29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

Synthesis,Structure and Electrochemical Properties of Acetamide- and Caprolactam-Containing Silicon Catecholates

Hexacoordinated heteroligand silicon catecholates, although being prospective as easily soluble compounds with high hydrolytic stability and diverse redox properties, have been insufficiently studied. The transesterification of 1-(trimethoxysilylmethyl)-2-oxohexahydroaze or N-methyl-N-(trimethoxysilylmethyl)acetamide by two equivalents of catechol derivatives in the presence of dicyclohexylamine afforded a series of target compounds in good yield. The complexes were characterized using elemental analysis, FTIR, ¹H, ¹³C and ²⁹Si NMR spectra, X-ray crystallography and cyclic voltammetry. X-ray diffraction confirmed that the silicon atom possesses the octahedral geometry of the SiCO₅ polyhedron that remains unchanged in solution as it follows from ²⁹Si NMR data. The compounds demonstrated up to three oxidation waves; and the reduction profile strongly depended on the nature of the substituents on a catecholate anion.     

Synthesis of oligosiloxanes with 3-aminopropyl groups and their testing as surfactants in the preparation of polymer microspheres

Polydimethyl-co-methyl(3-aminopropyl)siloxanes differing in the content of amino groups and molecular weight were synthesized by two pathways, namely, by hydrosilylation of oligodimethyl-co-methylhydrоsiloxane with N-(trimethylsilyl)allylamine (TMSAA) and by copolymerization of octamethylcyclotetrasiloxane with methyl(3-aminopropyl)cyclosiloxanes. The colloid-chemical properties of the synthesized oligomers were studied. The oligosiloxanes possess high surface activity and can reduce the interfacial tension in the interface surfactant solution in toluene–water to 6.7–4.2 mJ m². Aggregatively stable polystyrene suspensions with particle of 0.5 and 0.8 μm in diameter were obtained in the presence of the synthesized oligomers.     

Synthesis and structure characterization of ladder-like polymethylsilsesquioxane (PMSQ) by isolation of stereoisomer

The stereo cyclic siloxane compounds having tetra hydroxyl and tetra methyl group were synthesized by hydrolysis of tetrahydrido tetramethyl cyclosiloxane consisting of four stereoisomers. Among the stereo cyclic isomers, cis-trans-cis tetrahydroxyl tetramethyl cyclosiloxane (2) was separated by recrystallization and was used as a monomer species for preparation of ladder-like polymethylsilsesquioxane (PMSQ). The isolated isomer was directly polymerized in THF with potassium carbonate (K₂CO₃) by systematic ring condensation. The hydrolyzed stereoisomers were characterized by ¹H and ²⁹Si NMR, and HPLC. The structure of the synthesized PMSQs were characterized by size exclusion chromatography (SEC), MALDI-TOF mass, ²⁹Si NMR, and small angle X-ray scattering (SAXS).     

Minimally adhesive polymer surfaces prepared from star oligosiloxanes and star oligofluorosiloxanes

The effects of the surface energy, storage modulus (G′), and glass‐transition temperature (T_g) on the biofouling behavior of siloxane and fluorosiloxane polymer surfaces (films) were studied. Irregular Si? H‐terminated tetrabranched star oligosiloxanes and star oligofluorosiloxanes were prepared by the acid‐catalyzed equilibration of octamethylcyclotetrasiloxane or 1,3,5‐trimethyl‐1,3,5‐tris(3′,3′,3′‐trifluoropropyl)cyclotrisiloxane with tetrakis(dimethylsiloxy)silane, respectively. Terminal epoxy groups were introduced via Pt‐catalyzed hydrosilylation with allyl glycidyl ether to yield compounds that were subsequently crosslinked with α,ω‐bis(3‐aminopropyl)poly(dimethylsiloxane). The resulting films were characterized by goniometry, dynamic mechanical thermal analysis, and thermogravimetric analysis. The foul‐release behavior was studied by the measurement of how strongly sporelings (young plants) of the green seaweed Ulva adhered. The corrosion protection of aluminum was evaluated by electrochemical impedance spectroscopy. Fluorosiloxane films displayed higher G′ and T_g values, decreased contact angles (with water), and more effectively released Ulva sporelings in comparison with siloxane films. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2551–2566, 2006     

