Ab initio molecular orbital calculations of the mechanism of oxidation of boron and aluminum monohalides

The mechanisms of BF, BCl, AlF and AlCl reacting with 3O2 have been investigated by ab initio molecular orbital methods at the PMP4/6–31G∗ and G2 levels of theory. The BX + 3O2 reactions proceed via an XBOO intermediate that can dissociate to give XBO + 3O, which can react further to yield BO2 + X. At the G2 level of theory, the initial barrier for BCl + 3O2 is 8.5 kcal/mol and for BF + 3O2 is 12.4 kcal/mol. These compare very favorably with the experimental barriers of 6.5 and 12 kcal/mol, obtained by fitting model transition states to the observed rate data. For AlX + 3O2, the barrier for XAlOO → XAlO + 3O is much higher than the barrier for decomposition back to reactants, and the reaction may proceed by a different mechanism.