Quantification of hydroxylic groups in a river humic substance by 29Si-NMR

Quantification of the hydroxylic functions of carboxylic, phenolic and alcoholic groups in a selected river humic substance (Ruhr/Germany) is performed by methylation/silylation and ²⁹Si-NMR analysis. The well known methylation/silylation procedure of Thorn et al. is modified by carrying out the diazomethane methylation in the presence of the proton catalyst p-toluene sulphonic acid. Additional experiments by both ¹³C-NMR-spectroscopy and potentiometric acid-base titration confirm the results obtained by ²⁹Si-NMR.     

Zur synthese von siloxanen: III. Induktive substituetenkonstanten für siloxyreste; 29Si-NMR-spektroskopische untersuchungen

For a series of siloxy groups at the silicon atom the inductive substituent constants σ^* and σ_I have been determined by means of ²⁹Si NMR spectroscopy.     

Beiträge zur Chemie der Silicium-Schwefel-Verbindungen. 46 [1]. 29Si-NMR-chemische Verschiebungen von Trialkoxysilylthioderivaten der Permethylpolysilane

Contributions to the Chemistry of Silicon-Sulphur Compounds. 46. ²⁹Si-N.M.R. Chemical Shifts of Trialkoxysilylthio Derivatives of Permethylpolysilanes ²⁹Si-N.M.R. chemical shifts of trialkoxysilythio derivatives of permethylpolysilanes of the two series: α, ω-(RO)₃SiS(SiMe₂)_nSSi(OR)₃, n = 2, 3, 4, 6 and 1-(RO)₃SiS(SiMe₂)_nMe, n = 2, 4; R = i-Pr, t-Bu and also ³¹C-NMR shifts are given. The relationship of ²⁹Si-NMR chemical shift from the netto charge at the silicon atom q(Si) which value has been corrected according to the Sandorfy C quantum-chemical model is discussed. The greater reduction of the electron density at silicon in compounds with Si? X bond (X = S, P, Cl) has been explained by a conjugation of the lone of sulphur with the Si? X bonding pair.     

Characterization of plasma polymers from tetramethylsilane,octamethylcyclotetrasiloxane, and methyltrimethoxysilane

The products in inductively coupled plasma excited in organosilicic compounds were studied on chemical structures by spectroscopic methods. Plasma-polymerized tetramethylsilane (PA) contains not only Si? CH₃ groups but also Si? H groups on IR and ²⁹Si-NMR analyses. ¹³C-NMR spectrum of PA reveals that the crosslinked structure of PA is mainly constructed of Ch₂ and CH groups. Plasma-polymerized octamethyleyclotetrasiloxane (PD₄) is composed of Si? CH₃ and Si? O groups. ¹³C- and ²⁹Si-NMR spectra of PD₄ point out the highly preservation of the structural units similar to the monomer. These data of PD₄ suggest that PD₄ is formed from the ring-opening polymerization by cleavages of Si? O bonds. ¹³C-NMR spectrum of poly[methyltrimethoxysilane] (PT) indicates the existence of CH₃? O groups, meanwhile its ²⁹Si-NMR analysis concludes that PT is chiefly composed of the structural units similar to the monomer. The surface analyses by XPS of PA, PD₄ and PT suggest that these organosilicic plasma polymers resemble their starting materials.     

Si-NMR spectral assignments of polydisilahydrocarbons synthesised from diorganodichlorosilanes and styrene

In order to unambiguously assign the ²⁹Si-NMR chemical shifts of polydisilahydrocarbons containing structural units -CH₂Si¹(R₁R₂)Si²(R₁R₂)CHPh-, where R₁=R₂=Me or Ph, a model polymer viz., poly(tetramethyldisilyleneethylene) was synthesised through the dechlorination of 1,2-bis(chlorodimethylsilyl)ethane using potassium in toluene. The ²⁹Si-NMR spectrum of this polymer shows only one resonance peak at δ=−15.1 ppm due to Si atoms in the structural units, -CH₂Si(Me)₂Si(Me)₂CH₂- which unambiguously reveals that the chemical shift in the up field region of polydisilahydrocarbons containing structural units -CH₂Si¹(R₁R₂)Si²(R₁R₂)CH(Ph)- is due to Si¹, i.e., silicon attached to -CH₂- and accordingly, the chemical shift in the down field region is due to Si², i.e., silicon attached to -CH(Ph)-.     

29Si-NMR-Untersuchungen an einigen Cyclosilanderivaten

²⁹Si-NMR spectras of some new cyclosilane derivatives were measured. First aspects of the dependence of chemical shift on ring size and kind of substituents are given.

     

Molecular structure and photoreaction of poly(methylpropylsilane)

The molecular structure and photoreaction of poly(methylpropylsilane) (PMPrS) are investigated. The ²⁹Si-NMR spectra reveal that PMPrS is a linear polymer with no blanches. Fine multiple splitting of ²⁹Si-NMR is observed at room temperature. This multiple splitting is thought to be due to molecular conformation. The photo-oxidation reaction of the PMPrS film is found to be caused by light irradiation at the UV absorption band, 330 nm. Quantum efficiencies of scission and crosslink related to photodegradation are evaluated. There is little difference in either the molecular structure or photoreactivities between the high and the low molecular weight portions of the bimodal molecular weight distributions of PMPrS.     

