Abstract:
The optimized geometrical structures, the interaction energies, the basis set
superposition errors (BSSEs) and the infrared spectra of the monomers,
homodimers and the molecular complexes formed between BF3 and a number
of bases have been determined by means of ab initio theoretical calculations
using the Gaussian-98W (Widows version) computer program package. Three
levels of theory have been used. These are the Restricted Hartree-Fock (RHF)
level, the second-order Moller-Plesset (MP2) perturbation theory and the
Density Functional Theory (OFT). The basis set used was the 6-3 !G(d, p)
split-valence polarized basis set in order to identify the most reliable
computational method in the prediction of their molecular properties.
Full geometry optimizations using the BERNY optimization procedure, were
carried out at the tight level of convergence using the tight (TIGHT)
convergence criterion.
The vibrational spectra of the monomers, dimers and molecular complexes
were also obtained using the same three levels of theory and same basis set.
The interaction energies of each binary species have been computed and
corrected for the basis set superposition error (BSSE) by using the full
counterpoise methods.
The monomers, dimers and molecular complexes, which were investigated for
their molecular properties, are as follows:
Molecular Complexes: BF 3_CO. BF3_C02_ BF 3_H20, Bf 3 N20, BF J 02. Bf 3.03
and BF3S02
Ab initio molecular orbital calculations have been employed in these
studies as very useful tools in the prediction of molecular parameters,
interaction energies, and the interpretation of the infrared spectra.
The good correlation that exist between the theoretical and the experimental
results obtained from the literature, emphasises the strength of using the
matrix isolation technique together with ab initio molecular orbital (MO)
calculations for studying molecular interactions. The structures, interaction
energies and infrared spectra of both dimeric isomers and molecular
complexes mentioned in this work have been predicted by means of ab initio
MO calculations at the HF, MP2 and DFT levels of theory with the standard
6-31 G( d, p) split-valence polarized basis set. The computed infrared spectra
obtained in this. way have been analysed and used as guides in the
assignment and interpretation of the matrix isolation infrared spectra
obtained from the literature, where available.
All the complexes studied in this work feature the dominant B ... 0 electron
donor-acceptor intermolecular interaction. For both the dimeric isomers and
the molecular complexes mentioned above, the calculated dimerization and
interaction energies, after being corrected for basis set superposition errors
by employing the full counterpoise method, present strong evidence that the
interactions are weak ones. All the wavenumber shifts for both the homodimers
and hetero-dimers showed small perturbation at all the three levels of
theory.
By correlating the calculated wavenumbers of the complex together with
those of the parent monomers, it has been established that the degree of the
magnitudes of the in-plane bending mode and the antisymmetric stretching
modes wavenumber shifts of the electron acceptor moiety in the complex
can be employed as guides for determining the strength of the binding
energy as well as the nature of the intermolecular interaction. The small
wavenumber shifts expressed in terms of the in-plane bending and
antisymmetric stretching modes signify very weak intermolecular
interactions accompanied by a large intermolecular interaction distances and
a low interaction energies, after the latter have been corrected for basis set
superposition errors (BSSE). For the future works the MP2 method should
be used since it is more reliable than the DFT approach in the prediction of
the experimental results, and It is virtually always an improvement on the
Hatree-Fock method.
