Heptasulfur
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The element
sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
exists as many
allotropes Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: th ...
. In number of allotropes, sulfur is second only to
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
.
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In addition to the allotropes, each allotrope often exists in polymorphs (different crystal structures of the same covalently bonded Sn molecules) delineated by Greek prefixes (α, β, etc.). Furthermore, because elemental sulfur has been an item of commerce for centuries, its various forms are given traditional names. Early workers identified some forms that have later proved to be single or mixtures of allotropes. Some forms have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them, e.g. "Muthmann's sulfur I" or "Engel's sulfur". Steudel, 17 The most commonly encountered form of sulfur is the
orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...
polymorph of , which adopts a puckered ring – or "crown" – structure. Two other polymorphs are known, also with nearly identical molecular structures.
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In addition to , sulfur rings of 6, 7, 9–15, 18, and 20 atoms are known.
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At least five allotropes are uniquely formed at high pressures, two of which are metallic. The number of sulfur allotropes reflects the relatively strong S−S bond of 265 kJ/mol. Furthermore, unlike most elements, the allotropes of sulfur can be manipulated in solutions of organic solvents and are analysed by
HPLC High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify specific components in mixtures. The mixtures can origina ...
.


Phase diagram

The pressure-temperature (P-T) phase diagram for sulfur is complex (see image). The region labeled I (a solid region), is α-sulfur.


High-pressure solid allotropes

In a high-pressure study at ambient temperatures, four new solid forms, termed II, III, IV, V have been characterized, where α-sulfur is form I. Solid forms II and III are polymeric, while IV and V are metallic (and are
superconductive Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases gr ...
below 10 K and 17 K, respectively). Laser irradiation of solid samples produces three sulfur forms below 200–300 kbar (20–30 GPa).


Solid ''cyclo'' allotrope preparation

Two methods exist for the preparation of the ''cyclo''-sulfur allotropes. One of the methods, which is most famous for preparing
hexasulfur Hexasulfur is an inorganic chemical with the chemical formula . This allotrope was first prepared by M. R. Engel in 1891 by treating thiosulfate with HCl. ''Cyclo''- is orange-red and forms a rhombohedral crystal. It is called ρ-sulfur, ε-sulfur ...
, is to treat hydrogen polysulfides with polysulfur dichloride: : A second strategy uses
titanocene pentasulfide Titanocene pentasulfide is the organotitanium compound with the formula (C5H5)2TiS5, commonly abbreviated as Cp2TiS5. This metallocene exists as a bright red solid that is soluble in organic solvents. It is of academic interest as a precursor to ...
as a source of the unit. This complex is easily made from polysulfide solutions: : Titanocene pentasulfide reacts with polysulfur chloride: :


Solid ''cyclo''-sulfur allotropes


''Cyclo''-hexasulfur, ''cyclo''-

This allotrope was first prepared by M. R. Engel in 1891 by treating
thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...
with HCl. ''Cyclo''- is orange-red and forms a
rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a special case of a parallelepiped in which all six faces are congruent rhombus, rhombi. It can be used to define the rhombohedral lattice system, a Ho ...
crystal.
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It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. Another method of preparation involves the reaction of a
polysulfane A polysulfane is a chemical compound of formula , where ''n'' > 1 (although disulfane () is sometimes excluded). Compounds containing 2 – 8 sulfur atoms have been isolated, longer chain compounds have been detected, but only in solution.R. Steud ...
with
sulfur monochloride Disulfur dichloride (or disulphur dichloride by the British English spelling) is the inorganic compound of sulfur and chlorine with the formula . It is an amber oily liquid. Sometimes, this compound is incorrectly named ''sulfur monochloride'' (o ...
: : (dilute solution in
diethyl ether Diethyl ether, or simply ether, is an organic compound with the chemical formula , sometimes abbreviated as . It is a colourless, highly Volatility (chemistry), volatile, sweet-smelling ("ethereal odour"), extremely flammable liquid. It belongs ...
) The sulfur ring in ''cyclo''- has a "chair" conformation, reminiscent of the chair form of
cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
. All of the sulfur atoms are equivalent.


''Cyclo''-heptasulfur, ''cyclo''-

It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of ''cyclo''-heptasulfur are known.
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Two forms (γ-, δ-) have been characterized. The ''cyclo''- ring has an unusual range of bond lengths of 199.3–218.1 pm. It is said to be the least stable of all of the sulfur allotropes. Steudel, 6


''Cyclo''-octasulfur, ''cyclo''-

Octasulfur contains puckered rings, and is known in three forms that differ only in the way the rings are packed in the crystal.


