Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2007. / In this study, ten selected boron-nitrogen compounds and three borane carbonyl
complexes were investigated by a number of computational methods. It is well known
that the B-N dative bond is shorter in the solid state than in the gas phase. The B-CO
distance, on the other hand, displays the opposite effect.
Quantum mechanical techniques at the Hartree-Fock, Møller-Plesset second-order and
Density Functional Theory level were used to calculate the geometries of the isolated
molecules and to compare them with those found in molecular clusters built to model the
solid state. It was found that calculated geometries were very sensitive to the choice of
the basis set.
The effects of dipole-dipole interactions were further investigated by applying an external
electric field with varying strength to isolated molecules, and by replacing the central
molecule in a cluster with a different compound. The B-N bond was found to respond
much more to the applied field than the B-CO bond.
An effort was made to correlate the lengthening or shortening of the dative bond to the
strength of the crystal field, the latter being calculated classically from point charges.
Unfortunately, large differences were noted between the charges calculated with common
methods like Mulliken or Merz-Kollman-Singh. Furthermore, an analysis of 67 crystal
structures taken from the Cambridge Structural Database did not reveal a correlation
between the length of the B-N bond and the crystal field calculated with Charge
Equilibration charges.
Finally, a valence force field was developed for H3N-BH3. It was shown that a much
better fit of the vibrational spectrum can be obtained if the B-N stretching mode is
assigned to the 603 cm-1 band rather than the peak observed at 968 cm-1.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2846 |
| Date | 12 1900 |
| Creators | Zhao, Hailiang |
| Contributors | Dillen, J., Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
Page generated in 0.0014 seconds