The selenocyanate dimer radical anion in water: Transient Raman spectra, structure, and reaction dynamics

The selenocyanate dimer radical anion (SeCN)₂ ^•-, prepared by electron pulse irradiation of selenocyanate anion (SeCN)^- in water, has been examined by transient absorption, time-resolved Raman spectra, and range-separated hybrid density functional (ωB97x and LC-ωPBE) theory. The Raman spectrum, excited in resonance with the 450 nm (λ_max) absorption of the radical, is dominated by a very strong band at 140.5 cm^-1, associated with the Se-Se stretching vibration, its overtones and combinations. A striking feature of the (SeCN)₂ ^•- Raman spectrum is the relative sharpness of the 140.5 cm^-1 band compared to the S-S band at 220 cm^-1 in thiocyanate radical anion (SCN)₂ ^•-, the difference of which is explained in terms of a time-averaged site effect. Calculations, which reproduce experimental frequencies fairly well, predict a molecular geometry with the SeSe bond length of 2.917 (±0.04) Å, the SeC bond length of 1.819 (±0.004) Å, and the CN bond length of 1.155 (±0.002) Å. An anharmonicity of 0.44 cm^-1 has been determined for the 140.5 cm^-1 Se-Se vibration which led to a dissociation energy of ∼1.4 eV for the SeSe bond, using the Morse potential in a diatomic approximation. This value, estimated for the radical confined in a solvent cage, compares well with the calculated gas-phase energy, 1.32 ± 0.04 eV, required for the radical to dissociate into (SeCN)^• and (SeCN)^- fragments. The enthalpy of dissociation in water has been measured (0.36 eV) and compared with the value estimated by accounting for the solvent dielectric effects in structural calculations.