²⁷Al in single pulse, DQ‐MAS, and even in triple‐quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ‐MAS methods for homo‐nuclear correlations between both ²⁷Al–²⁷Al and ²⁹Si–²⁹Si and by CP‐HETCOR for hetero‐nuclear correlations between ²⁷Al–²⁹Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina‐silicate minerals. Copyright © 2012 John Wiley & Sons, Ltd.     

7Li and 29Si solid state NMR and chemical bonding of La2Li2Si3

The ternary silicide La₂Li₂Si₃ was synthesized from the elements in a sealed niobium tube. La₂Li₂Si₃ was characterized by powder and single crystal X-ray diffraction: Ce₂Li₂Ge₃ type, Cmcm, a = 450.03(8), b = 1880.3(4), c = 689.6(1) pm, wR2 = 0.0178, 597 F² values, and 26 parameters. The La₂Li₂Si₃ structure contains two crystallographically independent silicon sites, both in slightly distorted trigonal prismatic coordination. The Si1 atoms are located in condensed La₆ prisms and form cis–trans chains (two-bonded silicon) with Si1–Si1 distances at 238 and 239 pm, indicating single bond character. The Si2 atoms are isolated within La₂Li₄ prisms. La₂Li₂Si₃ might be formally considered as an electron precise Zintl phase with an electron partition (2La³⁺)(2Li⁺)(2Si1^2–)(Si2^4–). Electronic structure calculations show a trend in this direction based on a charge density analysis with large electron localization around the Si1–Si1 chains. The compound is found weakly metallic with chemical bonding reminiscent of LaSi and additional features brought in by Li and Si2. High resolution solid state ⁷Li and ²⁹Si MAS-NMR spectra are in agreement with the crystal structural information, however, the ²⁹Si resonance shifts observed suggest strong Knight shift contributions, at variance with the Zintl concept. Variable temperature solid state ⁷Li spectra indicate the absence of motional narrowing on the kHz timescale within the temperature range 300K < T < 400 K.     

The structure of phosphate and borosilicate glasses and their structural evolution at high temperatures as studied with solid state NMR spectroscopy: Phase separation,crystallisation and dynamic species exchange

In this contribution we present an in-depth study of the network structure of different phosphate based and borosilicate glasses and its evolution at high temperatures. Employing a range of advanced solid state NMR methodologies, complemented by the results of XPS, the structural motifs on short and intermediate length scales are identified. For the phosphate based glasses, at temperatures above the glass transition temperature T_g, structural relaxation processes and the devitrification of the glasses were monitored in situ employing MAS NMR spectroscopy and X-ray diffraction. Dynamic species exchange involving rapid P–O–P and P–O–Al bond breaking and reforming was observed employing in situ ²⁷Al and ³¹P MAS NMR spectroscopy and could be linked to viscous flow. For the borosilicate glasses, an atomic scale investigation of the phase separation processes was possible in a combined effort of ex situ NMR studies on glass samples with different thermal histories and in situ NMR studies using high temperature MAS NMR spectroscopy including ¹¹B MAS, ²⁹Si MAS and in situ ²⁹Si{¹¹B} REAPDOR NMR spectroscopy.     

Spectroscopic study on chemical structure of plasma-polymerized hexamethyldisiloxane

A plasma-polymerized material was produced from hexamethyldisiloxane vapor by a glow discharge polymerization technique. Spectroscopic interpretation of the chemical structure of the polymerized hexamethyldisiloxane was studied by spectroscopic means such as IR, XPS, and NMR. The plasma polymer was barely soluble in the usual organic solvents, although it contained a small amount of the monomer and its oligomers. The IR spectrum indicated that the polymer consisted of Si? CH₃, Si? O, Si? CH₂, and Si? H groups. The surface of the polymer was found to retain structural units similar to the monomer from the XPS measurement. On the other hand, the ¹³C and ²⁹Si high-resolution, solid-state NMR measurements revealed that the plasma polymer was highly crosslinked with a variety of conformations and a number of O atoms surrounding a Si atom. Results from the XPS and NMR spectra suggested that the bulk of the polymer was more oxidized than the surface layer; Si atom was preferentially oxidized. A hypothetical chemical structure was proposed for the polymerized hexamethyl-disiloxane.     

Conformations and 13C NMR chemical shifts of some polyamides in the solid state as studied by high-resolution 13C NMR spectroscopy

High-resolution ¹³Carbon nuclear magnetic resonance (NMR) spectra of Nylons 4, 6, and 66 in the solid state were measured over a wide range of temperature. From the results, it was found that resonance lines of crystalline and noncrystalline components were separable and their chemical shifts were determined. The ¹³C chemical shift behavior is closely related to their conformation. The origin of the conformational effects on the chemical shifts is discussed.