Lower sulfur oxides

The lower sulfur oxides are a group of chemical compounds consisting of:

• SO, sulfur monoxide and its dimer S₂O₂
• S₂O
• cyclic sulfur monoxides, S_nO where n is 5-10
• S₇O₂
• polymeric oxides, polysulfuroxides sometimes termed PSO's

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In these the average oxidation state of sulfur is 2 or less. Interest in the lower sulfur oxides has increased because of the need to understand terrestrial atmospheric sulfur pollution and the finding that the extraterrestrial atmospheres of Io, one of Jupiters moons, and Venus contain significant amounts of sulfur oxides. Some compounds reported by early workers such as the blue "sesquioxide", S₂O₃, formed by dissolving sulfur in liquid SO₃ appears to be a mixture of polysulfate salts of the S₄²⁺ and S₈²⁺ ions.

Sulfur monoxide, monomer (SO) and dimer (S₂O₂)

These are unstable molecules that have been trapped at low temperature (see sulfur monoxide article).

Disulfur monoxide, S₂O

S₂O has a non linear structure, like sulfur dioxide, SO₂, ozone, O₃ and S₃. The S-S bond is 188.4pm, the S-O bond is 146.5pm and the SSO angle is 117.88. The two dipole moment components are μa = 0.875D and μb = 1.18D^[1]

Trisulfur monoxide, S₃O

This unstable neutral molecule has been found in the gas phase using neutralization-reionization mass spectrometry. Both ring and chain structures were found.^[2]

S_nO

A number of cyclic monoxides S_nO are known where n= 5-10. They can be prepared by oxidising the related cyclo-S_n elemental sulfur rings (these are allotropes of sulfur) with trifluoroperoxoacetic acid, CF₃C(O)OOH. The compounds are all dark coloured and decompose to give sulfur and sulfur dioxide.

S₆O₂, S₇O₂

S₆O₂ and S₇O₂ can be prepared by oxidising cyclo-S₆ and cyclo-S₇ respectively, with trifluoroperoxoacetic acid, although crystalline S₆O₂ has not been isolated.

Polymeric sulfuroxides

These have been studied to determine whether they are a factor in the observed colour of Io.^[3]

References

Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 

1. ^ Meschi, D.J.; Myers R.J. (1959). "The microwave spectrum, structure, and dipole moment of disulfur monoxide". Journal of Molecular Spectroscopy 3 (1-6): 405-416. doi:10.1016/0022-2852(59)90036-0.
2. ^ de Petris, G; Rosi M Troiani A (2006). "S₃O and S₃O⁺ in the gas phase: ring and open-chain structures". Chem. Commun: 4416 - 4418. doi:10.1039/b609646h.
3. ^ Baklouti D., D; Schmitt, B.; Brissaud, O. (11 2004). "Infrared study of lower sulfur oxides on Io's surface". Bulletin of the American Astronomical Society Chem. Commun 36: 1099.