Ab initiostudies of ClOx reactions. VIII. Isomerization and decomposition of ClO2 radicals and related bimolecular processes

The isomerization and decomposition of ClOO and OClO radicals and related Cl+O2 and O+ClO reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the PW91PW91/6-311+G(3df ) level and their energies refined by single-point calculations with the modified Gaussian-2 method. Predicted bond-dissociation energies of ClOO and OClO, D0(Cl–OO)=4.6 and D0(O–ClO)=58.5 kcal/mol, agree well with experimental values. Calculated rate constants for the Cl+O2→ClOO reaction in 160–1000 K at the high- and low-pressure limits can be expressed by k1∞=1.8±0.1×10−10 cm3 molecule−1 s−1 and k10(He)=1.66×10−19 T−5.34 exp(−675/T) and k10(O2)=1.26×10−16 T−6.22 exp(−943/T) cm6 molecule−2 s−1. For Ar and N2, theory underpredicts k10(M) below room temperature due to significant contributions from the “chaperon” mechanism involving Cl–M complexes. The corresponding rate constants for O+ClO→OClO are predicted to be: k2∞=4.33×10−11 T−0.03 exp(43/T) cm3 molecule−1 s−1 and k20=8.60×10−21 T−4.1 exp(−420/T) cm6 molecule−2 s−1 for 200–1000 K with N2 as the third body. The O+ClO reaction producing Cl+O2 via ClOO was found to be pressure-independent with k3=4.11×10−11 T−0.06 exp(42/T) cm3 molecule−1 s−1. For the dissociation of ClOO, the rate constants are predicted to be: k−1∞=6.17×1015 T−0.46 exp(−2570/T) s−1 and k−10=1.89×107 T−5.88 ×exp(−3280/T) cm3 molecule−1 s−1 for 160–500 K with O2 as the third-body. The corresponding rate constants for OClO dissociation can be given by: k−2∞=1.11×1016 T−0.28exp(−29600/T) s−1 and k−20=1.64×10−47 T11.0 exp(−16700/T) cm3 molecule−1 s−1 for 200–2500 K with N2 as the third body. All of the predicted rate constants, with the exception mentioned above, are in close agreement with the available experimental results.