Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: I. Methane Chemisorption on a Reactive Silica Surface

Experimental and quantum chemical study of reactive silica surface methylation is carried out. The main product of the reaction is the (Si–O)₂Si(H)(CH₃) groups, which are formed via a radical-chain process with the participation of methane molecules and paramagnetic and diamagnetic defects on the oxide surface. Spectral (optical and IR) characteristics of the groups participating in the process (Si–O)₂Si^·–CH₃, (Si–O)₂Si(H)(CH₃), and (Si–O)₂Si(CH₃)(CH₃) are determined. Information on the kinetics of separate steps of the process is obtained including rate constants and the activation energies of steps.     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: III. Products of >Si

Permenov  D. G.  Radzig  V. A. 《Kinetics and Catalysis》2004,45(2):273-278

Improvement on multiparameter equations of state for dimethylsiloxanes by adopting more accurate ideal-gas isobaric heat capacities: Supplementary to P. Colonna, N.R. Nannan, A. Guardone, E.W. Lemmon, Fluid Phase Equilib. 244, 193 (2006)

A recent paper by the authors reports ideal-gas isobaric heat capacities () for several siloxanes. These values were determined using ad hoc speed-of-sound measurements and ab initio calculations. Thermodynamic models for some siloxanes documented in an earlier work by the same authors adopted less accurate estimations for . This note reports coefficients for the substance-specific Aly–Lee correlations for which ensure higher accuracy when used with the multiparameter equations of state for fluids [(CH₃)₂–Si–O]₄ (D₄, octamethylcyclotetrasiloxane), [(CH₃)₂–Si–O]₅ (D₅, decamethylcyclopentasiloxane), (CH₃)₃–Si–O–Si–(CH₃)₃ (MM, hexamethyldisiloxane), and (CH₃)₃–Si–O–[(CH₃)₂–Si–O]₄–Si–(CH₃)₃ (MD₄M, tetradecamethylhexasiloxane), as described in Colonna et al. [P. Colonna, N.R. Nannan, A. Guardone, E.W. Lemmon, Multiparameter equations of state for selected siloxanes, Fluid Phase Equilib. 244 (2) (2006) 193–211].     

Crystal structure of organosilicon compounds. LXIV. Structure of four unsaturated cyclocarbosilanes

An x-ray structural investigation of four permethylcyclocarbosilanes, whose heterocycles are formed by –Si–CH₂–CH₂–, –Si–CH=CH–, and –Si–CC– fragments, has been carried out. Strong distortions of some bond lengths and bond angles (caused by the shortness of the Si...Si distances between the neighboring Si atoms and the absence of intermolecular contancts with the participation of atoms of the heterocycles), which attest to statistical disordering of the molecules in the crystals, whose character could be established unequivocally for three of the compounds, have been observed in all the structures.For part LXIII, see [1].A. N. Nesmeyanov Institute of Organometallic Compounds, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 116–124, March–April, 1991.     

Kinetics of Intramolecular Hydrogen Atom Transfer in the Radicals >Si(O·)(R) (R = H,D, CH3, CD3, and C2H5)

Methods are developed for obtaining oxy radicals by the photodecomposition and thermal decomposition of precursors (Si–O)₂Si(N=N–O^·)(R) and (Si–O)₂Si(O–C^·=O)(R). The mechanism of these processes is established. Kinetic data are obtained for the reaction of hydrogen atom transfer in oxy radicals (Si–O)₂Si(O^·)(R) (R = H, D, CH₃, CD₃, and C₂H₅). The activation energies of hydrogen atom transfer are found for three-, four-, and five-membered transition states: 13.5 ± 1, 18 ± 1, and <10 kcal/mol, respectively. For the reaction of H(D) atom transfer in the (Si–O)₂Si(O^·)(H(D)) radical, the kinetic isotope effect is found. Quantum-chemical calculations were used to determine the structures of transition states in the studied processes. Experimental studies were carried out using ESR spectroscopy.     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: II. Methylation of >Si=O Groups