Methyltrimethoxysilane Sol-Gel Polymerization in Acidic Ethanol Solutions Studied by 29Si NMR Spectroscopy

²⁹Si solution NMR has been used to study the acid-catalyzed polymerization of methyltrimethoxysilane (MTMS) in ethanol. The complex reaction pathways have been analyzed, and the general trends described. New interpretations are given to the rate constants of condensation reactions. The steric effects and inductive effects are found to work in coordination to reduce the reactivity of the silicon sites with the increasing connectivity. The extensive cyclization is attributed to the low steric effects of a tetramer ring closure compared to other end/end species condensations.     

Application of solid-state NMR (13C and 29Si CP/MAS NMR) spectroscopy to the characterization of alkenyltrialkoxysilane and trialkoxysilyl-terminated polyisoprene grafting onto silica microparticles

The solid-state Nuclear Magnetic Resonance (NMR) was used to characterize surfaces of silica gels chemically modified by alkenyltrialkoxysilanes and trialkoxysilyl terminated 1,4-polyisoprenes. The formation of covalent bonds created between alkoxy functional groups from alkenyltrialkoxysilane or trialkoxysilyl-terminated 1,4-polyisoprene and silanol groups on silica was clearly demonstrated by means of ¹³C and ²⁹Si CP/MAS NMR spectroscopy. Quantitative data, including calculation of the grafting yields in relation with the initial silanol concentrations, were also obtained by using solid-state ²⁹Si-NMR leading to a final well-defined characterization of the silica surfaces. A relatively good agreement was noticed between the grafting yields calculated from ²⁹Si-NMR spectra and those determined from other analytical techniques such as Wijs titration or elementary analysis. The reactivity of the various silica silanols towards each coupling agent was clearly characterized and estimated, as were the proportions of the various grafted structures formed at the silica surface. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36 : 437–453, 1998     

29Si-NMR-Spektroskopie an Silicatlösungen. II. Zur Abhängigkeit der Struktur der Silicatanionen in wäßrigen Natriumsilicatlösungen vom Na : Si-Verhältnis

²⁹Si-NMR Spectroscopy of Silicate Solutions. II. On the Dependence of the Structure of Silicate Anions in Water Solutions from the Na : Si Ratio The ²⁹Si-FT-NMR spectra of 13 sodiumsilicate solutions with different Na:Si ratios are investigated with regard to the structure of the silicate anions in the solutions. The 5 typical building units (Monosilicate, end-, middle-, branching and crosslinking groups) show characteristic and non overlapping ranges of the ²⁹Si chemical shift with many signals splittings caused by different neighbouring group effects. The relative intensities of the signals give informations on the concentrations and the equilibrium of condensation of the silicate anions in dependence of the Na:Si ratio. It is shown, that many types of anions with different degree of condensation coexist in an equilibrium, which is shiftet to the polymer silicate anions with high content of branching and crosslinking groups on the decrease of the Na:Si ratio.     

Sol-Gel Synthesis of Protoenstatite and a Study of the Factors That Affect Crystallization

Three different magnesium silicate sols were prepared from Mg(OMe)₂ and TEOS for which hydrolysis with H₂O₂ and H₂O was under stoichiometric, stoichiometric, or over stoichiometric. Xerogels were prepared from the sols by simple evaporation, spray-drying, or freeze-drying. The freeze-dried precursor formed mostly protoenstatite, a high-temperature polymorph of enstatite (MgSiO₃) that is generally not stable at room temperature; the other precursors formed mixtures of protoenstatite and clinoenstatite. The three xerogels and their calcined products were studied with XRD, HTXRD, BET, carbon analysis, TG/DTA and²⁹ Si-NMR. Residual carbon was found to be an important factor in the crystallization, and ²⁹Si-NMR spectroscopy was found to be a better tool than XRD to assess the phase purity of protoenstatite.     

NMR study on microstructure of polymer produced by hydrosilation of styrene with poly(hydrogenmethylsiloxane)

The microstructure of the polymer formed by the hydrosilation of styrene with poly(hydrogenmethylsiloxane) was studied by ¹H- and ¹³C-NMR spectra. Two modes of addition in hydrosilation were found to occur randomly without the influence of the adjacent monomer unit by statistical analysis of sequence distribution of ²⁹Si-NMR spectrum.     

Nmr study of the VixSiX4−x compounds with X = Cl,Br; x = 0, 1, 2, 3

¹H, ¹³C and ²⁹Si NMR data for the compounds Vi_xSiX_4?x are reported. While the ¹H and ¹³C resonances from the π system are indicative of the electron-withdrawing inductive effect (-I) of the halogens, the ²⁹Si chemical shift data reveal not only a shift contribution originating from this inductive effect but also the important influence of a {d, σ^*-π} hyperconjugation [1]. This back-donation originates from the vinyl π system and not from the halogens. The chemical shift data and the coupling constants also show an important influence from steric interaction and even from an electric field effect caused by polarization of the silicon—halogen bond.     

