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Title: Study of 1,3-interactions to silicon by experiment and theory
Author: Foerster, Thomas
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2007
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Molecular structure determination plays a very important role in our understanding of both chemical and physical properties of molecules. Knowing the molecular geometries of reagents, it may be possible to rationalise, for example, why some combinations of materials may undergo a chemical reaction in preference to other combinations, i.e. the chemical reactivity. Structure can explain the occurrence of certain bonding motifs in crystalline solids or even the lack of a crystalline phase. It is known that the family of silyl compounds that have SiON or SiNN motifs are very reactive and most are not stable at room temperature. The key feature of these materials is often the presence of an SiO/NN bond angle (a angle) of less than 90°, which cannot be fully understood by VSEPR theory alone. This work presents new insights into the mechanism and limitations of the a effect with reference to the molecular structures of six silyl compounds as determined by gas-phase electron diffraction (GED) and with the aid of theoretical methods, i.e. ab initio, hybrid-DFT and DFT methods. The GED structures of three chain-type silyl compounds with an A,./V-dimethylamine donor group are presented, two of which contain the SiON motif and one the SiCN motif. An additional compound with an SiNN motif was studied by high-level ab initio and hybrid-DFT methods. The experimentally determined molecular structures of three anchored silyl compounds, where the bridging atom and the donor atom are contained within a heterocyclic ring, namely l-Me3Si-l,2,4-triazole, l-Me3Si-l,2,3-benzotriazole and 1- Me3Si-thiazole, are presented, along with calculations using high-level ab initio and hybrid-DFT methods. An additional 27 compounds have been studied by theoretical methods only to exemplify the presence of the a effect in constrained molecules. Ten chain-type silyl compounds with three different heterocyclic donors groups, namely pyrrole, pyrrolidine and piperidine, were studied by high-level theoretical methods to establish a relationship between the CNC bond angle, SiONC dihedral angle and the a effect. The constraints imposed on the CNC angles and SiONC torsion angles by the ring systems allowed the relationship between these parameters and the angle contraction of the SiON angle to be quantified.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available