Chemistry of NO x and HNO 3 Molecules with Gas‐Phase Hydrated O .− and OH − Ions

The gas‐phase reactions of O . − (H 2 O) n and OH − (H 2 O) n , n= 20–38, with nitrogen‐containing atmospherically relevant molecules, namely NO x and HNO 3 , are studied by Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry and theoretically with the use of DFT calculations. Hydrated O . − anions oxidize NO . and NO 2 . to NO 2 − and NO 3 − through a strongly exothermic reaction with enthalpy of −263±47 kJ mol −1 and −286±42 kJ mol −1 , indicating a covalent bond formation. Comparison of the rate coefficients with collision models shows that the reactions are kinetically slow with 3.3 and 6.5 % collision efficiency. Reactions between hydrated OH − anions and nitric oxides were not observed in the present experiment and are most likely thermodynamically hindered. In contrast, both hydrated anions are reactive toward HNO 3 through proton transfer from nitric acid, yielding hydrated NO 3 − . Although HNO 3 is efficiently picked‐up by the water clusters, forming (HNO 3 ) 0–2 (H 2 O) m NO 3 − clusters, the overall kinetics of nitrate formation are slow and correspond to an efficiency below 10 %. Combination of the measured reaction thermochemistry with literature values in thermochemical cycles yields Δ H f (O − (aq.))=48±42 kJ mol −1 and Δ H f (NO 2 − (aq.))=−125±63 kJ mol −1 .