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Argon compounds, the chemical compounds that contain the element
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
, are rarely encountered due to the inertness of the
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and
molecular ion A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry, ...
s containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar1C60 is stable at room temperature. Ar1C60 was discovered by the
CSIRO The Commonwealth Scientific and Industrial Research Organisation (CSIRO) is an Australian Government agency that is responsible for scientific research and its commercial and industrial applications. CSIRO works with leading organisations arou ...
. Argon ionises at 15.76 eV, which is higher than hydrogen, but lower than helium, neon or fluorine. Molecules containing argon can be van der Waals molecules held together very weakly by
London dispersion force London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, fluctuating induced dipole bonds or loosely as van der Waals forces) are a type of intermolecular force acting between at ...
s. Ionic molecules can be bound by charge induced dipole interactions. With gold atoms there can be some covalent interaction. Several boron-argon bonds with significant covalent interactions have been also reported. Experimental methods used to study argon compounds have included inert gas matrices,
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...
to study stretching and bending movements,
microwave spectroscopy Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter. History The ammonia molecule NH3 is shaped like a pyramid 0.38 Å in height, with an equilatera ...
and
far infrared Far infrared (FIR) or long wave refers to a specific range within the infrared spectrum of electromagnetic radiation. It encompasses radiation with wavelengths ranging from 15 μm ( micrometers) to 1 mm, which corresponds to a freque ...
to study rotation, and also visible and
ultraviolet spectroscopy Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of th ...
to study different electronic configurations including
excimer An excimer (originally short for excited dimer) is a short-lived polyatomic molecule formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an elec ...
s.
Mass spectroscopy Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
is used to study ions. Computation methods have been used to theoretically compute molecule parameters, and predict new stable molecules. Computational ''ab initio'' methods used have included CCSD(T), MP2 (
Møller–Plesset perturbation theory Møller–Plesset perturbation theory (MP) is one of several quantum chemistry post-Hartree–Fock ab initio methods in the field of computational chemistry. It improves on the Hartree–Fock method by adding electron correlation effects by me ...
of the second order), CIS and CISD. For heavy atoms, effective core potentials are used to model the inner electrons, so that their contributions do not have to be individually computed. More powerful computers since the 1990s have made this kind of ''
in silico In biology and other experimental sciences, an ''in silico'' experiment is one performed on a computer or via computer simulation software. The phrase is pseudo-Latin for 'in silicon' (correct ), referring to silicon in computer chips. It was c ...
'' study much more popular, being much less risky and simpler than an actual experiment. This article is mostly based on experimental or observational results. The
argon fluoride laser The argon fluoride laser (ArF laser) is a particular type of excimer laser, which is sometimes (more correctly) called an exciplex laser. With its 193-nanometer wavelength, it is a deep ultraviolet laser, which is commonly used in the production ...
is important in
photolithography Photolithography (also known as optical lithography) is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer. The process begins with a photosensiti ...
of silicon chips. These lasers make a strong ultraviolet emission at 192 nm.


Argonium

Argonium (ArH+) is an ion combining a proton and an argon atom. It is found in interstellar space in diffuse
atomic hydrogen A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral hydrogen atom contains a single positively charged proton in the nucleus, and a single negatively charged electron bound to the nucleus by the Coulomb for ...
gas where the fraction of
molecular hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
H2 is in the range of 0.0001 to 0.001. Argonium is formed when H2+ reacts with Ar atoms: :Ar + → ArH+ + H and it is also produced from Ar+ ions produced by cosmic rays and X-rays from neutral argon: :Ar+ + H2 → *ArH+ + H 1.49 eV. When ArH+ encounters an electron, dissociative recombination can occur, but it is extremely slow for lower energy electrons, allowing ArH+ to survive for a much longer time than many other similar protonated cations. :ArH+ + e → ArH* → Ar + H Artificial ArH+ made from earthly Ar contains mostly the isotope 40Ar rather than the cosmically abundant 36Ar. Artificially it is made by an electric discharge through an argon-hydrogen mixture.