The methods of optical and IR spectroscopy and quantum chemistry were used to obtain data on the direction and kinetics of the reaction of a silanone (SiO)₂Si=O with a CH₄ molecule and a methyl radical. Two mechanisms of methylation of silanone groups, molecular and free-radical, are studied. Both processes are accompanied by the formation of (SiO)₂Si(OH)(CH₃) groups. The rate constant of the molecular process is determined and its activation energy is estimated (17 kcal/mol). A methyl radical adds to the silicon atom in a silanone group to form the oxy radical (Si–O)₂Si(O)(CH₃). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (Si–O)₂Si(OH)(CH₃) groups is their decomposition with the abstraction of a methane molecule. The activation energy of this process is 70 kcal/mol. Quantum chemical methods were employed to obtain data on possible intermediates in the processes studied and these results are used to interpret spectral and kinetic data.     

Die Schwingungsspektren von Trimethylsilyl-alkyl-bzw.-phenyl-aminen

Zusammenfassung Die Infrarot- und die Raman-Spektren der Silylamine (CH₃)₃Si–NH–R (R=CH₃, C₂H₅ und C₆H₅) sowie der analogen N-deuterierten Verbindungen werden mitgeteilt und analysiert. Starke Kopplungen führen zu einer Mischung vonv SiN bei etwa 700 cm^–1 mit anderen Schwingungen des C₃Si–NHR-Skelettes.

---

The Infrared and Raman spectra of the silylamino compounds (CH₃)₃Si–NH–R (R=CH₃, C₂H₅, and C₆H₅) and the analogous N-deuterated species are reported and assigned. The SiN stretching mode at about 700 cm^–1 is strongly coupled with other vibrations of the molecules.

     

Untersuchungen in den Systemen: Mangan—{Vanadin,Rhenium, Eisen}—silicium

Zusammenfassung Die strukturchemischen Verhältnisse werden in den Dreistoffen: Mn–{V, Re, Fe}–Si für einen Zustand –1000° C homogenisiert und abgeschreckt — untersucht. Die neu aufgefundene Mn–Re--Phase ist hinsichtlich der Ordnung mit der Re–Nb--Phase vergleichbar. Mn₅Si₂ wird durch Mn/Re-Austausch bei 1000° C stabilisiert. Einer ternären Kristallart (X) mit erheblichem Homogenitätsgebiet kommt die ungefähre Formel Mn_2–3Re_2–1Si zu. Der Austausch Mn/Re in Mn₅Si₃ erfolgt nur in der 6g-Lage. MnSi und ReSi sind lückenlos mischbar.Gitterparameter für die Mn–Re–Si- und V–Mn–Si--Phasen werden bestimmt. V₃Si löst etwa 50 Mol% Mn₃Si. Die lückenlose Mischreihe Mn₃Si–Fe₃Si wird bestätigt; gegenüber Literaturbefunden besteht jedoch im ganzen Bereich ein Ordnungszustand (BiF₃-Typ).

---

The ternary systems: Mn–{V, Re, Fe}–Si have been studied after anneal at 1000° C followed by a quench by means of X-ray methods. The newly found Mn–Re--phase compares with the Re–Nb--phase as far as the ordering is concerned. Mn₅Si₂ can be stabilized by Re/Mn-substitution up to higher temperatures. A ternary phase X having a large homogeneous region, can roughly be described by a formula Mn_2–3Re_2–1Si. Re substitutes Mn within the Mn₅Si₃-phase occupying the 6g positions only. Lattice parameters of the Mn–Re–Si- and V–Mn–Si--phases have been determined. V₃Si dissolves about 50 mole% Mn₃Si. The solid solutions Mn₃Si–Fe₃Si can be confirmed; however there is an ordering throughout the whole domain (BiF₃-structure type).

