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dc.contributor.advisorDillen, J.
dc.contributor.authorVenter, Gerhard
dc.contributor.otherStellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.en
dc.date.accessioned2012-05-09T10:41:44Z
dc.date.available2012-05-09T10:41:44Z
dc.date.issued2005-12
dc.identifier.urihttp://hdl.handle.net/10019.1/20944
dc.descriptionThesis (PhD)--University of Stellenbosch, 2005.en_ZA
dc.description.abstractENGLISH 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.en_ZA
dc.description.abstractAFRIKAANSE 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.af
dc.format.extentix, 160 leaves : ill. (some col.)
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch University
dc.subjectChemistry, Physical and theoreticalen_ZA
dc.subjectChemical bondsen_ZA
dc.subjectCrystallographyen_ZA
dc.subjectTheses -- Polymer scienceen_ZA
dc.subjectDissertations -- Polymer scienceen_ZA
dc.titleThe effect of the crystalline state on the properties of the dative bonden_ZA
dc.typeThesisen_ZA
dc.rights.holderStellenbosch Universityen_ZA


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