Transient Raman observation and bond properties of a mixed dihalide radical anion in water

An elusive short-lived mixed dihalide radical anion (BrCl·-), produced in water by radiation-induced reactions, has been observed and analyzed by time-resolved resonance Raman spectroscopy (TRRR) and density functional theory. The Raman spectrum of the radical consists of a prominently enhanced stretching vibration, represented by a band at 212 cm-1 and its overtones. Calculations by range-separated hybrid density functionals (ωB97x and LC-ωPBE) reproduce the experimental vibrational frequency well, with a corresponding bond length of 2.713 (±0.028) Å. A first order anharmonicity of ∼1.42 cm-1 determined for the Br-Cl stretching mode in the radical suggests a convergence of vibrational states at an energy of ∼1.01 eV, using the Birge-Sponer extrapolation. This value, estimated for the radical confined in the solvent cage, compares fairly well with the calculated gas-phase energy, 1.06 eV, required for the radical to dissociate into Br· and Cl- fragments. The Raman detection of a closely related radical, ICl·-, in water was unsuccessful. The reaction pathways and equilibria have been analyzed to discuss the observability of mixed dihalide and analogous radical species.