Ab initio molecular orbital theoretical studies of the molecular complexes of boron trifluoride and oxygen electron donor ligands

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.