Complexes of HNO₃ and NO₃^- with NO₂ and N₂O₄, and their potential role in atmospheric HONO formation

Calculations were performed to determine the structures, energetics, and spectroscopy of the atmospherically relevant complexes (HNO₃)·(NO₂), (HNO₃)·(N₂O₄), (NO₃^-)·(NO₂), and (NO₃^-)·(N₂O₄). The binding energies indicate that three of the four complexes are quite stable, with the most stable (NO₃^-)·(N₂O₄) possessing binding energy of almost -14 kcal mol^-1. Vibrational frequencies were calculated for use in detecting the complexes by infrared and Raman spectroscopy. An ATR-FTIR experiment showed features at 1632 and 1602 cm^-1 that are attributed to NO₂ complexed to NO₃^- and HNO₃, respectively. The electronic states of (HNO₃)·(N₂O₄) and (NO₃^-)·(N₂O₄) were investigated using an excited state method and it was determined that both complexes possess one low-lying excited state that is accessible through absorption of visible radiation. Evidence for the existence of (NO₃^-)·(N₂O₄) was obtained from UV/vis absorption spectra of N₂O₄ in concentrated HNO₃, which show a band at 320 nm that is blue shifted by 20 nm relative to what is observed for N₂O₄ dissolved in organic solvents. Finally, hydrogen transfer reactions within the (HNO₃)·(NO₂) and (HNO₃)·(N₂O₄) complexes leading to the formation of HONO, were investigated. In both systems the calculated potential profiles rule out a thermal mechanism, but indicate the reaction could take place following the absorption of visible radiation. We propose that these complexes are potentially important in the thermal and photochemical production of HONO observed in previous laboratory and field studies.