Ab initio studies of ClO(x) reactions: prediction of the rate constants of ClO+NO2 for the forward and reverse processes.

The potential-energy surface for the reaction of ClO with NO2 has been constructed at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Six ClNO3 isomers are located; these are ClONO2, pc-ClOONO, pt-ClOONO, OClNO2, pt-OClONO, pc-OClONO, with predicted energies relative to the reactants of -25.6, -0.5, 1.0, 1.9, 12.2 and 13.6 kcal mol-1, and heats of formation at 0 K of 7.8, 32.9, 34.4, 35.5, 45.6 and 47.0 kcal mol-1, respectively. Isomerizations among them are also discussed. The rate constants for the low-energy pathways have been computed by statistical theory calculations. For the association reaction producing exclusively ClONO2, the predicted low- and high-pressure-limit rate constants in N2 for the temperature range of 200-600 K can be represented by: (N2)=3.19 x 10-17 T-5.54 exp(-384 K/T) cm6 molecule-2 s-1 and =3.33 x 10-7 T-1.48 exp(-18 K/T) cm3 molecule-1 s-1. The predicted low- and high-pressure-limit rate constants for the decomposition of ClONO2 in N2 at 200-600 K can be expressed, respectively, by =6.08 x 1013 T-6.54 exp(-13813 K/T) cm3 molecule-1 s-1 and =4.59 x 1023 T-2.43 exp(-13437 K/T) s-1. The predicted values compare satisfactorily with available experimental data. The reverse Cl+NO3 reaction was found to be independent of the pressure, giving exclusively ClO+NO2; the predicted rate constant can be expressed as k(Cl+NO3)=1.19 x 10-9 T-0.60 exp(58 K/T) cm3 molecule-1 s-1..