Water-Mediated Isomerization in the Stepwise Hydration of the Nitrobenzene Radical Cation with 1-6 Water Molecules

The gas-phase stepwise hydration of the nitrobenzene radical cation with 1-6 water molecules has been investigated by means of ion mobility mass spectrometry and density functional theory (DFT) calculations. The stepwise binding energies (ΔH°_n-1,n) were determined by equilibrium measurements for C₆H₅NO₂^·+(H₂O)_n with n = 1-3 as 13.4, 12.7, and 10.8 kcal/mol, respectively. DFT calculations indicate that the first three hydration steps are facilitated by noncovalent interactions of the associating water molecules with calculated enthalpy changes of 13.1, 12.8, and 10.7 kcal/mol, respectively, in excellent agreement with the experimentally determined ΔH° values. However, the fourth hydration step involves irreversible addition of water to the C₆H₅NO₂^·+(H₂O)₃ cluster, resulting in the formation of a covalently bonded ion hydrated by three water molecules [C₆H₇NO₃^·+(H₂O)₃]. DFT calculations indicate that the covalently bonded ion C₆H₇NO₃^·+ is formed by the insertion of a water molecule between the ortho carbon and the oxygen atom of the nitro group of C₆H₅NO₂^·+, resulting in a nitronic phenol-type structure hydrated by three water molecules [(OH)C₆H₅(NOOH)^·+(H₂O)₃]. The fifth and sixth hydration steps were demonstrated to be in equilibrium reversible associations with experimental -ΔH° values of 11.7 and 12.1 kcal/mol, respectively, in good agreement with the calculated values of 11.9 and 11.4 kcal/mol corresponding to the formation of (OH)C₆H₅(NOOH)^·+(H₂O)₄ and (OH)C₆H₅(NOOH)^·+(H₂O)₅ clusters, respectively, of the hydrated nitronic phenol-type radical cation (OH)C₆H₅(NOOH)^·+. The observed transition constitutes the first example of water-mediated isomerization of the noncovalent microhydrated nitrobenzene, leading to a favorable pathway for the formation of the covalently bonded nitronic phenol-type radical cation. The formation of the nitronic phenol-type radical cation has significant implications for the production of gas-phase reactive hydrated organic nitrates (RONO₂)^·+(H₂O)_n in atmospheric organic aerosols, which could contribute to air quality in a variety of environments.