On the constituents of aqueous polyselenide electrolytes: A combined theoretical and Raman spectroscopic study

The speciation of aqueous polyselenide solutions at high pH values has been investigated by Raman and UV/vis absorption spectroscopy. Complementary, ab initio molecular orbital calculations at the HF/6-31+G* level of theory have been carried out on the vibrational frequencies of polyselenide dianions Se-x(2-) (x = 2-4), polyselenide radical anions Se-x(-) (x = 2, 3), and protonated polyselenides HSex- (x = 1-3) in order to substantiate the assignment of the experimental Raman spectra. Although the optical spectra suggest the successive formation of Se-2(2-), Se-3(2-), and Se-4(2-) with progressive oxidation of the solutions, only two characteristic Raman bands are observed at 269 and 324 cm(-1). A sequence of overtones of the 269 cm(-1) band is observed after excitation with blue light. Likewise strong overtones of the 324 cm(-1) band appear after excitation with green light. The 269 cm(-1) feature is assigned to Se-4(2-) while uhe 324 cm(-1) band is attributed to the radical anion Se-2(-). The occurrence of polyselenide radicals in aqueous solution is without precedent, and the formation of Se-2(-) is ascribed to a photolytic process in the electrolyte. The only protonated species observed in this study is HSe-, which is characterized by a Raman band at 2303 cm(-1).