Electron localization function view of bonding in selected aluminum fluoride molecules

A series of calculations has been performed on the AIF(4)(-) and CaAlF5 spe cies in order to precise their equilibrium geometry as well as to get some insight onto their bonding properties. These species play an important role in electrochemical industrial processes such as the electrowinning of alum inum (i.e. the Heroult-Hall process). The calculations have been performed within the density functional approach using the B3LYP hybrid functional an d the 6-311 + + G(3df,2p) all-electron basis sets on all the atomic centres . The optimized geometries and the calculated frequencies of AIF(4)(-) are found to be in agreement with those previously obtained at a lower level of calculation and with experiments. The bonding has been investigated by the topological analysis of the electron localization function gradient field. The analysis indicates that the Al-F bond presents a noticeable covalent c haracter testified by the presence of a disynaptic basin between the Al and F cores. The basins' populations and their fluctuations have been calculat ed indicating an important delocalization between the AI-F bond and the flu orines lone pairs. Moreover, the structural role of the counter cation Ca2 has been emphasized. A structural and topological analysis of CaAlF5 has b een carried out. This study has clearly established that the CaAlF5 molecul e can be described as purely ionic interactions between the F-, AlF3 and Ca 2+ species. (C) 1999 Published by Elsevier Science B.V. All rights reserved .