Beiträge zur Chemie der Silicium-Schwefel-Verbindungen. XXIX. 29Si-NMR-Untersuchungen

Contributions to the Chemistry of Silicon-Sulphur Compounds. XXIX. ²⁹Si-N.M.R. Investigations Three series of silicon-sulphur compounds (RO)₃SiSR′ (I), (i-PrO)_4?nSi(SEt)_n (II) and cyclic Si? S compounds (III) were prepared, some of them at the first time and their ²⁹Si-N.M.R. spectra were measured. In the series of trialkoxysilylthio derivatives (I) were the steric and inductive effects of the RO and R'S groups evaluated. In the series II were the ²⁹Si-N.M.R. chemical shifts related to the relative paramagnetic screening constants σ^* and netto charge at the silicon atom q(Si) using the EN-quantum-chemical model discussed. In the series III were the shift contribution of the (SiS)₂ and 1-sila-2,5-dithiacyclopentan rings determined.     

Synthesis and characterization of sol–gel derived CNSL based hybrid anti-corrosive coatings

Thermally cured cashew nut shell liquid based hybrid coatings have been successfully developed using three step processes of malenization, silane modification and subsequent hydrolysis and condensation with tetra ethyl orthosilicate for corrosion protection of mild steel. The synthesized precursor was then characterized by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy (¹H-NMR, ¹³C-NMR and ²⁹Si-NMR) for structural elucidation. Four different coating formulations were developed on the basis of silane content (5, 10, 15 and 20 %) in the coating and cured with hexabutoxymethylmelamine at 120 °C for 1 h. The completely cured coatings were evaluated for mechanical properties, solvent resistance, chemical resistance, hydrolytic stability and accelerated weathering properties. Scanning electron microscope/energy dispersive spectroscopy was used to analyse the morphological behaviour and elemental distribution of the coating. Results revealed that 20 % silane modification showed better overall properties as compared to other formulations due to formation of more metal–oxygen–silicon covalent bond at metal-coating interface.     

29Si and 13C NMR Studies of organofunctional di- and trisilanes

A series of di- and trisilanes of general structure Ph₃SiSiMe₂R and (Ph₃Si)₂SiR′R″ were synthesized, and the ²⁹Si and ¹³C chemical shifts and one-bond siliconsilicon coupling constants (¹J_SiSi) were measured. The coupling constants of the disilanes were found to be primarily dependent upon the inductive effect of the alkyl group, R, as measured by the Taft o^★ constant. In both series of compounds, increasing alkyl substitution at silicon led to a decrease in ¹J_SiSi.     

Influence of preparation method on the structure and catalytic activity of supported solid sulfuric acid

Summary FT-IR and ²⁹Si-NMR studies suggested the interaction of H₂SO₄ with the SiO₂ support in the material synthesized by the sol-gel method. ¹H-NMR results showed that the acid strength of solid sulfuric acid was almost the same as that of liquid sulfuric acid. The catalyst showed higher catalytic activity and stability than that prepared by impregnation method.     

Oligosilanylated Silocanes

A number of mono- and dioligosilanylated silocanes were prepared. Compounds included silocanes with 1-methyl-1-tris(trimethylsilyl)silyl, 1,1-bis[tris(trimethylsilyl)silyl], and 1,1-bis[tris(trimethylsilyl)germyl] substitution pattern as well as two examples where the silocane silicon atom is part of a cyclosilane or oxacyclosilane ring. The mono-tris(trimethylsilyl)silylated compound could be converted to the respective silocanylbis(trimethylsilyl)silanides by reaction with KO^tBu and in similar reactions the cyclosilanes were transformed to oligosilane-1,3-diides. However, the reaction of the 1,1-bis[tris(trimethylsilyl)silylated] silocane with two equivalents of KO^tBu leads to the replacement of one tris(trimethylsilyl)silyl unit with a tert-butoxy substituent followed by silanide formation via KO^tBu attack at one of the SiMe₃ units of remaining tris(trimethylsilyl)silyl group. For none of the silylated silocanes, signs of hypercoordinative interaction between the nitrogen and silicon silocane atoms were detected either in the solid state. by single crystal XRD analysis, nor in solution by ²⁹Si-NMR spectroscopy. This was further confirmed by cyclic voltammetry and a DFT study, which demonstrated that the N-Si distance in silocanes is not only dependent on the energy of a potential N-Si interaction, but also on steric factors and through-space interactions of the neighboring groups at Si and N, imposing the orientation of the p_z(N) orbital relative to the N-Si-X axis.     

Functionalized poly(methylphenyl)silane: End group reactivity

A Cl ended poly(methylphenyl) silane is synthesized and analyzed by ²⁹Si-NMR spectroscopy. The end groups are extremely reactive with small compounds as alcohols and amines. By this way, copolysilanes containing some nitrogen atoms can be synthesized. © 1996 John Wiley & Sons, Inc.