| dc.description.abstract | 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 N,N-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 1-Me₃Si-1,2,4-triazole, 1-Me₃Si-1,2,3-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. | en |