Natural occurrence

In the
Crab Nebula The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus (constellation), Taurus. The common name comes from a drawing that somewhat resembled a crab with arm ...
, ArH+ occurs in several spots revealed by
emission line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
s. The strongest place is in the Southern Filament. This is also the place with the strongest concentration of Ar+ and Ar2+ ions. The
column density The area density (also known as areal density, surface density, superficial density, areic density, column density, or density thickness) of a two-dimensional object is calculated as the mass per unit area. The SI derived unit is the "kilogram p ...
of ArH+ in the Crab Nebula is between 1012 and 1013 atoms per square centimeter. Possibly the energy required to excite the ions so that then can emit, comes from collisions with electrons or hydrogen molecules. Towards the Milky Way centre the column density of ArH+ is around .


Cluster argon cations

The diargon cation, has a binding energy of 1.29 eV. The triargon cation is linear, but has one Ar−Ar bond shorter than the other. Bond lengths are 2.47 and 2.73
ångström The angstrom (; ) is a unit of length equal to m; that is, one ten- billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814� ...
s. The dissociation energy to Ar and Ar2+ is 0.2 eV. In line with the molecule's asymmetry, the charge is calculated as +0.10, +0.58 and +0.32 on each argon atom, so that it greatly resembles bound to a neutral Ar atom. Larger charged argon clusters are also detectable in mass spectroscopy. The tetraargon cation is also linear.
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrical tha ...
clusters have an core, whereas is dioctahedral with an core. The linear core has +0.1 charge on the outer atoms, and +0.4 charge on each or the inner atoms. For larger charged argon clusters, the charge is not distributed on more than four atoms. Instead the neutral outer atoms are attracted by induced electric polarization. The charged argon clusters absorb radiation, from the near infrared, through visible to ultraviolet. The charge core, , or is called a
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
. Its spectrum is modified by the first shell of neutral atoms attached. Larger clusters have the same spectrum as the smaller ones. When photons are absorbed in the
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
, it is initially electronically excited, but then energy is transferred to the whole cluster in the form of
vibration Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...
. Excess energy is removed by outer atoms evaporating from the cluster one at a time. The process of destroying a cluster by light is called photofragmentation. Negatively-charged argon clusters are thermodynamically unstable, and therefore cannot exist. Argon has a negative
electron affinity The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy This differs by si ...
.


Argon monohydride

Neutral argon hydride, also known as argon monohydride (ArH), was the first discovered noble gas hydride. J. W. C. Johns discovered an emission line of ArH at 767 nm and announced the find in 1970. The molecule was synthesized using
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
irradiation of mixtures of argon with hydrogen-rich molecules such as H2, H2O, CH4 and CH3OH. The X-ray excited argon atoms are in the 4p state. Argon monohydride is unstable in its ground state, 4s, as a neutral inert gas atom and a hydrogen atom repel each other at normal intermolecular distances. When a higher-energy-level ArH* emits a photon and reaches the ground state, the atoms are too close to each other, and they repel and break up. However a van der Waals molecule can exist with a long bond. However, excited ArH* can form stable Rydberg molecules, also known as
excimer An excimer (originally short for excited dimer) is a short-lived polyatomic molecule formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an elec ...
s. These Rydberg molecules can be considered as a protonated argon core, surrounded by an electron in one of many possible higher energy states. :Formation: Ar + ''ν'' → Ar*; Ar* + H2 → ArH* + H Instead of dihydrogen, other hydrogen containing molecules can also have a hydrogen atom abstracted by excited argon, but note that some molecules bind hydrogen too strongly for the reaction to proceed. For example,
acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...
will not form ArH this way. In the van der Waals molecule of ArH, the bond length is calculated to be about 3.6 Å and the dissociation energy calculated to be 0.404 kJ/mol (33.8 cm−1). The bond length in ArH* is calculated as 1.302 Å. The spectrum of argon monohydride, both ArH* and Ar D*, has been studied. The lowest bound state is termed A2Σ+ or 5s. Another low lying state is known as 4p, made up of C2Σ+ and B2π states. Each transition to or from higher level states corresponds to a band. Known bands are 3p → 5s, 4p → 5s, 5p → 5s (band origin ), 6p → 5s (band origin ) 3dσ → 4p, 3dπ → 4p (6900 cm−1), 3dδ → 4p (8200–8800 cm−1), 4dσ → 4p (), 6s → 4p (7400–7950 cm−1), 7s → 4p (predicted at , but obscured), 8s → 4p (), 5dπ → 4p (), 5p → 6s (band origin 3681.171 cm−1), 4f → 5s ( and band origin for ArD and ArH), 4f → 3dπ (7548.76 and 7626.58 ccm−1), 4f → 3dδ (6038.47 and 6026.57 cm−1), 4f → 3dσ (4351.44 cm−1 for ArD). The transitions going to 5s, 3dπ → 5s and 5dπ → 5s, are strongly predissociated, blurring out the lines. In the UV spectrum a continuous band exists from 200 to 400 nm. This band is due to two different higher states: B2Π → A2Σ+ radiates over 210–450 nm, and E2Π → A2Σ+ is between 180 and 320 nm. A band in the near infrared from 760 to 780 nm. Other ways to make ArH include a
Penning Penning may refer to: __NOTOC__ Currency *Norwegian penning *Swedish penning People *Mike Penning (born 1957), British politician *Frans Michel Penning (1894–1953), Dutch physicist *Edmund Penning-Rowsell (1913–2002), British journalist *Lou ...
-type discharge tube, or other electric discharges. Yet another way is to create a beam of ArH+ (argonium) ions and then neutralize them in laser-energized
caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...
vapour. By using a beam, the lifetimes of the different energy states can be observed, by measuring the profile of electromagnetic energy emitted at different wavelengths. The E2π state of ArH has a radiative lifetime of 40 ns. For ArD the lifetime is 61 ns. The B2Π state has a lifetime of 16.6 ns in ArH and 17 ns in ArD.


