Computational study of the boron-nitrogen dative bond

[摘要] In this study, ten selected boron-nitrogen compounds and three borane carbonylcomplexes were investigated by a number of computational methods. It is well knownthat the B-N dative bond is shorter in the solid state than in the gas phase. The B-COdistance, on the other hand, displays the opposite effect.Quantum mechanical techniques at the Hartree-Fock, Møller-Plesset second-order andDensity Functional Theory level were used to calculate the geometries of the isolatedmolecules and to compare them with those found in molecular clusters built to model thesolid state. It was found that calculated geometries were very sensitive to the choice ofthe basis set.The effects of dipole-dipole interactions were further investigated by applying an externalelectric field with varying strength to isolated molecules, and by replacing the centralmolecule in a cluster with a different compound. The B-N bond was found to respondmuch 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 thestrength of the crystal field, the latter being calculated classically from point charges.Unfortunately, large differences were noted between the charges calculated with commonmethods like Mulliken or Merz-Kollman-Singh. Furthermore, an analysis of 67 crystalstructures taken from the Cambridge Structural Database did not reveal a correlationbetween the length of the B-N bond and the crystal field calculated with ChargeEquilibration charges.Finally, a valence force field was developed for H3N-BH3. It was shown that a muchbetter fit of the vibrational spectrum can be obtained if the B-N stretching mode isassigned to the 603 cm-1 band rather than the peak observed at 968 cm-1.

[发布机构] Stellenbosch University