AB-INITIO MO STUDY OF SELECTED ALUMINUM AND BORON CHLORIDES AND FLUORIDES - COMPARISON WITH B-11 NMR-SPECTRA OF A TETRACHLOROBORATE MELT

Molecular geometries were fully optimized for AlCl3, AlCl4-, Al2Cl6, A l2Cl7-, AlF3, AlF4-, Al2F6, Al2F7-, BCl3, BCl4-, B2Cl6, B2Cl7-, BF3, B F4-, B2F6, and B2F7-, as well as a few mixed halogen species, at the H artree-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 p ossible, the computed geometries were compared to known structures fro m electron or X-ray diffraction. The agreement between these was quite good for the neutral species, and somewhat poorer for the anions. Vib rational frequencies were calculated for all species at the HF level w ith 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 calcu lations on the BF3/BF4- system indicate the structures and enthalpies are nearly converged with respect to basis set size and level of corre lation treatment. The previously unknown species B2Cl7- is predicted t o be energetically stable on the basis of the calculations. Some featu res of the B-11 NMR spectra of room temperature melts consisting of mi xtures of boron trichloride with 1-methyl-3-ethylimidazolium chloride are presented. These features suggest that these melts may contain sma ll amounts of B2Cl7- as an intermediate in an exchange reaction. (C) 1 996 by John Wiley & Sons, Inc.