The effect of the crystalline state on the properties of the dative bond

[摘要] ENGLISH ABSTRACT: Density functional theory (DFT) has been used to investigate the effect ofthe surrounding molecules on the structure of selected boron-nitrogen compounds.It was found that a very limited number of molecules, orientatedaccording to the experimental crystal structure, are needed to successfullyreproduce the large changes in structure witnessed when HCN–BF3and CH3CN–BF3 crystallises. Specifically, the addition of seven moleculesshortens 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 largechanges in the N–B–F angle. Investigation of the structure of these complexesin 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 bondchange. Through delocalisation of the fluorine lone pairs the antibondingσ∗(B–N) orbital becomes increasingly occupied as the N–B–F angle lessensand vice versa.Further, an investigation of the specific effects of dipole-dipole interactionswas performed by applying uniform electric fields of varying strengthalong 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 showsensitivity to the external electric field, however, only the compounds havingnitrile donors and acceptors with fluorine atoms produce large changes,which in turn are dominated by a very sudden large change in B–N bondlength occurring in a very narrow range of changing field strength. Analysisof the changes in bond character reveals that HCN–BF3 and CH3CN–BF3have long bonds in the gas phase, formed primarily through electrostatic interactionbetween the donor and acceptor. In the short bond in the condensedphase the bond character changes considerably through the introduction ofstrong electron sharing interactions, i.e. covalent or orbital interactions.Fundamental changes in the nature of the bond, catalysed by surroundingmolecules, thus lie at the heart of the large phase-dependent changes in thesespecies.

[发布机构] Stellenbosch University