Disulfur is the
diatomic molecule
Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...
with the formula S
2. It is analogous to the
dioxygen
There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are:
* Ato ...
molecule but rarely occurs at room temperature. This violet gas is the dominant species in hot sulfur vapors. S
2 is one of the minor components of the atmosphere of
Io, which is predominantly composed of
SO2. The instability of S
2 is usually described in the context of the
double bond rule
In chemistry, the double bond rule states that elements with a principal quantum number (''n'') greater than 2 for their valence electrons ( period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). Do ...
.
Synthesis
This violet gas is generated by heating sulfur and is the predominant species above 720 °C, comprising 80% of all vapor species at 530°C and 100 mm Hg, and 99% of the vapor at low pressure (1 mm Hg) at 730 °C.
Disulfur can be produced when an atmosphere of
COS is irradiated with UV light using a mercury
photosensitizer or when CS
2, H
2S
2, S
2Cl
2 or C
2H
4S, PSF
3 or COS are irradiated.
Natural occurrence
Gaseous disulfur has been detected emanating from the surface of Jupiter's moon
Io, from the vicinity of
Pele volcano.
Properties
The ground state of S
2 is a
triplet: a
diradical
In chemistry, a diradical is a chemical species, molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate energy level, degenerate. The term "diradical" is mainly used to describe organic compounds, where most ...
, with two unpaired electrons like O
2 and
SO. It has the S–S bond length of 189 pm, much shorter than the S–S single bonds in S
8, which are 206 pm long. Its
Raman spectrum consists of a band at 715 cm
−1. The corresponding O–O band for O
2 is found at 1556 cm
−1. The S-S
bond energy
In chemistry, bond energy (''BE'') is one measure of the strength of a chemical bond. It is sometimes called the mean bond, bond enthalpy, average bond enthalpy, or bond strength. IUPAC defines bond energy as the average value of the gas-phase b ...
is 430 kJ/mol compared to 498 kJ/mol for O
2.
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-
Diels–Alder cycloadditions, not because of any spectroscopic evidence.
Triplet disulfur readily
photodissociates,
with a mean lifespan of 7.5 min in sunlight.
References
External links
*
{{Diatomic elements
Allotropes of sulfur
Homonuclear diatomic molecules
Gases with color