Argon polyhydrides

The argon dihydrogen cation has been predicted to exist and to be detectable in the
interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
. However it has not been detected . is predicted to be
linear In mathematics, the term ''linear'' is used in two distinct senses for two different properties: * linearity of a '' function'' (or '' mapping''); * linearity of a '' polynomial''. An example of a linear function is the function defined by f(x) ...
in the form Ar−H−H. The H−H distance is 0.94 Å. The dissociation barrier is only 2 kcal/mol (8 kJ/mol), and readily loses a hydrogen atom to yield ArH+. The force constant of the ArH bond in this is 1.895 m
dyne The dyne (symbol: dyn; ) is a derived units of measurement, unit of force (physics), force specified in the centimetre–gram–second system of units, centimetre–gram–second (CGS) system of units, a predecessor of the modern International S ...
2 (). The argon trihydrogen cation has been observed in the laboratory. ArH2D+, and have also been observed. The argon trihydrogen cation is planar in shape, with an argon atom off the vertex of a triangle of hydrogen atoms.


Argoxonium

The argoxonium ion ArOH+ is predicted to be
bent molecular geometry In chemistry, molecules with a non-collinear arrangement of two adjacent bonds have bent molecular geometry, also known as angular or V-shaped. Certain atoms, such as oxygen, will almost always set their two (or more) covalent bonds in non-collin ...
in the 11A′ state. 3Σ is a triplet state 0.12 eV higher in energy, and 3A″ is a
triplet state In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = � ...
0.18 eV higher. The Ar−O bond is predicted to be 1.684 Å long and to have a force constant of 2.988 mdyne/Å2 ().


ArNH+

ArNH+ is a possible ionic molecule to detect in the lab, and in space, as the atoms that compose it are common. ArNH+ is predicted to be more weakly bound than ArOH+, with a force constant in the Ar−N bond of 1.866 mdyne/Å2 (). The
angle In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...
at the nitrogen atom is predicted to be 97.116°. The Ar−N lengths should be 1.836 Å and the N−H bond length would be 1.046 Å


Argon dinitrogen cation

The argon dinitrogen linear cationic complex has also been detected in the lab: :Ar + → Ar+ + N2. The dissociation yields Ar+, as this is a higher-energy state. The binding energy is 1.19 eV. The molecule is linear. The distance between two nitrogen atoms is 1.1 Å. This distance is similar to that of neutral N2 rather than that of ion. The distance between one nitrogen and the argon atom is 2.2 Å. The vibrational band origin for the nitrogen bond in (''V'' = 0 → 1) is at 2272.2564 cm−1 compared with N2+ at 2175 and N2 at 2330 cm−1. In the process of
photodissociation 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 ...
, it is three times more likely to yield Ar+ + N2 compared to Ar + .


has been produced in a supersonic jet expansion of gas and detected by

Fourier transform In mathematics, the Fourier transform (FT) is an integral transform that takes a function as input then outputs another function that describes the extent to which various frequencies are present in the original function. The output of the tr ...
microwave spectroscopy Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter. History The ammonia molecule NH3 is shaped like a pyramid 0.38 Å in height, with an equilatera ...
. The molecule is linear, with the atoms in the order Ar−H−N−N. The Ar−H distance is 1.864 Å. There is a stronger bond between hydrogen and argon than in ArHCO+. The molecule is made by the following reaction: :ArH+ + N2 → .


