Covalency and ionicity do not oppose each other : relationship between Si-O bond character and basicity of siloxanes

Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R3Si-(O-SiR2)(n)-O-SiR3] that both covalency and ionicity of the Si-O bonds impact on the basicity of the Si-O-Si linkage. The relationship between the siloxane basicity and the Si-O bond character has been under debate since previous studies have presented conflicting explanations. It has been shown with natural bond orbital methods that increased hyperconjugative interactions of LP(O)->sigma*(Si-R) type, that is, increased orbital overlap and hence covalency, are responsible for the low siloxane basicity at large Si-O-Si angles. On the other hand, increased ionicity towards larger Si-O-Si angles has been revealed with real-space bonding indicators. To resolve this ostensible contradiction, we perform a complementary bonding analysis, which combines orbital-space, real-space, and bond-index considerations. We analyze the isolated disiloxane molecule H3SiOSiH3 with varying Si-O-Si angles, and n-membered cyclic siloxane systems Si2H4O(CH2)(n-3). All methods from quite different realms show that both covalent and ionic interactions increase simultaneously towards larger Si-O-Si angles. In addition, we present highly accurate absolute hydrogen-bond interaction energies of the investigated siloxane molecules with water and silanol as donors. It is found that intermolecular hydrogen bonding is significant at small Si-O-Si angles and weakens as the Si-O-Si angle increases until no stable hydrogen-bond complexes are obtained beyond phi(SiOSi) = 168 degrees, angles typically displayed by minerals or polymers. The maximum hydrogen-bond interaction energy, which is obtained at an angle of 105 degrees, is 11.05 kJ mol(-1) for the siloxane-water complex and 18.40 kJ mol(-1) for the siloxane-silanol complex

Theoretical Studies of the Mechanism for the Synthesis of Silsesquioxanes. 1. Hydrolysis and Initial Condensation

The mechanisms for the hydrolysis of SiHCl3 to form HSi(OH)3 and the condensations of SiH3OH and HSi(OH)3 are studied by using ab initio electronic structure methods including electron correlation via second and fourth order perturbation theory and coupled cluster calculations. In the gas phase, the barrier heights for the hydrolysis and silanol condensation reactions are quite high, ranging from 20 to 30 kcal/mol. The barrier for the condensation of HSi(OH)3 is much smaller as a result of hydrogen bond stabilization of the transition state. Addition of just one extra water molecule is sufficient to reduce the calculated barriers to very small values or zero

Some silyl anions

Two attempts to módify the ammonium salts of silyl thiol and silyl selenol were made in an effort to make them easily soluble. Both attempts were unsuccessful. The first was to try to form the potassium salt by using an ion exchange resin; the second was to prepare the methylsilyl derivative in the hope that it would be soluble. The ammonium salt of silyl tellurol was also prepared by an analogous route to the sulphur and selenium compounds.2H₂Te + (SiH₃)₃N → NH₄TeSiH₃ + (SiH3)₃Te ¹.Methyl silyl telluride was prepared from it by adding methyl iodide.CH₃I + NH₄ TeSiH₃ → CH₃TeSiH₃ + NH₄ I ².Lithium silyl oxide, sulphide and selenide was prepared by the reaction of methyl lithium, as a solution in diethyl ether at -64°, with disiloxane, disilyl sulphide and disilyl selenide.CH₃Li + (SiH₃)₂Y → LiYSiH₃ + CH₃SiH₃ ³.Y = (0, S, Se)Similar reactions were carried out with methyl lithium and trisilyl phosphine and trisilyl arsine.CH₃Li + (SiH₃)₃Z→LiZ(SiH₃)₂ + CH₃SiH₃'⁴Z= (P, As)Yields of the lithium derivatives were all of the order of 80% and they were characterised by i.r., raman and n.m.r. spectroscopy. In some cases a reaction with trimethylsilyl chloride was also carried out.Reactions of these compounds, as a solution in diethyl ether, were carried out with a variety of reagents but most were unsuccessful in that the desired products were not isolated. Instead, the main volatile silyl product was generally (SiH₃)₃Z or (SiH₃)₂Y. This was attributed to the side-reactions:SiH₃ - Y - Q +Bθ → SiH₃B + θY - Q ⁵.and (SiH₃)₂Z - Q + Bθ → SiH₃B + SiH₃QZθ ⁶.where Q is any group of interestand Bθ is a base - in this case YSiH₃ or Z(SiH₃)₂This conclusion was confirmed by reactions of the silyl anions and closely related molecules producing exchange situations. Attempts to limit the extent of reactions (5) and (6) consisted of the use of benzene as a solvent, the use of trimethylamine and the absence of solvent. The first two were unsuccessful but the latter worked. It was possible to prepare disilyl phosphine and 1,1,1-trimethyl disiloxane by this method.2LiP(SiH₃)₂ + H₂S -- HP(SiH₃)₂ + Li₂S ⁷.Li0SiH₃ + (CH₃)₃SiCl → (CH₃)₃0SiH₃ + LiCl ⁸

