Ab initio chemical kinetics for the hydrolysis of N₂O ₄ isomers in the gas phase

The mechanism and kinetics for the gas-phase hydrolysis of N ₂O₄ isomers have been investigated at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level of theory in conjunction with statistical rate constant calculations. Calculated results show that the contribution from the commonly assumed redox reaction of sym-N₂O ₄ to the homogeneous gas-phase hydrolysis of NO₂ can be unequivocally ruled out due to the high barrier (37.6 kcal/mol) involved; instead, t-ONONO₂ directly formed by the association of 2NO ₂, was found to play the key role in the hydrolysis process. The kinetics for the hydrolysis reaction, 2NO₂ + H₂O HONO + HNO₃ (A) can be quatitatively interpreted by the two step mechanism: 2NO₂ → t-ONONO₂, t-ONONO₂ + H₂O → HONO + HNO₃. The predicted total forward and reverse rate constants for reaction (A), k_tf = 5.36 ×10^-50T ^3.95exp(1825/T) cm⁶ molecule^-2 s^-1 and k_tr = 3.31 ×10^-19T^2.478exp(-3199/T) cm³ molecule^-1 s^-1, respectively, in the temperature range 200-2500 K, are in good agreement with the available experimental data.