Bis(dinitrogen) argon cation

The argon ion can bond two molecules of dinitrogen (N2) to yield an ionic complex with a linear shape and structure N=N−−N=N. The N=N bond length is 1.1014 Å, and the nitrogen to argon bond length is 2.3602 Å. 1.7 eV of energy is required to break this apart to N2 and . The band origin of an infrared band due to antisymmetric vibration of the N=N bonds is at 2288.7272 cm−1. Compared to N2 it is redshifted 41.99 cm−1. The ground state rotational constant of the molecule is . is produced by a supersonic expansion of a 10:1 mixture of argon with nitrogen through a nozzle, which is impacted by an
electron beam Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy. Mechani ...
.


ArN2O+

ArN2O+ absorbs photons in four violet–ultraviolet wavelength bands leading to breakup of the molecule. The bands are 445–420, 415–390, 390–370, and 342 nm.


ArHCO+

ArHCO+ has been produced in a supersonic-jet expansion of gas and detected by Fabry–Perot-type Fourier transform microwave spectroscopy. The molecule is made by this reaction ArH+ + CO → ArHCO+.


Ar''n''BO+

BO+ forms four complexes with argon: ArBO+; two isomers of Ar2BO+ (one with equidistant Ar-B bonds and another with a short and long bond); and Ar3BO+. These ions were formed by firing a green laser at a boron target in a gaseous mixture of helium, argon and nitrous oxide.


Carbon dioxide–argon ion

can be excited to form * where the positive charge is moved from the carbon dioxide part to the argon. This molecule may occur in the upper atmosphere. Experimentally the molecule is made from a low-pressure argon gas with 0.1%
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, irradiated by a 150 V
electron beam Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy. Mechani ...
. Argon is ionized, and can transfer the charge to a carbon dioxide molecule. The dissociation energy of is 0.26 eV. + CO2 → Ar + (yields 0.435 eV.)


Van der Waals molecules

Neutral argon atoms bind very weakly to other neutral atoms or molecules to form van der Waals molecules. These can be made by expanding argon under high pressure mixed with the atoms of another element. The expansion happens through a tiny hole into a vacuum, and results in cooling to temperatures a few degrees above absolute zero. At higher temperatures the atoms will be too energetic to stay together by way of the weak
London dispersion forces London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, fluctuating induced dipole bonds or loosely as van der Waals forces) are a type of intermolecular force acting between at ...
. The atoms that are to combine with argon can be produced by evaporation with a laser or alternatively by an electric discharge. The known molecules include AgAr, Ag2Ar, NaAr, KAr, MgAr, CaAr, SrAr, ZnAr, CdAr, HgAr, SiAr, InAr, CAr, GeAr, SnAr, and BAr. SiAr was made from silicon atoms derived from Si(CH3)4. In addition to the very weakly bound van der Waals molecules, electronically excited molecules with the same formula exist. As a formula these can be written ArX*, with the "*" indicating an
excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...
. The atoms are much more strongly bound with a covalent bond. They can be modeled as an ArX+ surrounded by a higher energy shell with one electron. This outer electron can change energy by exchanging photons and so can fluoresce. The widely used
argon fluoride laser The argon fluoride laser (ArF laser) is a particular type of excimer laser, which is sometimes (more correctly) called an exciplex laser. With its 193-nanometer wavelength, it is a deep ultraviolet laser, which is commonly used in the production ...
makes use of the ArF* excimer to produce strong ultraviolet radiation at 192 nm. The argon chloride laser using ArCl* produces even shorter ultraviolet at 175 nm, but is too feeble for application. The argon chloride in this laser comes from argon and chlorine molecules.


