Theoretical study of the isomerisation of the [B11H12O](-) oxaborate cluster

The structural complexity and chemical stability of the oxaborate [B11H12O] (-) ion made us use a quantum chemical calculation which is an indispensabl e tool to complete the experimental data. Therefore, we have optimised the geometries of six possible structures of the anion [B11H12O](-) using the s emi-empirical method AM1. The latter method gives satisfactory results for the boron clusters. A comparison of the heat of formation and the frontier molecular orbital energies shows that the most stable structure is that whe re the oxygen atom is bound to three boron atoms in the open face of the ca ge. This structure is in good agreement with that proposed by analysing NMR spectrum data. We have also calculated the B-B, B-H and B-O bond lengths, the bond index and the charge distribution for this conformation. Finally, we have theoretically studied the isomerisation mechanism of the stablest s tructure in order to obtain the other structures. The highest activation ba rrier energy calculated for the first stage explains the stability of this compound. The isomerisation of the stablest isomer of the [B11H12O](-) ion is found to occur by the migration of the bridged hydrogen on to the oxygen atom in order to provide the stablest conformation containing the OH funct ion. (C) 1999 Elsevier Science B.V. All rights reserved.