Si-O-M bonds

Figure 1 shows Fourier transforms of EXAFS spectra of a few samples prepared. The radial distribution functions of these samples are different from that of nickel oxide or cobalt oxide [7]. All the Fourier transforms showed two peaks at similar distances (phase uncorrected) the peak between 1 and 2 A is ascribed to the M-0 bond (M divalent cation) and the peak between 2 and 3 A is ascribed to the M-O-M and M-O-Si bonds. The similar radial distribution functions in Figure 1 indicate that the local structures of X-ray absorbing atoms (Ni, Co, and Zn) are similar. No other bonds derived from metal oxides (nickel, cobalt and zinc oxides) were observed in the EXAFS Fourier transforms of the samples calcined at 873 K, which suggests that the divalent cations are incorporated in the octahedral lattice. 

The kinetics of the hydrolysis and condensation of organic functional trialkoxy silanes has been reported by Pohl and Osterholtz [17-19]. The silane coupling agents used as adhesion promoters [1-3] usually have a trialkoxy silane as one of the functional groups, i.e. (MeO)3Si—(CH,),—0,CC(Me)=CH2. If this attaches to a glass substrate, it will form Si—O—Si bonds or if it attaches to metal substrates, it can form M—O—Si bonds. Thus, the work described here can be applicable to providing additional understanding for those processes. 

IR-spectra of the transition metal containing mesoporous silicas are similar to that of amorphous silica. Only an supplementary absorption band at around 940 nm is present in all the spectra of the mono- and bimetallic incorporated samples but its intensity is variable. This band can be attributed to the M-O-Si bond and often used as a prove of the incorporation of metal ions in the tfamework of silicas. 

Oxane bonds, M—O—Si, are hydroly2ed during prolonged exposure to water but reform when dried. Adhesion in composites is maintained by controlling conditions favorable for equiUbrium oxane formation, ie, maximum initial oxane bonding, minimum penetration of water to the interface, and optimum morphology for retention of silanols at the interface. The inclusion of a hydrophobic silane, such as phenyltrimethoxysilane [2996-92-17, with the organofunctional silane increases thermal stabiUty of the silane and make the bond more water resistant (42). 

Figure 5.31. Network (megamer) formation through the hydrolysis/crosslinMng of neighboring PAMAMOS dendrimer units. Hydrolysis of the C-O-Si bond may also be exploited for the controlled-release of entrained agents (e.g, cancer drugs, etc.). It should be noted that subsequent thermal annealing to remove the PAMAM cores results in a nanoporous network that has a dielectric constant (k) of ca. 1.5 - of extreme interest for next-generation IC interconnect applications. Reproduced with permission from Dvornic, P. R. Li, J. de Leuze-Jallouli, A. M. Reeves, S. D. Owen, M. J. Macromolecules, 2002, 35, 9323. Copyright 2002 American Chemical Society.

Pelmenschikov A., Strandh H., Pettersson L. G. M., and Leszczyynski J. (2000) Lattice resistance to hydrolysis of Si-O-Si bonds of silicate minerals Ab initio calculations of a single water attack onto the (001) and (111) beta-quistobalite surfaces. J. Phys. Chem. B 104, 5779-5783. 

Two excellent reviews that detail procedures for the preparation of bonded phase supports have recently been published by Leonard - and Buchmeiser. One of the most popular methods of surface chemical modification involves the use of organosilanes. These organosilanes react with the surface metal hydroxyl groups and form a surface, which may be represented as M-O-R, where R represents an alkyl chain and M represents the metal (i.e., silica, zirconia, titania, etc.). One important factor that must be stated, however, is that the order of stability of M-O-R bonds increases in the order of M=Si > Zr > Ti > Improvements in the hydrolytic... 

Q Figure 2.7 shows the formation of four M-O single bonds from two M=0 double bonds, where M = Si and C. Consider the enthalpy change for this reaction, and hence predict the preferred form of the oxide for each of Si and C. 

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