Argon clusters

Cooled argon gas can form clusters of atoms. Diargon, also known as the argon dimer, has a binding energy of 0.012 eV, but the Ar13 and Ar19 clusters have a sublimation energy (per atom) of 0.06 eV. For liquid argon, which could be written as Ar, the energy increases to 0.08 eV. Clusters of up to several hundred argon atoms have been detected. These argon clusters are
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrical tha ...
in shape, consisting of shells of atoms arranged around a central atom. The structure changes for clusters with more than 800 atoms to resemble a tiny crystal with a
face-centered cubic In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties o ...
(fcc) structure, as in solid argon. It is the surface energy that maintains an
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrical tha ...
shape, but for larger clusters internal pressure will attract the atoms into an fcc arrangement. Neutral argon clusters are transparent to visible light.


Diatomic van der Waals molecules

: ArO* is also formed when
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 ...
trapped in an argon matrix is subjected to
vacuum ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of the ...
. It can be detected by its luminescence: :O2 + ''hv'' → + e; + e → 2O*; O* + Ar → ArO*. Light emitted by ArO* has two main bands, one at 2.215 eV, and a weaker one at 2.195 eV. Argon sulfide, ArS* luminesces in the
near infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those o ...
at 1.62 eV. ArS is made from UV irradiated OCS in an argon matrix. The excited states lasts for 7.4 and 3.5 μs for spectrum peak and band respectively.


Triatomic van der Waals molecules

Cluster molecules containing dichlorine and more than one argon atom can be made by forcing a 95:5 mixture of helium and argon and a trace of chlorine though a nozzle. ArCl2 exists in a T shape. Ar2Cl2 has a distorted tetrahedron shape, with the two argon atoms 4.1 Å from each other, and their axis 3.9 Å from the Cl2. The van der Waals bond energy is 447 cm−1. Ar3Cl2 also exists with a van der Waals bond energy of 776 cm−1. The linear Ar·Br2 molecule has a continuous spectrum for
bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...
molecule X → B transitions. The spectrum of bromine is blue-shifted and spread out when it binds an argon atom. ArI2 shows a spectrum that adds satellite bands to the higher vibrational bands of I2. The ArI2 molecule has two different isomers, one shape is linear, and the other is T-shaped. The dynamics of ArI2 is complex. Breakup occurs through different routes in the two isomers. The T shape undergoes intramolecular vibrational relaxation, whereas the linear one directly breaks apart. Diiodine clusters, I2Ar''n'' have been made. The ArClF cluster has a linear shape. The argon atom is closest to the chlorine atom. Linear ArBrCl can also rearrange to ArClBr, or a T-shaped isomer. Multiple argon atoms can " solvate" a
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
molecule forming a monolayer around the H2O. Ar12·H2O is particularly stable, having an
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrical tha ...
shape. Molecules from Ar·H2O to Ar14·H2O have been studied. ArBH was produced from boron monohydride (BH) which in turn was created from
diborane Diborane(6), commonly known as diborane, is the chemical compound with the formula . It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has att ...
by way of an ultraviolet 193 nm laser. The BH-argon mixture was expanded through a 0.2 mm diameter nozzle into a vacuum. The gas mixture cools and Ar and BH combine to yield ArBH. A band spectrum that combines the A1Π←X1Σ+ electronic transition, with vibration and rotation can be observed. The BH has singlet spin, and this is the first known van der Waals complex with a singlet spin pair of atoms. For this molecule the rotational constant is 0.133 cm−1, The dissociation energy is 92 cm−1 and distance from argon to boron atom is 3.70 Å. ArAlH is also known to exist. MgAr2 is also known.


Polyatomic van der Waals molecules

Some linear polyatomic molecules can form T-shaped van der Waals complexes with argon. These include NCCN,
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
,
nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...
,
acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...
, carbon oxysulfide, and ClCN. Others attach the argon atom at one end to continue to be linear, including HCN. Other polyatomic van der Waals compounds of argon, include those of
fluorobenzene Fluorobenzene is an aryl fluoride and the simplest of the fluorobenzenes, with the formula C6H5F, often abbreviated Phenyl group, PhF. A colorless liquid, it is a precursor to many fluorophenyl compounds. Preparation PhF was first reported in 18 ...
, formyl radical (ArHCO), 7-azaindole,
glyoxal Glyoxal is an organic compound with the chemical formula OCHCHO. It is the smallest dialdehyde (a compound with two aldehyde groups). It is a crystalline solid, white at low temperatures and yellow near the melting point (15 °C). The liquid ...
, sodium chloride (ArNaCl), ArHCl, and
cyclopentanone Cyclopentanone is the organic compound with the formula (CH2)4CO. This cyclic ketone is a colorless volatile liquid. Preparation Ketonic decarboxylation of adipic acid gives cyclopentanone. The reaction is conducted at elevated temperatures in t ...
. :


