Trisulfur

The S3 molecule, known as trisulfur, sulfur trimer, thiozone, or triatomic sulfur, is a cherry-red allotrope of sulfur. It comprises about 10% of vaporised sulfur at 713 K and 1333 Pa. It has been observed at cryogenic temperatures as a solid. Under ordinary conditions it converts to cyclooctasulfur.


 * 8 S3 → 3 S8

Structure and bonding
In terms of structure and bonding S3 and ozone (O3) are similar. Both adopt bent structures and are diamagnetic. Although represented with S=S double bonds, the bonding situation is more complex.

The S–S distances are equivalent and are $191.7 pm$, and with an angle at the central atom of $117.36 °$. However, cyclic S3, where the sulfur atoms are arranged in an equilateral triangle with three single bonds (similar to cyclic ozone and cyclopropane), is calculated to be lower in energy than the bent structure experimentally observed.

The name thiozone was invented by Hugo Erdmann in 1908 who hypothesized that S3 comprises a large proportion of liquid sulfur. However its existence was unproven until the experiments of J. Berkowitz in 1964. Using mass spectrometry, he showed that sulfur vapour contains the S3 molecule. Above 1200 °C S3 is the second most common molecule after S2 in gaseous sulfur. In liquid sulfur the molecule is not common until the temperature is high, such as 500 °C. However, small molecules like this contribute to most of the reactivity of liquid sulfur. S3 has an absorption peak of $425 nm$ (violet) with a tail extending into blue light.

S3 can also be generated by photolysis of S3Cl2 embedded in a glass or matrix of solid noble gas.

Natural occurrence
S3 occurs naturally on Io in volcanic emissions. S3 is also likely to appear in the atmosphere of Venus at heights of 20 to 30 km, where it is in thermal equilibrium with S2 and S4. The reddish colour of Venus' atmosphere at lower levels is likely to be due to S3.

Reactions
S3 reacts with carbon monoxide to make carbonyl sulfide and S2.

Formation of compounds with a defined number of sulfur atoms is possible:


 * S3 + S2O → S5O (cyclic)

Radical anion
Although S3 is elusive under ordinary conditions, the radical anion S3− is abundant. It exhibits an intense blue colour. The anion is sometimes called thiozonide, by analogy with the ozonide anion, O3-, to which it is valence isoelectronic. The gemstone lapis lazuli and the mineral lazurite (from which the pigment ultramarine is derived) contain S3-. International Klein Blue, developed by Yves Klein, also contains the S3- radical anion. The blue colour is due to the C2A2 transition to the X2B1 electronic state in the ion, causing a strong absorption band at 610–$$ or $620 nm$ (in the orange region of the visible spectrum). The Raman frequency is $2.07 eV$ and another infrared absorption is at $523 cm-1$.

The S3- ion has been shown to be stable in aqueous solution under a pressure of 0.5 GPa, and is expected to occur naturally at depth in the Earth's crust where subduction or high pressure metamorphism occurs. This ion is probably important in movement of copper and gold in hydrothermal fluids.

Lithium hexasulfide (which contains S6-, another polysulfide radical anion) with tetramethylenediamine solvation dissociates acetone and related donor solvents to S3-.

The S3- radical anion was also made by reducing gaseous sulfur with Zn(2+) in a matrix. The material is strongly blue-coloured when dry and changes colour to green and yellow in the presence of trace amounts of water. Another way to make it is with polysulfide dissolved in hexamethylphosphoramide where it gives a blue colour.

Other methods of production of S3- include reacting sulfur with partially hydroxylated magnesium oxide at 400°C.

Raman spectroscopy can be used to identify S3-, and it can be used non-destructively in paintings. The bands are $580 cm-1$ for symmetric stretch, $$ for asymmetric stretch, and $$ for bending. Natural materials can also contain S2- which has an optical absorption at $549 cm-1$ and Raman band at $585 cm-1$.

Trisulfide ion
The trisulfide ion, S3(2-) is part of the polysulfide series. The sulfur chain is bent at an angle of 107.88°. Strontium trisulfide (SrS3) has a S–S bond length of $259 cm-1$. The bonds are single. It is isoelectronic to sulfur dichloride.