IR Spectroscopy of 4-Aminobenzonitrile+–Arn (n = 0–2): Determination of the Activation Barrier for the π → NH Site-Switching Reaction

Information about the intermolecular potential energy surface for the interaction between solute and solvent molecules is required to understand the impact of solvation on reaction mechanisms and dynamics. In this study, we measured vibrational-specific infrared (IR) spectra of 4-aminobenzonitrile–(argon)_n cation clusters, 4ABN⁺–Ar_n (n = 1, 2), in the NH stretching range to elucidate the energetics of the photoionization-induced π → NH migration of Ar. The IR spectra of 4ABN⁺–Ar_n generated by resonant photoionization of neutral π-bonded clusters display the hydrogen-bonded NH₂ stretching vibration (ν_NH2) only when intermolecular vibrational levels are excited. This is the first observation of Ar migration from the aromatic ring toward the NH₂ group upon photoionization in the n = 1 cluster. From the vibrational-level dependence of the IR spectra, the activation barrier heights are determined to be 21–47 (34 ± 13) and <27 cm^–1 for 4ABN⁺–Ar₁ and 4ABN⁺–Ar₂, respectively. The potential energy surfaces and mechanism of the Ar migration are discussed with the help of complementary density functional theory calculations.