Effect of the linear siloxane chain in cyclic silsesquioxane (CSSQ) on the mechanical/electrical property of the thin films

Several kinds of cyclic silsesquioxane (CSSQ) precursors containing linear siloxane chain were prepared to improve both the mechanical properties of their thin films and the compatibility with heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (tCD) as a porogen. The precursors were synthesized using a hydrolysis/condensation reaction with 2,4,6,8-tetramethyl-2,4,6,8-tetra (trimethoxysilylethyl) cyclotetrasiloxane (cyclic monomer) and three kinds of linear siloxane monomers. As the linear siloxane chain length increases in the CSSQ precursors, the compatibility between the CSSQ precursor and tCD molecules improved due to the chain flexibility of the precursor. Moreover, the mechanical strength of the CSSQ precursor (4ST37) containing linear tetrasiloxane was the best among the prepared precursors. The enhancement of mechanical property might also be attributed to the content of Si-OH groups as well as the chain flexibility, which could help the crosslinking reaction of Si-OH groups in the film curing process.     

Microwave spectroscopy and nuclear magnetic resonance study of the molecular geometry of 2-fluorophenylisocyanate

Analysis of the microwave spectra of the vibrational ground state of 2-fluorophenyl-isocyanate in the gaseous state shows the existence of a single planar cis conformer. Rotational spectra of the excited torsional states out of the plane and spectra of in-plane vibrations have also been studied. MINDO III type theoretical calculations corroborate the experimental results. Similarly, the configuration of 2-fluorophenylisocyanate in the liquid phase has been studied by NMR using the chemical shift reagent, Eu(fod)₃. Induced paramagnetic shifts have been measured on the ¹³C spectra, for different quantities of the reagent. The results clearly show a higher paramagnetic shift for the carbon bonded to the fluorine atom than for the other nuclei of the aromatic cycle (with the exception of the quaternary carbon), indicating the probable presence of the cis isomer.     

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.     

The stereochemistry of 2,4-disubstituted γ-butyrolactones and 2,4,6-trisubstituted-5,6-dihydro-4H-1, 3-oxazines

2,4-Disubstituted butyrolactones and 2,4,6-trisubstituted-5,6 dihydro-4H-1,3-oxazines show similar features in their ¹H and ¹³C- NMR spectra. Two geminal ring hydrogens of cis isomers give rise to a complex ABXY spectra when the substituent is alkyl or aryl. In spectra of trans isomers these patterns are degenerated. When R is OMe(in 4) or OCOMe (in 6) the difference in chemical shifts of geminal protons and vicinal coupling constants cannot be used for diagnosis. In ¹³C spectra ring carbons C-2 and C-3 in lactones and C-4 and C-5 in oxazine of trans isomers show a small but consistent shift to higher fields.     

Synthesis, structural characterization and electrochemical study of 1,1′-ferrocenylene labeled amino acids

Unsymmetrical 1,1′-disubstituted ferrocenes bearing an amino acid moiety and a conjugated electron density controlling substituent were synthesized conveniently starting from 1,1′-ferrocenedicarbaldehyde. The novel ferrocene amino acid derivatives were completely characterized from their MS, ¹H NMR and ¹³C NMR spectra. Their electrochemical behavior was studied by cyclic voltammetry. Their formal redox potentials E_f were slightly influenced by the nature of the amino acid and mainly by the kind of the ethenyl substituent. Furthermore all the (Z)-isomers exhibited a slight anodic shift compared with the corresponding (E)-isomers.     

