Observation of the ν3 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 C_2V 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 ν₁ and the bending mode ν₂ have been observed, whereas the anti‐symmetric stretching mode ν₃ has never been observed whatever the system. In this work, we confirm that ν₃ 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 ν (S₂⁻) and the second to ν₃ (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 ν (S₂⁻) and ν₃ (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 ν₃ (S₃⁻). Furthermore, ν₃ (S₃⁻) is observed in green UP and in pigments of zeolite structure. The ν₃ 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 β‐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. Copyright © 2007 John Wiley & Sons, Ltd.