Pseudohalides, absorption

Sulfur free-radical chemistry is largely governed by the ability of sulfur to form three-electron bonded intermediates. A case in point is the complexation of a thiyl radical with a thiolate ion (for an analogy with the halide and other pseudohalide systems, see Chap. 5.2). These disulfide radical anions are characterized by strong absorptions in the UV-Vis (Adams et al. 1967). Complexation can occur both intermolecularly as well as intramolecularly. For GSH, for example, the stability constant of the disulfide radical anion is 2900 dm3 mol1 (Mezyk 1996a). The protonated disulfide radical anion is not stable, but such intermediates are known in the cases of the intramolecular complexes [reactions (39) and (40) Akhlaq and von Sonntag 1987]. 

In the alkali metal pseudohalides the contribution of cationic wave functions to the valence band structure can be neglected. The optical absorption spectra can therefore be correlated to transitions involving excited states of the anions. However, one can see solid state effects like the superposition of vibronic structure on the molecular symmetry forbidden transition at 5.39 eV in the crystal spectra of the alkali metal azides (76). In the more complex heavy metal and divalent azides, a whole range of optical transitions can occur both due to crystal field effects and the enhanced contributions from cationic states to the valence band. Detailed spectral measurements on a-PbNe (80), TIN3 (57), AgNs (52), Hg(CNO)2 (72) and AgCNO (72) have been made but the level assignments can at best be described as tentative since band structure calculations on these materials are not available at present. 

The reduction of other mercuric halides or pseudohalides HgXj (X = Br, T, CN , or SCN, respectively) by hydrated electrons or alcohol radicals yields as well transient HgX monomers. . They dimerize except HgCN which disproportionate. It was found that the HgjXj absorption spectra are at shorter wavelengths that those of HgX (Table 6). The monomer are strong electron donor towards O2, tetranitromethane and p-benzoquinone. On the contrary, in diluted solutions of HgClj, added with more concentrated salts such as Cd, ... 

The end-point of a precipitation titration of substances (usually halides and pseudohalides) with silver ions can be detected by the change in colour of certain compounds adsorbed on the precipitate. The colour change results from a change of the charge on the precipitate surface at the equivalence point, which leads to a change in the light absorption properties of the adsorbed indicator. The best-known example is Fluorescein, which changes from colourless to pink when silver ions become in excess in the solution. 

---