Ab initio MO study of selected aluminum and boron chlorides and fluorides: Comparison with11B NMR spectra of a tetrachloroborate melt

Abstract

Molecular geometries were fully optimized for AlCl3, AlCl4-, Al2Cl6, Al2Cl7-, AlF3, AlF4-, Al2F6, Al2F7-, BCl3, BCl4-, B2Cl6, B2Cl7-, BF3, BF4-, B2F6, and B2F7-, as well as a few mixed halogen species, at the Hartree-Fock (HF) level, using basis sets from STO-3G to 6–311 + G(d). In some cases geometries were also optimized at the MP2 level. Where possible, the computed geometries were compared to known structures from electron or X-ray diffraction. The agreement between these was quite good for the neutral species, and somewhat poorer for the anions. Vibrational frequencies were calculated for all species at the HF level with the largest basis set. The geometries were characterized as minima or transition structures. Various formation reaction enthalpies were calculated; these compare well with known values. More extensive calculations on the BF3/BF4- system indicate the structures and enthalpies are nearly converged with respect to basis set size and level of correlation treatment. The previously unknown species B2Cl7- is predicted to be energetically stable on the basis of the calculations. Some features of the 11B NMR spectra of room temperature melts consisting of mixtures of boron trichloride with 1-methyl-3-ethylimidazolium chloride are presented. These features suggest that these melts may contain small amounts of B2Cl7- as an intermediate in an exchange reaction. © 1996 by John Wiley & Sons, Inc.