Observation of the nu3 Raman band of S3- inserted into sodalite cages

The S3- radical anion is observed in several systems: non-aqueous polysulfides solutions, doped alkali halides, ultramarine pigments (UP) for which S3- is the blue chromophore and S2- is the yellow one and pigments of zeolite 4A structure. The S3- ion has C2V symmetry, and therefore its three vibrational modes should be observed in the Raman and in IR spectra. In resonance Raman spectroscopy, only the symmetric stretching mode 1 and the bending mode 2 have been observed, whereas the anti-symmetric stretching mode 3 has never been observed whatever the system. In this work, we confirm that 3 is not observed in solutions with resonance Raman spectroscopy. However, our investigation of several blue UP, with various concentrations of S2-, shows that there is a superposition of two bands at ca 590 cm-1: the first is assigned to (S2-) and the second to 3 (S3-). With the 457.9 nm excitation line, for which the resonance conditions are simultaneously fulfilled for S2- and S3-, the band at ca 590 cm-1 is the sum of the contributions of both (S2-) and 3 (S3-) vibrations, while, with the 647.1 nm line, which only satisfies the resonance conditions of S3-, the band at ca 584 cm-1 must be assigned only to 3 (S3-). Furthermore, 3 (S3-) is observed in green UP and in pigments of zeolite structure. The 3 vibration of S3-, which is observed neither in polysulfide solutions nor in doped alkali halides in resonance Raman conditions, can therefore be observed when this species is inserted into the -cages of the sodalite or of the zeolite 4A structures. So, the band at ca 590 cm-1 cannot always be assigned to S2- in these systems. This implies that the concentration of S2- in UP must be reconsidered.