The cis–trans isomerization easily occurs, and the barrier of conversion tran- to cis-ONO–NO2 is 7.57 kcal/mol, which can compare with the number reported by Liu and Goddard (2012), 4.4 kcal/mol. Such a low barrier could lead to a fast equilibrium between cis- and trans-ONO–NO2 at room temperature.
3.4. Hydrolysis of N2O4 isomers
In the marine environment, simulation of the heterogeneous reaction in the presence of water vapor much more resembles the real atmospheric environment than a dry one. Thus, in this BIBW 2992 work, the reactions of H2O with sym-O2N–NO2, trans-ONO–NO2, and cis-ONO–NO2 on the (100) surface of NaCl have been considered.
At the (100) surface of NaCl, the interaction of H2O with sym-O2N–NO2 further forms one complex, H2O–sym-O2N–NO2 (NaCl), with − 7.78 kcal/mol association energies. The H2O–sym-O2N–NO2 (NaCl) dissociates to HNO3 + HONO via a five-membered ring transition state TS4 with 33.79 kcal/mol energy above H2O + sym-O2N–NO2. As the reaction takes place, the two NO2 units of sym-O2N–NO2 start to depart from each other. The O atom of H2O approaches an N atom of one NO2 unit on the surface of NaCl, while an H atom of H2O approaches to an O atom of the other NO2 unit. At the transition state, as shown as TS4 in Fig. 4, the two NO2 units match the H and OH fragments of H2O, respectively, to form the HONO and HNO3 product molecules. As a result, the reaction is not kinetically favorable. Moreover, the reaction is endothermic by 9.39 kcal/mol, and is not thermo-dynamically favorable.