Thesis (PhD)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Density functional theory (DFT) has been used to investigate the effect of
the surrounding molecules on the structure of selected boron-nitrogen compounds.
It was found that a very limited number of molecules, orientated
according to the experimental crystal structure, are needed to successfully
reproduce the large changes in structure witnessed when HCN–BF3
and CH3CN–BF3 crystallises. Specifically, the addition of seven molecules
shortens the B–N distance by 0.735 °A in (HCN–BF3)8 and 0.654A° in (CH3CN–
BF3)8.
Accompanying the large changes in B–N bond length are equally large
changes in the N–B–F angle. Investigation of the structure of these complexes
in terms of localised electron pairs shows that the availability of lone pairs,
in close proximity to the B–N bond axis, plays an important role in the bond
change. Through delocalisation of the fluorine lone pairs the antibonding
σ
∗(B–N) orbital becomes increasingly occupied as the N–B–F angle lessens
and vice versa.
Further, an investigation of the specific effects of dipole-dipole interactions
was performed by applying uniform electric fields of varying strength
along the donor-acceptor bond axis of a series of compounds of the form X–Y;
X=H3N, HCN, CH3CN; Y = BF3, BH3, SO3. All complexes investigated show
sensitivity to the external electric field, however, only the compounds having
nitrile donors and acceptors with fluorine atoms produce large changes,
which in turn are dominated by a very sudden large change in B–N bond
length occurring in a very narrow range of changing field strength. Analysis
of the changes in bond character reveals that HCN–BF3 and CH3CN–BF3
have long bonds in the gas phase, formed primarily through electrostatic interaction
between the donor and acceptor. In the short bond in the condensed
phase the bond character changes considerably through the introduction of
strong electron sharing interactions, i.e. covalent or orbital interactions.
Fundamental changes in the nature of the bond, catalysed by surrounding
molecules, thus lie at the heart of the large phase-dependent changes in these
species. / AFRIKAANSE OPSOMMING: ’n Kohn-Sham elektrondigtheidsteorie (DFT) studie is gedoen op die effek
van die omliggende molekules in die kristalstruktuur van sekere molekules
wat boor-stikstof bindings bevat. Daar is gevind dat ’n klein aantal
molekules, georienteer soos in die eksperimentele kristalstruktuur, benodig
word om die groot veranderinge in stuktuur te veroorsaak wat eksperimenteel
waargeneem word wanneer HCN–BF3 en CH3CN–BF3 kristaliseer.
Spesifiek, die byvoeging van sewe molekules verminder die B–N bindingslengte
met 0.735 °A in (HCN–BF3)8 en 0.654 A° in (CH3CN–BF3)8.
Die groot veranderinge in B–N bindingslengte gaan saam met ewe groot
veranderinge in die N–B–F hoek. ’n Ondersoek van die struktuur van die
molekules in terme van gelokaliseerde elektronpare wys dat die beskikbaarheid
van alleenpare, wat naby die B–N bindingsas lˆe, ’n belangrike rol speel
in the verandering in bindingslengte. Deur delokalisasie van die fluoor alleenpare
word die antibindende σ
∗(B–N) orbitaal toenemend beset soos die
N–B–F hoek afneem en omgekeerd.
Verder is die spesifieke effek van dipool-dipool interaksies ondersoek
deur uniforme elektriese velde aan te lˆe langs the donor-akseptor bindingsas
van ’n reeks komplekse van die vorm X–Y; X = H3N, HCN, CH3CN; Y = BF3,
BH3, SO3. Al die komplekse toon sensitiwiteit teenoor die eksterne elektriese
veld, maar net die verbindings wat nitriel akseptore en fluoor atome aan the
donor fragmente het, toon groot veranderinge, wat op hulle beurt weer oorskadu
word deur ’n skielike verandering in the B–N bindingslengte in ’n
nou band van veranderende veldsterkte. Analise van die veranderinge in
bindingskarakter toon dat HCN–BF3 en CH3CN–BF3 lang bindings in die
gasfase het, wat hoofsaaklik gevorm word deur elektrostatiese interaksies
tussen die donor en akseptor fragmente. In die kort binding in die kristalfase
is daar ’n aansienlike verandering in the karakter as gevolg van die intrede
van sterk elektrondelingsinteraksies, m.a.w. kovalente of orbitaalinteraksies.
Fundamentele veranderinge in the manier wat die binding saamgestel
word, wat gekataliseer word deur omliggende molekules, is dus die oorsaak
van die groot faseafhanklike veranderinge.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20944 |
| Date | 12 1900 |
| Creators | Venter, Gerhard |
| 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 | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | ix, 160 leaves : ill. (some col.) |
| Rights | Stellenbosch University |
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