Disulfur is the diatomic molecule consisting of two sulfur atoms with the formula S<sub>2</sub>. It is analogous to the dioxygen molecule but rarely occurs at room temperature. This violet gas is the dominant species in hot sulfur vapors. S<sub>2</sub> is one of the minor components of the atmosphere of Io, which is predominantly composed of SO<sub>2</sub>. The instability of S<sub>2</sub> is usually described in the context of the double bond rule.
This violet gas is generated by heating sulfur and is the predominant species above 720 ðC, comprising 80% of all vapor species at 530ðC and 100 mm Hg, and 99% of the vapor at low pressure (1 mm Hg) at 730 ðC.
Disulfur can be produced when an atmosphere of COS is irradiated with UV light using a mercury photosensitizer or when CS<sub>2</sub>, H<sub>2</sub>S<sub>2</sub>, S<sub>2</sub>Cl<sub>2</sub> or C<sub>2</sub>H<sub>4</sub>S, PSF<sub>3</sub> or COS are irradiated.
Gaseous disulfur has been detected emanating from the surface of Jupiter's moon Io, from the vicinity of Pele volcano.
The ground state of S<sub>2</sub> is a triplet: a diradical, with two unpaired electrons like O<sub>2</sub> and SO. It has the SâÂÂS bond length of 189 pm, much shorter than the SâÂÂS single bonds in S<sub>8</sub>, which are 206 pm long. Its Raman spectrum consists of a band at 715 cm<sup>âÂÂ1</sup>. The corresponding OâÂÂO band for O<sub>2</sub> is found at 1556 cm<sup>âÂÂ1</sup>. The S-S bond energy is 430 kJ/mol compared to 498 kJ/mol for O<sub>2</sub>.
Singlet disulfur is believed to be the product of trisulfides with triphenylphosphine dibromide, or the thermal decomposition of dialkoxy disulfides. However, the disulfur produced in that reaction is believed singlet because it undergoes formal hetero-DielsâÂÂAlder cycloadditions, not because of any spectroscopic evidence.
Triplet disulfur readily photodissociates, with a mean lifespan of 7.5 min in sunlight.