---

---

Mit 2 Abbildungen     

Synthesis of Bioactive Organic-Inorganic Hybrids with γ-Methacryloxypropyltrimethoxysilane

Bioactive organic-inorganic hybrids were synthesized through sol-gel processing starting from -methacryloxypropyltrimethoxysilane. NMR-spectroscopic studies showed the presence of silanol groups (Si–OH) and Si–O–Si bonds. In vitro tests of the hybrids for bioactivity with a simulated body fluid (Kokubo solution) indicated that only calcium-containing hybrids could form apatite on their surfaces. Thus the presence of calcium ions was no less important to deposit apatite than the formation of silanol groups or Si–O–Si bonds.     

IR Photolysis of Liquid Hexamethyldisiloxane by Pulse CO2Laser Radiation

The IR photolysis of liquid hexamethyldisiloxane [(CH₃)₃Si]₂O by pulse CO₂laser radiation was studied. The experiments were performed both with and without a graphite powder as a sensitizer. The presence of the graphite sensitizer increases the efficiency of dissociation. To provide the highest penetration of the laser emission into a medium, the irradiation of samples were carried out with tuning far from the maximum of the Si–O bond absorption band (1055 cm^–1) using the 10P16 line (_las= 947.78 cm^–1). This tuning makes it possible to separate the process of IR multiphoton dissociation of the vapor over the liquid phase and to carry out the reaction in the liquid at a low (1 J/cm²) energy fluence of laser radiation. The IR spectra of irradiated liquid samples indicate the formation of linear and cyclic polystructures.     

Intramolecular Rearrangements of >Si(O–C·=O)(CH2–CH3) and >Si(CH2–CH·–CH3)(CH2–CH3) Radicals

Using ESR and IR spectroscopy, the structures of >Si(O–C^·=O)(CH₂–CH₃) (1) and >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) (2) radicals were deciphered. The directions and kinetic parameters of reactions of intramolecular rearrangements in these radicals were determined. The reactions of hydrogen atom abstraction in radical (1) from the CH₂ and CH₃ groups were studied. It was found that the endothermic reaction of hydrogen atom abstraction from the methyl group occurs at a higher rate than the exothermic reaction with the methylene group. The differences are determined by changes in the size of a cyclic transition state. Based on the experimental data, the strengths of separate C–H bonds in surface fragments are compared. The rearrangement >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) >Si(C^·(CH₃)₂)(CH₂–CH₃) was discovered and its mechanism was determined. One of its steps is the skeletal isomerization Si- (2)- _. (1)Si- (1)- _. (2). Experimental data are analyzed using the results of quantum-chemical calculations of model systems.     

Convenient approach to Si–H-containing polymers

New synthetic routes to organosilicon polymers containing SiH₂ groups and organic -electron units in the polymer main chain are described. These polymers are expected to be useful precursors for ceramics. The polymer backbone is formed by condensation of ,-bis(trifluoromethylsulfonyloxy)-substituted organo-silicon compounds containing SiH₂ groups with the organometallic dinucleophiles Li₂C₂, Li₂C₄, and 1,4-BrMg–C₆H₄–MgBr. We could confirm the formation of the silicone polymers at low temperatures, in short reaction times, and with high yields. The structural characterization is based on ²⁹Si, ¹³C, and ¹H NMR spectroscopy. The narrow ²⁹Si NMR signals of the products indicate the regular alternating arrangement of the building blocks in the polymer backbone resulting from the fact that the condensation reactions are not accompanied by exchange processes analogous to metal halogen exchange. Weight-average molecular weights in the range of M_w = 10000–20000, relative to polystyrene standards, were found by GPC. The pyrolytic behaviour of [H₂Si–H₂Si–C C–]_n 2a is compared with the behaviour of the methylated derivative [Me₂Si–Me₂Si–C C–]_n.     