Influence of ligand donor-acceptor properties on the structure,optical, and electrochemical characteristics of Ir(III) complexes with cyclometallated 2-phenylbenzotiazole

The cis-C,C-structure of a series of cyclometallated Ir(III) complexes with 2-phenylbenzotiazole was determined by the methods of X-ray diffraction (XRD) analysis, ¹H, ¹³C, ³¹P NMR, and IR spectroscopy. Long-wave absorption bands, phosphorescence, and processes of electrochemical oxidation and reduction of the complexes were assigned to the transfer of electrons presumably located on d _π and π* orbitals of cyclometallated 2-phenylbenzotiazole. The decrease in donor and acceptor properties of the ligands leads to the blue shift of absorption spectra and phosphorescence and to the anodic shift of oxidation and reduction potentials of the complexes.     

Optimization of the conditions of acid hydrolysis of tetraethoxysilane and identification of primary products of sol-gel synthesis

The effect of the reactant ratio in the Si(OEt)₄-H₂O-EtOH-HCl system on the hydrolytic polycondensation of tetraethoxysilane and gelation rate was examined. A mass-spectrometric study showed that the primary products of the sol-gel process are linear and cyclic oligosiloxanes containing hydroxy and ethoxy groups at the silicon atoms.     

Polymerization behavior and gel properties of ethane,ethylene and acetylene-bridged polysilsesquioxanes

Soluble bridged polysilsesquioxanes with a range of molecular weight were synthesized from bis(triethoxysilyl)ethane, ethylene, and acetylene (BTES-E1, -E2, and -E3) via hydrolysis and polycondensation reaction by adjusting the water amount. Polymerization behavior of these three trialkoxysilanes was investigated by monitoring the reaction progress by GPC, and ²⁹Si NMR spectrometry of the resulting polymers, poly(BTES-E1), poly(BTES-E2), and poly(BTES-E3), showing that BTES-E1 generated cyclic oligomers at the early stage. In contrast, polymerization of BTES-E2 and BTES-E3 provided no detectable amounts of cyclic oligomers, but afforded linear polymers only. Bulk gels were also prepared by curing the polymers. The gel from poly(BTES-E3) exhibited high thermal stability derived from the rigid acetylene spacer with respect to thermogravimetric analysis. On the other hand, the polymer film of BTES-E1 showed the highest pencil hardness index among the polymers, indicating the tight siloxane network of poly(BTES-E1).     

Conformational analysis of cyclic phosphates derived from 5-C′ substituted 1,2-O-isopropylidene-α-d-xylofuranose derivatives

Twelve 2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes fused with a 1,2-O-isopropylidene-α-d-xylofuranose moiety in cis orientation and substituted at the C′5 position were prepared in two steps from commercially available diacetone-α-d-glucose. Their conformations, and configurations were determined by ¹H and ³¹P NMR and X-ray crystallographic techniques. Both, chair-twisted-chair and chair-boat equilibria were observed in solution. We observed that the strong anisotropic shielding effect of the benzene ring in the phenoxy group generates an upfield shift of the H¹ hydrogen atom, when the cyclic phosphates adopt a boat conformation. This is due to a relative cis-orientation of the P-phenoxy group and the H¹ proton of the 1,2-O-isopropylidene-α-d-xylofuranose moiety. Therefore, the configuration of the phosphorus center (S_P or R_P) can be determined by ¹H NMR spectroscopy. Interestingly, the crystal structure of one of the cyclic phosphates exhibits two independent molecules in the asymmetric unit, one with a chair and the other one with a boat conformation.     

Synthesis and NMR spectra of the syn and anti isomers of substituted cyclobutanes—evidence for steric and spatial hyperconjugative interactions

The syn and anti isomers of cis,cis-tricyclo[5.3.0.0^2,6]dec-3-ene derivatives have been synthesized and their ¹H and ¹³C NMR spectra unequivocally analyzed. Both their structures and their ¹H and ¹³C NMR chemical shifts were calculated by DFT, the latter two calculations employing the GIAO perturbation method. Additionally, calculated NMR shielding values were partitioned into Lewis and non-Lewis contributions from the bonds and lone pairs involved in the molecules by accompanying NBO and NCS analyses. The differences between the syn and anti isomers were evaluated with respect to steric and spatial hyperconjugation interactions.     