Novel High Capacity Oligomers for Low Cost CO2 Capture

The novel concept of using a molecule possessing both physi-sorbing and chemi-sorbing properties for post-combustion CO2 capture was explored and mixtures of aminosilicones and hydroxyterminated polyethers had the best performance characteristics of materials examined. The optimal solvent composition was a 60/40 blend of GAP-1/TEG and a continuous bench-top absorption/desorption unit was constructed and operated. Plant and process models were developed for this new system based on an existing coal-fired power plant and data from the laboratory experiments were used to calculate an overall COE for a coal-fired power plant fitted with this capture technology. A reduction in energy penalty, from 30% to 18%, versus an optimized 30% MEA capture system was calculated with a concomitant COE decrease from 73% to 41% for the new aminosilicone solvent system

Synthesis of Disila-Crown Ether Complexes and Heteroatomic Bridged Paracyclophanes

Der erste Abschnitt dieser kumulativen Dissertation behandelt die Synthese von hybriden Kronenethern, in denen Tetramethyldisilan-Gruppen sowie Glycol-Gruppen vorliegen. Umsetzungen mit Alkali- und Erdalkalimetallsalzen führen zu Koordinaitonsverbindungen, die unter anderem kristallographisch untersucht wurden. DFT-Rechnungen und dynamische Proton-NMR Experimente zeigten, dass die Lithium-Komplexe von 1,2-Disila-12-Krone-4 und 1,2,7,8-Tetrasila-12-Krone-4 eine ähnliche Komplexstabilität aufweisen im Vergleich zu dem Lithium-Komplex des organischen 12-Krone-4. Im zweiten Abschnitt der Arbeit werden diverse Synthesewege zu heteroatomverbrückten Paracyclophane vorgestellt. Hierbei fanden insbesondere Silicium, Phosphor, Stickstoff und Gallium Anwendung als verbrückende Atome

Silsesquioxanes : alternative approaches and methods of synthesis.

The synthesis of silsesquioxanes from chlorosilanes has been investigated in this thesis using the “non aqueous” hydrolysis approach. DMSO (dimethylsulfoxide) has been used as a source of oxygen for the “non aqueous” hydrolysis involving di- and trichlorosilanes. With dichlorosilanes the reaction led to D rings and in particular to a D3 ring according to the following scheme: O Cl Me2 SiCL, + H3 C-S-CH 3 (Me2 SiO)n + HjC-S^CH, Cl' Under the conditions of the reaction, D3 is converted into larger and more stable rings such as D4 and D5. Other oxygen donor compounds have been studied and, as for DMSO, the result was the formation of D3 which was subsequently converted into larger rings. The mechanism has also been investigated and we found that D ring formation occurs via the formation of a,co-dichlorosiloxanes which are converted to D rings via an intramolecular cyclisation. DMSO has also been used to react with trichlorosilanes leading to cages. As for the conventional hydrolysis we expected the formation of T8 cages but to our surprise we obtained T6. Several trichlorosilanes are commercially available and so we performed the “non aqueous” hydrolysis using trichlorosilanes with a range of substituents. This provided a route to different T6s from the trichlorosilane which have previously been unobtainable. We also synthesised trichloroilanes via hydrosilylation in order to obtain trichlorosilanes carrying other functionalities which could be used to synthesise a new set of T6s which again could not be prepared by ordinary hydrolysis. We performed the “non aqueous” hydrolysis of trichlorosilanes using the other oxygen donor compounds but the results were not as satisfactory as with DMSO. Our last aim was the synthesis of cages (in particular T8 cages) carrying different arms on he silicon atoms. The synthesis of cyclic cis-polyols was the first step to achieve this aim. We used a range of approaches to the synthesis of the polyols but the most satisfactory was Shchegolikhina’s method. With this method we were able to isolate a number of new product such as the cis cyclopentyltriol and the cis cyclohexyltetrol

Computational study of the molecular and crystal structure and selected physical properties of octahydridosilasequioxane–(Si2O3H2)4. I. Electronic and structural aspects

