Separated Energy Domains in the Sequential Hydration of a Benzonitrile Radical Cation

The sequential hydration of organic radical cations provides a model system to investigate the intriguing interaction between the radical and solvent with unconventional carbon-based ionic hydrogen bonds. In this context, a benzonitrile radical cation (BN^•+) bonded to water (H₂O) has been experimentally investigated by infrared (IR) spectrum with a single water molecule and mass spectra with up to seven water molecules. In this work, we performed a comprehensive potential energy surface search at the highly accurate doubly hybrid density functional theory (XYGJ-OS) and couple-cluster level to exhaustively explore the low-lying structures of BN^•+-(H₂O) _n=1-6. For n = 1, an isomer where water binds to the ortho-C-H group is determined as the most populated configuration at room temperature due to its low energy and chirality. Calculated thermal-averaged IR spectrum reveals that this ortho-isomer plays an essential role in quantitative reproduction of the experimentally measured counterpart of BN^•+-H₂O. As the number of water molecules increases, a kind of distonic cation including a cycle that consists of a protonated water cluster, cyano group, and hydroxyl substituent is identified as the kinetically dominant species at ambient conditions, which requires overcoming an energy barrier as high as 30 kcal/mol to isomerize to the most stable configurations. The characteristic protonated water cluster moiety in the kinetically stable isomers results in strong ionic hydrogen bonds, which are responsible for the absence of signals of BN^•+-(H₂O) _n≤3 in experimental mass spectra. Calculated IR spectra demonstrate that the range between 1700 and 2700 cm^-1 is the diagnostic region for distinguishing kinetically and thermodynamically stable isomers. These findings establish a paradigm for the interaction between BN^•+ and water, paving the way for a further understanding of the hydration of organic radical cations.