α-Sulfur

α-Sulfur is the form most commonly found in nature. When pure it has a greenish-yellow colour (traces of ''cyclo''- in commercially available samples make it appear yellower). It is practically insoluble in water and is a good electrical insulator with poor thermal conductivity. It is quite soluble in
carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is an inorganic compound with the chemical formula and structure . It is also considered as the anhydride of thiocarbonic acid. It is a colorless, flammable, neurotoxic liquid that is used as ...
: 35.5 g/100 g solvent at 25 °C. It has an orthorhombic crystal structure. α-Sulfur is the predominant form found in "flowers of sulfur", "roll sulfur" and "milk of sulfur". It contains puckered rings, alternatively called a crown shape. The S–S bond lengths are all 203.7 pm and the S-S-S angles are 107.8° with a dihedral angle of 98°. At 95.3 °C, α-sulfur converts to β-sulfur.


β-Sulfur

β-Sulfur is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. It is unusual because it is only stable above 95.3 °C; below this temperature it converts to α-sulfur. β-Sulfur can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur. It has a melting point variously quoted as 119.6 °C and 119.8 °C but as it decomposes to other forms at around this temperature the observed melting point can vary. The 119 °C melting point has been termed the "ideal melting point" and the typical lower value (114.5 °C) when decomposition occurs, the "natural melting point".


γ-Sulfur

γ-Sulfur was first prepared by F.W. Muthmann in 1890. It is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. It crystallises in pale yellow monoclinic needles. It is the densest form of the three. It can be prepared by slowly cooling molten sulfur that has been heated above 150 °C or by chilling solutions of sulfur in
carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is an inorganic compound with the chemical formula and structure . It is also considered as the anhydride of thiocarbonic acid. It is a colorless, flammable, neurotoxic liquid that is used as ...
,
ethyl alcohol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an alcohol, with its formula also written as , or EtOH, where Et is the pseudoelement symbol ...
or
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
s. It is found in nature as the mineral rosickyite. Steudel, 7 It has been tested in carbon fiber-stabilized form as a cathode in lithium-sulfur (Li-S) batteries and was observed to stop the formation of polysulfides that compromise battery life.


''Cyclo''- (''n'' = 9–15, 18, 20)

These allotropes have been synthesised by various methods for example, treating
titanocene pentasulfide Titanocene pentasulfide is the organotitanium compound with the formula (C5H5)2TiS5, commonly abbreviated as Cp2TiS5. This metallocene exists as a bright red solid that is soluble in organic solvents. It is of academic interest as a precursor to ...
and a
dichlorosulfane Sulfur dichloride is the chemical compound with the formula . This cherry-red liquid is the simplest sulfur chloride and one of the most common, and it is used as a precursor to organosulfur compounds. It is a highly corrosive and toxic substance, ...
of suitable sulfur chain length, : : or alternatively treating a
dichlorosulfane Sulfur dichloride is the chemical compound with the formula . This cherry-red liquid is the simplest sulfur chloride and one of the most common, and it is used as a precursor to organosulfur compounds. It is a highly corrosive and toxic substance, ...
, and a
polysulfane A polysulfane is a chemical compound of formula , where ''n'' > 1 (although disulfane () is sometimes excluded). Compounds containing 2 – 8 sulfur atoms have been isolated, longer chain compounds have been detected, but only in solution.R. Steud ...
, : : , , and can also be prepared from . With the exception of ''cyclo''-, the rings contain S–S bond lengths and S-S-S bond angle that differ one from another. ''Cyclo''- is the most stable ''cyclo''-allotrope. Its structure can be visualised as having sulfur atoms in three parallel planes, 3 in the top, 6 in the middle and three in the bottom.
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Two forms (α-, β-) of ''cyclo''- are known, one of which has been characterized. Two forms of ''cyclo''- are known where the conformation of the ring is different. To differentiate these structures, rather than using the normal crystallographic convention of α-, β-, etc., which in other ''cyclo''- compounds refer to different packings of essentially the same
conformer A conformer is a clear acrylic shell fitted after an enucleation of the eye Enucleation is the removal of the eye that leaves the eye muscles and remaining orbital contents intact. This type of ocular surgery is indicated for a number of oc ...
, these two
conformers In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one or more single bonds. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as ...
have been termed endo- and exo-.


''Cyclo''-·''cyclo''- adduct

This adduct is produced from a solution of ''cyclo''- and ''cyclo''- in . It has a density midway between ''cyclo''- and ''cyclo''-. The crystal consists of alternate layers of ''cyclo''- and ''cyclo''-. This material is a rare example of an allotrope that contains molecules of different sizes. Steudel, 9


''Catena'' sulfur forms

The term "''Catena'' sulfur forms" refers to mixtures of sulfur allotropes that are high in catena (polymer chain) sulfur. The naming of the different forms is very confusing and care has to be taken to determine what is being described because some names are used interchangeably.


