Theoretical investigation of the isomerization and dissociation reactions of all the HOONO2 isomers

The energies and structures of various peroxynitric acid (HOONO₂) isomers as well as their isomerization and dissociation reactions have been investigated at the CBS-QB3 level of theory. One HOONO₂, five HOOONO, three HONO₂O, and one HONO₃ isomers were found here. Among them, the HOONO₂ configuration (isomer 1) is the most stable one in both the gas phase and water, while the configuration HONO₃ (isomer 9) is the energetically highest. Moreover, four HONO⋯O₂ complexes, i.e., isomers a, b, c, and d, were found. Calculated results indicate that different isomers of HOONO₂ can rearrange into each other via one-step or multi-step isomerization. The isomerization and dissociation reactions involving isomer 1 were found to be hard to occur, which implies that isomer 1 is kinetically stable in the above reactions. For other isomers, their isomerization is relatively easy to occur. Additionally, the effects of aqueous solvation of water on the isomerization and dissociation reactions were also investigated.