DFT and direct ab-initio MD study on hyperfine coupling constants of methyl radicals adsorbed on model surface of silica gel

Methyl radicals interacting with silica gel surfaces have been investigated by means of DFT and direct ab-initio molecular dynamics (MD) methods using cluster models. Four typical binding sites of CH₃ on the cluster models were found in the geometry optimization from several initial geometries of CH₃ around the silica gel clusters. These were two silanol Si–OH sites and two siloxane Si–O–Si sites. In both sites, magnitude of hyperfine coupling constants of the methyl radical (a_H) was smaller than that of free CH₃ (a_H = 23.04 G). Temperature effects on a_H of the methyl radical were investigated by means of the direct ab-initio MD method. The hyperfine coupling constant of CH₃ interacting with the SiOH group decreased with increasing temperature. The methyl radical interacting with alkali metal supported silica gel was also investigated for comparison. The electronic states of methyl radicals on silica gel were discussed on the basis of theoretical results.     

Mechanism of the grafting of organosilanes on mineral surfaces. IV. Phenylderivatives of sepiolite and poly (organosiloxanes)

Grafting reactions of phenyl groups on silica substrates using as reagents various phenylsilanes with variable distances between the silicon atoms and the aromatic ring by 0, 1, and 2 methylene groups [Si–(CH₂) _n –C₆H₅] were studied. Two different silicates have been selected as source of silica: sepiolite and tetraethyl-orthosilicate (TEOS). Sulfonic- and nitro-derivatives prepared from these phenyl compounds by electrophilic substitution reactions, have been obtained. The mechanism of these processes has been studied in relation to the number of methylene separating groups belonging to the starting reagents. The characterization of such systems has been achieved by XRD,²⁹Si and¹³C NMR/MAS, IR, and laser microprobe mass spectrometry.Parts I, II and III published in this Journal (1978) 256:135–139, (1979) 257:178–181 and (1985) 263:1025–1030     

Phosphororganische Verbindungen, 7. Mitt.: Über die Oxydation von β-Oxo-alkylenphosphoniumsalzen mit Bleitetraacetat

Zusammenfassung Es wird gezeigt, daß Phosphoniumsalze des Typs (C₆H₅)₃PCH₂COCH₃X (X=Cl, Br, J. SCN und SeCN) mit Bleitetraacetat zu Ketophosphorylenen (C₆H₅)₃P=CXCOR oxydiert werden können. R kann auch ein ungesättigter Rest sein. Eine derartige Wirkungsweise des Bleitetraacetats ist unseres Wissens noch nicht beschrieben worden. Die Oxydation von Phosphoniumsalzen der allgemeinen Struktur (C₆H₅)₃PCH(R₁)COCH₂R₂ X mit Bleitetraacetat wurde vorläufig für X=Cl und SCN untersucht. In ersterem Fall erhält man Triphenylphosphinoxid und R₁CHXCOCH₂R₂. Dies entspricht einer übersichtlichen Darstellungsmöglichkeit bestimmter -Halogenketone. Im zweiten Fall entsteht neben Triphenylphosphinoxid R₁–CC–CH(R₂)–N=C=S. Diese Senföle verdanken ihre Entstehung einer interessanten neuartigen Umlagerungsreaktion der primär entstehenden Allenrhodanide R₁–C(SCN)=C=CHR₂.

---

It is shown that phosphonium salts of the type (C₆H₅)₃PCH₂COCH₃X (X=Cl, Br, J, SCN and SeCN) can be oxidised with lead tetraacetate to ketophosphorylenes (C₆H₅)₃P=CXCOR. R can be an unsaturated group. To the best of our knowledge reactions of this nature with lead tetraacetate have not hitherto been described. A preliminary investigation of the lead tetraacetate oxydation of phosphonium salts of the general formula (C₆H₅)₃PCH(R₁)COCH₂R₂X was made for X=Cl and SCN. In the former case triphenylphosphine oxide and R₁CHXCOCH₂R are formed, offering a method of preparing certain -haloketones. In the latter case R₁–CC–CH(R₂)–N=C=S results, together with triphenylphosphine oxide. These mustard oils owe their formation to an interesting new type of rearrangement of the initially formed allene rhodanides R₁–C(SCN)=C=CHR₂.