The free molecule octahydridosilasequioxane, Si8O12H8, was computationally studied, as well as embedded in the unit cell. The point group of the free molecule is indeed Oh, and its crystal symmetry is reduced to C3i, thus confirming the occurrence of two different types of Si−O−Si bond lengths found experimentally. The molecular orbitals of the free molecule show that some electron density occurs in the cubic cavity, thus contributing to the opening of the Si−O−Si angle. A study of the packing in the unit cell identifies a new type of packing scheme in which eight (partial) molecules participate: each apex H atom of one protruding Si−H bond of every molecule points to the corner of an equilateral triangle having 2.631Å sides. All hydrogen atoms in both the free molecule and in the solid state carry negative partial charges. The reason for this is also explored, as well as its consequences for the unique packing scheme.http://rspa.royalsocietypublishing.org.nf201

Theoretical Studies of the Mechanism for the Synthesis of Silsesquioxanes. 2. Cyclosiloxanes (D3 and D4)

Several possible mechanisms for the synthesis of small ring systems, (H(OH)SiO)n, (n = 3 and 4), the three- (D3) and four- (D4) membered cyclosiloxanes, respectively, are investigated with ab initio molecular orbital methods including electron correlation effects. It is found that the substantial potential energy barriers that must be overcome for these species to form are reduced to nearly zero in the presence of a water molecule that represents the solvent

Analysis of an Adhesion Promoter for Rubber to Metal Bonding

The intermolecular and intramolecular changes induced by thermal stress in an industrial rubber to metal coupling agent (the ‘green molecule’ or GM) are the subject of this thesis. The GM was analysed in-situ in a model application environment using vibrational spectroscopy. NMR spectroscopy was used in order to analyse the solution chemistry of the compound and how this changed as a result of thermal stress. The interaction of the GM and the substrate was analysed using a range of surface analysis techniques including XPS, AFMIR and EDX. An example of a complex substrate, the zinc phosphate conversion coating, was analysed using vibrational spectroscopy and EDX to determine the manner in which it behaves in the application environment. The effect of the industrial application environment was examined by preparing adhesion test pieces and analysing the manner in which the application temperature affected their performance. In tandem with the adhesion testing, the interaction of the substrate, the GM and the rubber in the application environment was examined. This was done by preparing test pieces that used the GM in isolation as the intermediate in rubber to metal bonding. It is typically used in a formulation. Stability testing of the GM in DMSO was carried out. Vibrational analysis of the GM revealed that urethane hydrogen bonding was playing an active role in directing the intermolecular state of the GM as a function of temperature. This intermolecular association was seen to have an effect on the manner in which the GM hydrolysed and condensed in the model application environment. Solution NMR of the GM before and after being subjected to thermal stress revealed that the GM was resistant to thermally induced hydrolysis at high concentration. This effect was a result of the urethane hydrogen bonding identified using the vibrational analysis. Surface analysis of mild steel substrates that were exposed to the GM at high temperature showed that the GM binds to the substrate very sparingly. The mirror polished mild steel surfaces were visibly unchanged after thermal treatment in the presence of the pure GM. XPS analysis gave the only indication that any of the GM had bonded to the surface. IR analysis showed that the Henkel ZPCC dehydrated when subjected to thermal stress. EDX of thermally treated ZPCC showed that oxidation was occurring at the ZPCC coating however it is unclear whether this oxidation was occurring to metallic species contained in the coating or to the steel substrate underneath the coating. Adhesion testing showed that the rubber to metal adhesive formulation containing the GM formed stronger adhesive joints between the rubber and the metal at lower processing and curing temperatures than those typically used in industry. Analysis of test pieces prepared using the GM as the sole intermediate between the rubber and the metal showed that the GM nitroso moiety reacted upon mixing with the rubber. This interaction between the GM and the rubber was accompanied by sulphur release from the rubber which deposited as a sulphate on the iron substrate. The morphology of the sulphate deposit was temperature dependent, changing from crystalline to amorphous as the sample preparation temperature increased. Stability testing, where the hydrolysis of the GM induced by minute concentrations of water in DMSO-d6 was compare with common alkoxysilane precursors GPTMS and MAPTMS, showed that the GM was relatively stable in comparison to the alkoxysilane precursors that did not possess a urethane moiety in their molecular structure. The GM structure incorporates a urethane moiety that acts to stabilise the GM thermally and chemically. The hydrolysis and condensation behaviour of the GM is novel in comparison to common alkoxysilane precursors. The incorporation of the urethane moiety in the design of novel alkoxysilane precursors and also the preparation of urethane functionalised analogues of common organofunctional alkoxysilane compounds may open a doorway to improved alkoxysilane based surface treatments. These treatments will also be more easily ‘tuned’ to meet the requirements of a given application