Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: There is limited experimental and computational information available on the
structures of compounds of the form R2GeX2, where X is a halogen and R an alkyl
group. Gas phase electron diffraction studies of the dihalo(dimethyl)germanes
(R=Me)consistently give C-Ge-C angles in the range of 120-125°, about 10° larger
than the corresponding C-C-C angles in the 2,2-dihalopropanes. However, in
dimethylgermane, where the halogen atoms are substituted by hydrogen, the
value of the C-Ge-C is very similar to the corresponding C-C-C angle in propane
and deviates little from the tetrahedral value of 109.47°.
The unusually large influence of atomic substituents on the value of the valence
angles in these compounds introduces a serious challenge to the development of
empirical force fields, where the size of an angle is traditionally determined only
by the atoms directly involved in the formation of the angle and not by the other
substituents attached to the central atom. Unfortunately, the large experimental
errors in the gas phase electron diffraction studies and the lack of representative
crystalline compounds in the Cambridge Structural Database make it impossible
to establish conclusively whether these large valence angles are significant or just
statistical anomalies.
A systematic ab initio study of a number of compounds of the general form
Me2AX2with A=C, Si or Ge and X=H, F, Cl, Br or I has been initiated to verify the
experimental results and to try to explain this observed deviation in valence angle
in terms of electronic effects and existing theories of structure and bonding. The
carbon and silicon analogs of the dimethylated germanes were included in the
calculations to ascertain whether the observed effect is an anomaly or merely a
periodic trend in the group IVelements. To obtain a clearer overall view, identical
calculations were also performed on compounds of the form AHnX4-n,MeAH2X,
MeAHX2and Me2AHX,where Aand X have the same meaning as before. The ab initio calculations confirmed that there is in fact a significant increase in
the C-A-C angle from A=C to A=Ge in the compounds Me2AX2,although the
calculated increase is smaller than the experimentally determined increase by a
few degrees. Together with this observed increase in the C-A-C angle there is a
corresponding decrease in the X-A-X angle. Calculation of the electron density of
three representative compounds revealed a significant difference in electron
distribution between the germanium compounds and their carbon analogs,
suggesting that the ionicity of the bonds and the electronegativity of the
substituents may playa role in the size of the C-A-C angle in compounds of this
form. This is supported by a statistical analysis of compounds in the Cambridge
Structural Database containing a C2GeYZ fragment, where Y and Z may be any
elements except carbon, which showed that the average C-Ge-C angle in
compounds where Y and Z are electronegative is approximately 7° larger than in
compounds where Y and Z are electropositive. The qualitative trends in the C-A-C
and X-A-X angles have also been discussed in terms of three different bonding
models.
To verify the results of the ab initio calculations experimentally, a representative
compound, dichlorobis(phenethyl)germane, has been synthesized and its crystal
structure determined by X-ray diffraction. The C-Ge-C angle was found to be
121.2°, which is in good agreement with both the ab initio and the gas phase
electron diffraction results.
Furthermore, a force field for halogenated organic carbon, silicon and germanium
compounds has also been developed based on the structural and vibrational data
obtained from the ab initio calculations. Molecules of the form AHnX4-nand
Me2AX2with A=C, Si, Ge and X=H, F, Cl and Br were used in the training set and
the bond lengths, bond angles and vibrational frequencies were used to optimize
the force field. Calculations performed with the force field reproduce the C-A-C
angles to within 1° of the observed values and the reproducibility for the rest of
the experimental data is also good. Force fields have been developed for some of
the simpler molecules in our training set and where this is the case, the force
field parameters have been compared to the previously determined values. / AFRIKAANSE OPSOMMING:
Sien volteks vir opsomming
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53178 |
| Date | 03 1900 |
| Creators | Rohwer, Heidi |
| Contributors | Dillen, J., Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 163 p. : ill. |
| Rights | Stellenbosch University |
Page generated in 0.0022 seconds