| Abstract |
The understanding of the basic reactions of SiΟ or Si and small molecules in the gas phase and the study of the physico-chemical processes that take place are very important for the production of novel silicic materials and generally for the science and technology of materials. Especially the knowledge of SiΟ and Si chemical reactivity towards small molecules that contain atoms with lone free pair of electrons (e.g. O, N, S), is of great scientific interest. The present thesis aims fundamentally to the study of chemical reactions of silicon monoxide (SiO) and elemental silicon (Si) with CH3OH in the gas phase, as well as to the understanding of the chemical mechanism that leads to the formation of novel solid deposited products containing complicated groups of Si/O/H, Si/C/O/H, και Si/C/O/D. SiΟ and Si fragments are born through evaporation when a CO2 laser beam is directed towards a SiO solid target. The experiments took place in a static reactor under vacuum or in presence of vapors of methanol (CH3OH), d3-methanol (CD3OH) and of an inert gas (Ar) in various concentrations. The extracted Si and SiO fragments during laser ablation react mainly with the molecules of methanol and lead to the formation of very reactive and vibrationally excited products, mainly silanones or they form various clusters as the following: (SiO)n, SixOy and Sin that deposit on to the surfaces of the reactor. The silanones that are produced collide mainly with methanol molecules or with each other and undergo either vibrational relaxation or reactions that produce several dimmers like cyc-SiOSiO and cyc-SiOCO which deposit onto the surfaces of the reactor as well. The placement of KBr substrates vertically and close to the point where the ablation takes place leads to the formation of thin deposits of the previous dimeric products which were characterized using FTIR spectroscopy. The volatile products of the previous primary and secondary reactions were characterized using FTIR and mass spectroscopies. Experiments were held using several laser outputs and several number of laser pulses. The IR spectra showed that the solid products contain the vibrational frequencies δ(Si-CH3), ρ(Si-CH3), v(OSi-H), v(Si-OH), v(C-H), as well as the basic vibrations v(SiO), v(SiOSi) και v(SiOC). The equivalent groups δ(Si-CD3), ρ(Si-CD3), v(OSi-H), v(Si-OH), v(C-H) resulted from the experiments with d3-methanol (CD3OH). The analysis of the final volatile products with mass spectroscopy showed that the main final products were H2, CH4, C2H2, C2H4, CO and CO2, which come along almost exclusively through the IR multiphoton decomposition of CH3OH in the gas phase. However the compounds (CH3)Si(OCH3)(OH)2 and cyclo-(CH3)(OH)SiO2Si(CH3)(OH), that stem from the secondary reaction between silanone and methanol exist as well in the final volatile products in very small concentrations. Theoretical calculations (DFT) were also carried out for the determination of thermochemistry, structures and vibrational frequencies of the reactants, the intermediate and the final products. The calculations proved that the most stable products of the SiO + CH3OH reaction are the molecules CH3Si(OH)=O, CH3OSi(H) and cyclo-CH2OSi(OH)H, while of the reaction Si + CH3OH are the molecules CH3Si(H)=O, H2C=Si(H)OH and cyclo-CH2OSiH2. The formation of these products is achieved through highly exothermic reactions that take place through several intermediate states. Among the previous exothermic pathways only those that lead to silanones CH3Si(OH)=O and CH3OSi(H)=O are the most probable as their energetic barriers are low. Generally the reactions of SiO and Si atoms with methanol molecules are achieved through intervention between the CH3O-H or CH3-OH bonds, leading to the formation of the corresponding sililenes CH3OSiH and CH3OSi(O)H or CH3SiOH and CH3Si(O)OH. Next the stabilization of the intermediate complexes happens (a) through hydrogen approach towards Si or through a COSi ring formation that leads to the formation of (a) CH3Si(H)O through dissociation of the C---O bond, (b) H2C=Si(H)OH through dissociation of C---O bond and transfer of hydrogen from the methyl towards Si.
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