Ab initio study of BF3+(HF)1–7 clusters

Liquid HF is a weak acid but the addition of BF3 greatly increases the acidity of the solution. In order to gain insight into this phenomenon we have used density functional theory to investigate the clusters BF3+(HF)1–7. We find that BF3+(HF)1–3 are weakly bound Van der Waals complexes, as indicated by long intermolecular B–F distances. However, the B–F distance is appreciably shortened in BF3+(HF)4 and BF3+(HF)5, which adopt ring structures stabilized by the cooperative multiple hydrogen bonding between hydrogen and fluorine. Further shortening of the B–F distance occurs in these large clusters when two additional HF molecules are bound to BF3 by hydrogen bonds. The superacidity of solutions of BF3 in liquid HF is discussed in the light of our calculations.