Water-Mediated Isomerization in the Stepwise Hydration of the Nitrobenzene Radical Cation with 1–6 Water Molecules
Year: 2026
Type: article
Abstract: High Resolution Image Download MS PowerPoint Slide 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 (Δ H ° n –1, n ) were determined by equilibrium measurements for C 6 H 5 NO 2 ·+ (H 2 O) 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 Δ H ° values. However, the fourth hydration step involves irreversible addition of water to the C 6 H 5 NO 2 ·+ (H 2 O) 3 cluster, resulting in the formation of a covalently bonded ion hydrated by three water molecules [C 6 H 7 NO 3 ·+ (H 2 O) 3 ]. DFT calculations indicate that the covalently bonded ion C 6 H 7 NO 3 ·+ is formed by the insertion of a water molecule between the ortho carbon and the oxygen atom of the nitro group of C 6 H 5 NO 2 ·+, resulting in a nitronic phenol-type structure hydrated by three water molecules [(OH)C 6 H 5 (NOOH) ·+ (H 2 O) 3 ]. The fifth and sixth hydration steps were demonstrated to be in equilibrium reversible associations with experimental −Δ H ° 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)C 6 H 5 (NOOH) ·+ (H 2 O) 4 and (OH)C 6 H 5 (NOOH) ·+ (H 2 O) 5 clusters, respectively, of the hydrated nitronic phenol-type radical cation (OH)C 6 H 5 (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 (RONO 2 ) ·+ (H 2 O) n in atmospheric organic aerosols, which could contribute to air quality in a variety of environments.
Institution: Virginia Commonwealth University
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: hybrid
Funder: National Science Foundation
OpenAlex ID: W7140074223