Observation of the ν 3 Raman band of S 3 − inserted into sodalite cages

The S 3 − radical anion is observed in several systems: non‐aqueous polysulfides solutions, doped alkali halides, ultramarine pigments (UP) for which S 3 − is the blue chromophore and S 2 − is the yellow one and pigments of zeolite 4A structure. The S 3 − ion has C 2 V 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 ν 1 and the bending mode ν 2 have been observed, whereas the anti‐symmetric stretching mode ν 3 has never been observed whatever the system. In this work, we confirm that ν 3 is not observed in solutions with resonance Raman spectroscopy. However, our investigation of several blue UP, with various concentrations of S 2 − , shows that there is a superposition of two bands at ca 590 cm −1 : the first is assigned to ν (S 2 − ) and the second to ν 3 (S 3 − ). With the 457.9 nm excitation line, for which the resonance conditions are simultaneously fulfilled for S 2 − and S 3 − , the band at ca 590 cm −1 is the sum of the contributions of both ν (S 2 − ) and ν 3 (S 3 − ) vibrations, while, with the 647.1 nm line, which only satisfies the resonance conditions of S 3 − , the band at ca 584 cm −1 must be assigned only to ν 3 (S 3 − ). Furthermore, ν 3 (S 3 − ) is observed in green UP and in pigments of zeolite structure. The ν 3 vibration of S 3 − , 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 β‐cages of the sodalite or of the zeolite 4A structures. So, the band at ca 590 cm −1 cannot always be assigned to S 2 − in these systems. This implies that the concentration of S 2 − in UP must be reconsidered. Copyright © 2007 John Wiley & Sons, Ltd.