Experimental and theoretical study of the reaction of POCl(3) (-) with O(2).

The oxidation of the trichlorooxyphosphorus anion (POCl(3) (-)), which takes place in combustion flames, has been examined experimentally at a variety of temperatures and theoretically via ab initio and density functional methods. The reaction was examined in a turbulent ion flow tube and kinetics was measured between 300 and 626 K, estimating an overall reaction barrier of 1.23 kcal/mol. Calculations at the density functional, Moller-Plesset second order perturbation, and coupled cluster levels of theory with basis sets up to augmented triple-zeta quality point to a multistep reaction mechanism involving an initial OP(Cl)(3)(OO) intermediate, an adduct between triplet O(2) with POCl(3) (-), subsequent formation of a four-membered nonplanar P-O-O-Cl ring transition state, with concomitant breaking of the P-Cl and O-O bonds to provide a transient intermediate OP(Cl)(2)OO...Cl, which, in turn, converts to the product complex (POCl(2) (-))(ClO) upon formation of the Cl-O bond without barrier. The calculated energy of the four-membered transition state is considered to be in good agreement with the small overall barrier found by experiment. The final step is responsible for the large exothermicity of the reaction.