Amorphous sulfur

Amorphous sulfur is the quenched product from molten sulfur hotter than the λ-transition at 160 °C, where polymerization yields ''catena'' sulfur molecules. (Above this temperature, the properties of the liquid melt change remarkably. For example, the viscosity increases more than 10000-fold as the temperature increases through the transition). As it anneals, solid amorphous sulfur changes from its initial glassy form, to a plastic form, hence its other names of ''plastic'', and ''glassy'' or ''vitreous'' sulfur. The plastic form is also called χ-sulfur. Amorphous sulfur contains a complex mixture of ''catena''-sulfur forms mixed with ''cyclo''-forms.


Insoluble sulfur

Insoluble sulfur is obtained by washing quenched liquid sulfur with . Steudel, 3 It is sometimes called polymeric sulfur, μ-S or ω-S.


Fibrous (φ-) sulfur

Fibrous (φ-) sulfur is a mixture of the allotropic ψ- form and γ-''cyclo''-. Steudel, 43


ω-Sulfur

ω-Sulfur is a commercially available product prepared from amorphous sulfur that has not been stretched prior to extraction of soluble forms with . It sometimes called "white sulfur of Das" or supersublimated sulfur. It is a mixture of ψ-sulfur and lamina sulfur. The composition depends on the exact method of production and the sample's history. One well known commercial form is "Crystex". ω-sulfur is used in the
vulcanization Vulcanization (British English: vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to ...
of rubber. Steudel, 15


λ-Sulfur

λ-Sulfur is molten sulfur just above the melting temperature. It is a mixture containing mostly ''cyclo''-. Cooling λ-sulfur slowly gives predominantly β-sulfur.


μ-Sulfur

μ-Sulfur is the name applied to solid insoluble sulfur and the melt prior to quenching.


π-Sulfur

π-Sulfur is a dark-coloured liquid formed when λ-sulfur is left to stay molten. It contains mixture of rings.


Biradical ''catena'' () chains

This term is applied to biradical ''catena''-chains in sulfur melts or the chains in the solid.
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Solid ''catena'' allotropes

The production of pure forms of ''catena''-sulfur has proved to be extremely difficult. Complicating factors include the purity of the starting material and the thermal history of the sample.


ψ-Sulfur

This form, also called fibrous sulfur or ω1-sulfur, has been well characterized. It has a density of 2.01 g·cm−3 (α-sulfur 2.069 g·cm−3) and decomposes around its melting point of 104 °C. It consists of parallel helical sulfur chains. These chains have both left and right-handed "twists" and a radius of 95 pm. The S–S bond length is 206.6 pm, the S-S-S bond angle is 106° and the dihedral angle is 85.3°, (comparable figures for α-sulfur are 203.7 pm, 107.8° and 98.3°).
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Lamina sulfur

Lamina sulfur has not been well characterized but is believed to consist of criss-crossed helices. It is also called χ-sulfur or ω2-sulfur.


High-temperature gaseous allotropes

Monatomic sulfur can be produced from
photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...
of
carbonyl sulfide Carbonyl sulfide is the chemical compound with the linear formula . It is a colorless flammable gas with an unpleasant odor. It is a linear molecule consisting of a carbonyl double bonded to a sulfur atom. Carbonyl sulfide can be considered to ...
.Eberhard Leppin and Klaus Gollnick (1970), "Direct photolysis of carbonyl sulfide in solution" I (Mechanism) and II (Reactions); ''Journal of the American Chemical Society'' vol. 92 issue 8, pp. 2217–2227. an
10.1021/ja00711a005


Disulfur,

Disulfur, , is the predominant species in sulfur vapour above 720 °C (a temperature above that shown in the phase diagram); at low pressure (1 mmHg) at 530 °C, it comprises 99% of the vapor. It is a triplet
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 ...
(like
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 ...
and
sulfur monoxide Sulfur monoxide is an inorganic compound with formula . It is only found as a dilute gas phase. When concentrated or condensed, it converts to S2O2 ( disulfur dioxide). It has been detected in space but is rarely encountered intact otherwise. St ...
), with an S−S bond length of 188.7 pm. The blue colour of burning sulfur is due to the emission of light by the molecule produced in the flame. The molecule has been trapped in the compound (E = As, Sb) for crystallographic measurements, produced by treating elemental
sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
with excess
iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...
in liquid
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
. The cation has an "open-book" structure, in which each ion donates the unpaired electron in the π*
molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...
to a vacant orbital of the molecule.


Trisulfur,

is found in sulfur vapour, comprising 10% of vapour species at 440 °C and 10 mmHg. It is cherry red in colour, with a bent structure, similar to
ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
, .


Tetrasulfur,

has been detected in the vapour phase, but it has not been well characterized. Diverse structures (e.g. chains, branched chains and rings) have been proposed. Theoretical calculations suggest a cyclic structure.


Pentasulfur,

Pentasulfur has been detected in sulfur vapours but has not been isolated in pure form.


List of allotropes and forms

Allotropes are in Bold.


References


Bibliography

* *


External links

* {{DEFAULTSORT:Sulfur allotropes Amorphous solids