NDDO AND ABINITIO MOLECULAR-ORBITAL STUDIES OF VAPOR-PHASE COMPLEXES BETWEEN HF AND ALF3 MOLECULES

SCF calculations have been performed on various structures of the new HAlF4 (AlF3:HF = 1:1) and H2AlF5 (AlF3:HF = 1:2) complexes. In addition to ab initio calculations at minimal and extended basis set levels, a verified NDDO formalism has been used to test its applicability for semiquantitative model calculations of such systems. The optimized geometry of the energetically preferred AlF3-FH complex (C3v) calculated with the NDDO method is in good agreement with the results of ab initio calculations at the extended basis set level. Using both ab initio and NDDO methods, the same energetic sequences of the different structure models for HAlF4 and H2AlF5 have been obtained. Hence, this agreement has encouraged the use of the NDDO method for qualitative structure predetermination of more complicated AlF3 complexes with HF molecules.