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

Abstract

The mechanisms of BF, BCl, AlF and AlCl reacting with 3O 2 have been investigated by ab initio molecular orbital methods at the PMP4/6–31G ∗ and G2 levels of theory. The BX + 3O 2 reactions proceed via an XBOO intermediate that can dissociate to give XBO + 3O, which can react further to yield BO 2 + X. At the G2 level of theory, the initial barrier for BCl + 3O 2 is 8.5 kcal/mol and for BF + 3O 2 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 + 3O 2, the barrier for XAlOO → XAlO + 3 O is much higher than the barrier for decomposition back to reactants, and the reaction may proceed by a different mechanism.