The structure and properties of the dihalo(dimethyl)germanes and related compounds
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.
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