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

The selenocyanate dimer radical anion (SeCN) , 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) absorption of the radical, is dominated by a very strong band at 140.5 cm, associated with the Se-Se stretching vibration, its overtones and combinations. A striking feature of the (SeCN) Raman spectrum is the relative sharpness of the 140.5 cm band compared to the S-S band at 220 cm in thiocyanate radical anion (SCN) , 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 has been determined for the 140.5 cm 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.