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.
Christensen et al. (Mon,) studied this question.