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

Liquid HF is a weak acid but the addition of BF, greatly increases the acid ity 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 i s appreciably shortened in BF3 + (HF)(4) and BF3 + (HF)(5), which adopt rin g structures stabilized by the cooperative multiple hydrogen bonding betwee n hydrogen and fluorine. Further shortening of the B-F distance occurs in t hese large clusters when two additional HF molecules are bound to BF, by hy drogen bonds. The superacidity of solutions of BF, in liquid HF is discusse d in the light of our calculations. (C) 1999 Published by Elsevier Science B.V. All rights reserved.