---

---

Mit 1 Abbildung

---

6. Mitt.:E. Zbiral undL. Fenz, Mh. Chem.96, 1983 (1965).

---

Auszugsweise vorgetragen am Österr. Chemikertreffen in Graz am 28. 9. 1965.     

Free Radical Reactions of N2O Molecules

Experimental data are presented on the spectral (ESR, IR, and optical) and thermochemical characteristics of a complex between the (Si–O)₃Si^.radical and an N₂O molecule. The rate constants of separate reactions in the systems (Si–O)₃Si^.+ N₂O and (Ge–O)₃Ge^.+ N₂O are found. The results of quantum chemical calculations of potential energy surfaces and spectral characteristics are presented for the following systems: H^.+ N₂O, H₃C^.+ N₂O, H₃Si^.+ N₂O, F₂HSi^.+ N₂O, F₃Si^.+ N₂O, and F₃Ge^.+ N₂O. The latter three systems served as molecular models for experimentally found systems. Based on experimental and theoretical data, the product of N₂O addition to (Si–O)₃Si^.has the structure Si–N=N–O^.. The reactions of free radicals H^., H₃C^., H₃Si^., F₂HSi^., F₃Si^., (Si–O)₃Si^., and (Ge–O)₃Ge^.with N₂O are compared. The spectrum of optical absorbance of the (Si–O)₃Si–O^.radical is recorded and qualitatively characterized.     

DFT study of the mechanism of alkane hydrogenolysis by transition metal hydrides. 1. Interaction of silica-supported zirconium hydrides with methane

Model reactions of silica-supported zirconium hydrides (Si—O—)₃ZrH and (Si—O—)₂ZrH₂ with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (Si—O—)₃ZrCH₃ and (Si—O—)₂Zr(H)CH₃ as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (Si—O—)₂ZrH₂ are more reactive toward the methane C—H bonds than zirconium monohydrides (Si—O—)₃ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (Si—O—)₂ZrH₂ are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.     

Insight into Mechanical Properties of CH3SiO3/2 Film

Young's modulus and stress-optical coefficient of self-sustained gel film of CH₃SiO_3/2 were measured. Young's modulus varied from 1.1 to 1.5 GPa, depending on the structural state of the film. It increased with an increase in the degree of polymerization that was induced by heat treatments, and also increased with subsequent exposure to the ambient condition. The stress-optical coefficient was 6.3 × 10^–12 Pa^–1 for a sample treated at 120°C. The compliance of the film seems to depend mainly on the intermolecular structure of Si–CH₃...CH₃–Si as well as hydrogen bonding due to silanols and adsorbed water.     

Mechanisms of Reactions of H2O and NH3 Molecules with (≡Si–O)3Si⋅and (≡ Si–O)3Si–⋅ Radicals on the Silica Surface

Spin traps, which are diamagnetic centers (SiO)₂Si, are used to register low-molecular radicals OH, NH₂, and H formed by the reactions of H₂O and NH₃ molecules with the radicals (Si–O)₃Si and (Si–O)₃Si–O stabilized on the silica surface. The experimental data and the results of quantum-chemical calculations for model systems are used to determine the mechanism and thermochemical characteristics of these reactions. A new paramagnetic center (Si–O)₂SiNH₂ was identified on the silica surface, and its radiospectroscopic characteristics are determined.     

Radical telomerization of vinyltrimethylsilane with diethyl phosphite

Telomerization of vinyltrimethylsilane with diethyl phosphite was carried out in the presence of tert-butyl peroxide (TBP), affording a series of regular telomers (EtO)₂P(O)(CH₂CHSiMe₃)_nH (n=1–3)(T_n) and the compounds Me₃Si· (CH₂)₂CH(CH₃)OP(O)(OEt)(CH₂)₂SiMe₃(T₂). Arguments are presented which support a rearrangement with 1,5-H migration in the first propagating radical along a chain including P and O atoms. The partial chain-transfer constant C₁ (6.2) was evaluated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1883–1887, August, 1990.