σ/σ* and π/π* Two-center three-electron bonding in complexes formed between charged thiocyanate and X radicals (X=Cl, Br, I and SCN)

Transient Raman spectra, structure, and thermochemistry of the thiocyanate dimer radical anion in water

Time-resolved resonance-enhanced Stokes and anti-Stokes Raman spectra of the thiocyanate dimer radical anion, (SCN)₂^•-, prepared by pulse radiolysis in water, have been obtained and interpreted in conjunction with theoretical calculations to provide detailed information on the molecular geometry and bond properties of the species. The structural properties of the radical are used to develop a molecular perspective on its thermochemistry in an aqueous solution. Twenty-nine Stokes Raman bands of the radical observed in the 120-4200 cm^-1 region are assigned in terms of the strongly enhanced 220 cm^-1 fundamental, weakly enhanced 721 cm^-1, and moderately enhanced 2073 cm^-1 fundamentals, their overtones, and combinations. Calculations by range-separated hybrid density functionals (ωB97x and LC-ωPBE) support the spectroscopic assignments of the 220 cm^-1 vibration to a predominantly SS stretching mode and the features at 721 cm^-1 and 2073 cm^-1 to CS and CN symmetric stretching modes, respectively. The corresponding bond lengths are 2.705 (±0.036) Å, 1.663 (±0.001) Å, and 1.158 (±0.002) Å. A first order anharmonicity of 1 cm^-1 determined for the SS stretching mode suggests a convergence of vibrational states at an energy of ∼1.5 eV, using the Birge-Sponer extrapolation. This value, estimated for the radical confined in solvent cage, compares well with the calculated gas-phase energy of 1.26 ± 0.04 eV required for the radical to dissociate into SCN^• and SCN^- fragments. The enthalpy of dissociation drops to 0.60 ± 0.03 eV in water when solvent dielectric effects on the radical and its dissociation products upon S-S bond scission are incorporated in the calculations. No frequency shift or spectral broadening was observed between light and heavy water solvents, indicating that the motion of solvent molecules in the hydration shell has no perceptible effect on the intramolecular dynamics of the radical. The Stokes and anti-Stokes Raman frequencies were found to be identical within the experimental uncertainty, suggesting that the frequency difference between the thermally relaxed and spontaneously created vibrational states of (SCN)₂^•- in water is too small to be observable.

Structure, energy, and IR spectra of I2∙−.nH2O clusters (n=1–8): A theoretical study

The authors report theoretical results on structure, bonding, energy, and infrared spectra of iodine dimer radical anion hydrated clusters, I(2) (-).nH(2)O (n=1-8), based on a systematic study following density functional theory. Several initial guess structures are considered for each size cluster to locate minimum energy conformers with a Gaussian 6-311++G(d,p) split valence basis function (triple split valence 6-311 basis set is applied for iodine). It is observed that three different types of hydrogen bonded structures, namely, symmetrical double hydrogen bonding, single hydrogen bonding, and interwater hydrogen bonding structures, are possible in these hydrated clusters. But conformers having interwater hydrogen bonding arrangements are more stable compared to those of double or single hydrogen bonded structures. It is also noticed that up to four solvent H(2)O units can reside around the solute in interwater hydrogen bonding network. At the maximum six H(2)O units are independently linked to the dimer anion having four double hydrogen bonding and two single hydrogen bonding, suggesting the hydration number of I(2) (-) to be 6. However, conformers having H(2)O units independently linked to the iodine dimer anion are not the most stable structures. In all these hydrated clusters, the odd electron is found to be localized over two I atoms and the two atoms are bound by a three-electron hemi bond. The solvation, interaction, and vertical detachment energies are calculated for all I(2) (-).nH(2)O clusters. Energy of interaction and vertical detachment energy profiles show stepwise saturation, indicating geometrical shell closing in the hydrated clusters, but solvation energy profile fails to show such behavior. A linear correlation is observed between the calculated energy of interaction and vertical detachment energy. It is observed that formation of I(2) (-)-water cluster induces significant shifts from the normal O-H stretching modes of isolated H(2)O. However, bending mode of H(2)O remains insensitive to the successive addition of solvent H(2)O units. Weighted average energy profiles and IR spectra are reported for all the hydrated clusters based on the statistical population of individual conformers at room temperature.