13C NMR spectroscopic study of epoxidized 1,4-polyisoprene and 1,4-polybutadiene

Partly epoxidized cis- and trans-1,4-polyisoprenes and cis-and trans-1,4-polybutadienes were prepared, and their ¹³C NMR spectra examined. All the prominent resonances in the spectra of the epoxidized polymers were assigned by using lanthanide shift reagent and off-resonance decoupling experiments. A ¹³C NMR method of quantitative assessment of the epoxide content was developed following determination of relative spin-lattice relaxation time (T₁) and nuclear Overhauser effect (NOE) parameters of the various carbons in the epoxidized polyisoprenes and polybutadienes.     

Studies on the stereochemistry of 1,2,6-trimethyl-4-piperidone

In the effort to create new derivatives of analgesically active spiropiperidines intermediate 1,2,6-trimethyl-4-piperidone was synthesized. The substitution of the skeleton gives rise to configurational as well as conformational isomerism. Despite the symmetry of 1,2,6-trimethyl-4-piperidone two different sets of signals were present in the ¹H and ¹³C NMR spectra. They were supposed to arise from a cis/trans mixture of 1,2,6-trimethyl-4-piperidone. In contrast to this explanation only two signals of the methyl groups and hydrogens at carbon atoms 2 and 6 were observed in the ¹H and ¹³C NMR spectra, normally expecting one for the cis- and two for the trans-isomer. To solve this discrepancy, the kind of isomeric mixture of 1,2,6-trimethyl-4-piperidone leading to the ¹H and ¹³C NMR spectra was examined. Energy differences between chair conformations of both the cis- and the trans-isomer of 1,2,6-trimethyl-4-piperidone and the potential energy surface of the equilibration process of the trans-isomer of 1,2,6-trimethyl-4-piperidone between its chair conformers were determined by quantum chemical calculations. The barrier height of the equilibration process was measured by high and low temperature NMR measurements to confirm the theoretical outcome. The results of all investigations agree nicely and proved a cis-/trans-mixture of 1,2,6-trimethyl-4-piperidone being present at room temperature.     

Lithium monosilicide (LiSi), a low-dimensional silicon-based material prepared by high pressure synthesis: NMR and vibrational spectroscopy and electrical properties characterization

Lithium monosilicide (LiSi) was formed at high pressures and high temperatures (1.0-2.5 GPa and 500-700°C) in a piston-cylinder apparatus. This compound was previously shown to have an unusual structure based on 3-fold coordinated silicon atoms arranged into interpenetrating sheets. In the present investigation, lowered synthesis pressures permitted recovery of large (150-200 mg) quantities of sample for structural studies via NMR spectroscopy (²⁹Si and ⁷Li), Raman spectroscopy and electrical conductivity measurements. The ²⁹Si chemical shift occurs at −106.5 ppm, intermediate between SiH₄ and Si(Si(CH₃)₃)₄, but lies off the trend established by the other alkali monosilicides (NaSi, KSi, RbSi, CsSi), that contain isolated Si₄⁴⁻ anions. Raman spectra show a strong peak at 508 cm⁻¹ due to symmetric Si-Si stretching vibrations, at lower frequency than for tetrahedrally coordinated Si frameworks, due to the longer Si-Si bonds in the 3-coordinated silicide. Higher frequency vibrations occur due to asymmetric stretching. Electrical conductivity measurements indicate LiSi is a narrow-gap semiconductor (E_b∼0.057 eV). There is a rapid increase in conductivity above T=450 K, that might be due to the onset of Li⁺ mobility.     

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.     

The Family of Ferrocene‐Stabilized Silylium Ions: Synthesis, 29Si NMR Characterization,Lewis Acidity,Substituent Scrambling,and Quantum‐Chemical Analyses

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene‐stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the ²⁹Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene‐stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear ¹H and ²⁹Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene‐bridged intermediate. Moreover, the quantum‐chemical analysis of the ²⁹Si NMR chemical shifts revealed a linear relationship between δ(²⁹Si) values and the Fe???Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three‐center two‐electron bonding attractor between the iron, silicon, and C′_ipso atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between ²⁹Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non‐intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe???Si interaction.