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.     

Nitrogen-Containing Paramagnetic Centers in Vitreous Silica

The results of a study on the structure and spectral characteristics of nitrogen-containing paramagnetic defects in vitreous silicas are presented. The UV irradiation, γ-irradiation, and mechanical destruction of nitrogen-doped vitreous silicas, which were synthesized using a plasma-chemical SPCVD technology, were used for the generation of paramagnetic centers. Another technique for producing nitrogen-containing paramagnetic centers was the target-oriented chemical modification of the surface defects of silica with the use of NH₃ (ND₃) molecules. Paramagnetic centers with free valences at silicon [(≡Si-O-)₂Si^· (-NH₂) and (≡Si-O-)₂Si^· -N(-Si≡)₂] and nitrogen atoms [≡Si-N^· -H and ≡Si-N^· -Si≡] were detected. The reactivity of nitrogen-centered radicals toward H₂ and CO molecules was studied. Quantum-chemical calculations of model systems were employed in the interpretation of the experimental data. The strengths of Si-O and Si-N bonds in vitreous silica were compared.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 615–634.Original Russian Text Copyright © 2005 by Radtsig.     

Hydrogenation of the silanone groups (≡Si-O)2Si=0. Experimental and quantum-chemical studies

The reactivity of bis(siloxy)silanone groups (Si-0)₂Si=O stabilized on a silica surface with respect to H₂ molecules was studied. The reaction was found to give the (Si-O)₂SiH(OH) groups. The rate constant for this process was determined. Its activation energy in the 300–580 K temperature range is 13.4±0.3 kcal mol^–1, and the enthalpy is 54±5 kcal mol^–1. The activation energy for the reverse reaction,viz., elimination of a hydrogen molecule, is equal to 65 kcal mol^–1. Quantum-chemical calculations of hydrogenation of F₂Si=O and (HO)₂Si=O, which are the simplest molecular models of the silanone groups, were performed. Data on the geometrical and electronic structures of transition states and on the effects of substituents at the silicon atom on the reactivity of the silanone groups in this process were obtained. The optical absorption band of the surface silanone groups was quantitatively characterized. Its maximum is located at 5.65±0.1 eV; the extinction coefficient at the maximum (220 nm) is (3±0.5) · 10^–18 cm² molec.^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1951–1958, August, 1996.     

SCF impregnation of polymer matrices with stable nitroxyl radicals

The ESR method is used to study the impregnation of a set of polymer matrices (co-polymer F-42, polyvinylchloride, polycarbonate, polymethylmethacrylate) with stable nitroxyl radicals of 2,2,6,6-tetramethyl-4-oxypiperidin-1-oxyl (TEMOPO) in a supercritical carbon dioxide medium. The amount of the substance incorporated into the polymer matrices increases with increase of the mass of TEMOPO samples used and converges to the limit values that for F-42 and polyvinylchloride samples are 9.6 × 10²⁰ and 7.5 × 10²⁰ paramagnetic centers per gram of the polymer, respectively. At the same time, no formation of TEMOPO clusters is observed; the spatial distribution of radicals absorbed by the polymer film is close to random and is independent from the film thickness.     

Inhomogeneous nature of UV absorption bands of bulk and surface oxygen-deficient centers in silica glasses

Photochemical and photoluminescence studies of oxygen-deficient centers stabilized in the bulk and on the surface of silica glasses clearly demonstrate the inhomogeneous nature of the absorption and luminescence spectra of oxygen-deficient centers. The conclusion is drawn that the inhomogeneity of the absorption spectra is due to the dispersion of the energy of the S₀-S₁ transition, while the inhomogeneity of the luminescence spectra is due to the dispersion of the energy barrier of intersystem crossing. The inhomogeneous nature of the oxygen-deficient centers in silica glasses is assumed to be caused by a small dispersion in the geometrical parameters of different groups of centers with similar chemical properties.     

Overton-Region IR Registration of (≡Si–O)2Si=O and (≡Si–O)2Si〈O2〉C=O Groups on the Silica Surface

IR spectroscopy in a range of 2050–4000 cm^–1 (the range of overtones and composite frequencies) is used to study the groups (Si–O)₂Si=O and (Si–O)₂SiO₂C=O with different isotopic compositions (¹⁶O, ¹⁸O, ¹²C, and ¹³C). Analysis of the experimental data and quantum-chemical calculations of vibrational spectra for the model compounds are used to identify the IR bands. New data are obtained on the vibrational spectra of these groups. Their identification is shown to be possible in the spectral range that is convenient for the study of silica samples.     

Mechanisms of reactions between > Si=O groups and CO2, N2O, and HC≡CH molecules: An experimental and quantum chemical study

IR spectroscopy and quantum chemical calculations are used to study the directions and kinetics of reactions between silanone groups (≡Si-O)₂Si=O and CO₂, N₂O, and acetylene molecules. IR bands are assigned on the basis of the calculation of vibrational spectra of model low-molecular systems. Quantum chemical methods are used to obtain the data on the shapes of potential energy surfaces of these systems (intermolecular complexes and transition states). These data are used to interpret kinetic data. The silanone group is inclined to the formation of relatively stable (~10 kcal/mol) intramolecular complexes with CO₂, N₂O, and acetylene molecules. Their geometries and electronic structures are determined.     

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.