Aqueous argon

Argon dissolved in water causes the pH to rise to 8.0, apparently by reducing the number of oxygen atoms available to bind protons. With ice, argon forms a
clathrate hydrate Clathrate hydrates, or gas hydrates, clathrates, or hydrates, are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped ins ...
. Up to 0.6 GPa, the clathrate has a cubic structure. Between 0.7 and 1.1 GPa the clathrate has a tetragonal structure. Between 1.1 and 6.0 GPa the structure is body centered orthorhombic. Over 6.1 GPa, the clathrate converts into solid argon and
ice VII Variations in pressure and temperature give rise to different phases of ice, which have varying properties and molecular geometries. Currently, twenty-one phases, including both crystalline and amorphous ices have been observed. In modern histor ...
. At atmospheric pressure the clathrate is stable below 147 K. At 295 K the argon pressure from the clathrate is 108 MPa.


Argon fluorohydride

Argon fluorohydride Argon fluorohydride (systematically named fluoridohydridoargon) or argon hydrofluoride is an inorganic compound with the chemical formula HArF (also written ArHF). It is a compound of the chemical element argon. Discovery The discovery of this ...
was an important discovery in the rejuvenation of the study of noble gas chemistry. HArF is stable in solid form at temperatures below 17 K. It is prepared by photolysis of
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
in a solid argon matrix. HArArF would have such a low barrier to decomposition that it will likely never be observed. However HBeArF is predicted to be more stable than HArF.


Uranium compounds

CUO in a solid argon matrix can bind one, or a few argon atoms to yield CUO·Ar, CUO·Ar3 or CUO·Ar4. CUO itself is made by evaporating
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
atoms into
carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
. Uranium acts as a strong
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...
in CUO and forms bonds with energies of about 3.2 kcal/mol (13.4 kJ/mol) with argon. The argon acts as a
Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. Its electron density is inserted into an empty 6d orbital on the uranium atom. The spectrum of CUO is changed by argon so that the U−O stretch frequency changes from 872.2 to 804.3 cm−1 and the U−C stretch frequency from 1047.3 to 852.5 cm−1. The significant change in the spectrum occurs because the CUO is changed from a singlet state (in gas phase or solid neon) to a triplet state, with argon or noble gas complexing. The argon–uranium bond length is 3.16 Å. This is shorter than the sum of atomic radii of U and Ar of 3.25 Å, but considerably longer than a normal covalent bond to uranium. For example, U−Cl in UCl6 is 2.49 Å. When
xenon Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
is included in the solid argon matrix up to a few percent, additional van der Waals molecules are formed: CUO·Ar3Xe, CUO·Ar2Xe2, CUO·ArXe3 and CUO·Xe4. Similarly krypton can substitute for argon in CUO·Ar3Kr, CUO·Ar2Kr2, CUO·ArKr3 and CUO·Kr4. The shape of these molecules is roughly
octahedral In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
, with a uranium centre and with the noble gas atoms around the equator. can bind up to five noble gas atoms in a ring around a linear O

O core. These molecules are produced when uranium metal is laser ablated into dioxygen. This produces UO, UO2, UO3, U+, and importantly . is then condensed into a noble gas matrix, either a pure element or a mixture. Heavier noble gas atoms will tend to displace the lighter atoms. Ionic molecules produced this way include , , , , , , , , , , , , and , which are identified by a shift in the U=O antisymmetric stretching frequency. Neutral UO2 condensed in solid argon is converted from one electronic state to another by the argon atom ligands. In argon the electron configuration is 5f2(δφ) whereas in neon it is 5f17s1 (the state 3H4g compared to 3Φ2u). This is because the argon atoms have a larger antibonding interaction with the 7s1 electron, forcing it into a different subshell. The argonated compound has a stretching frequency of 776 cm−1 compared to 914.8 cm−1 in
neon Neon is a chemical element; it has symbol Ne and atomic number 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with approximately two-thirds the density of ...
. The argon uranium dioxide molecule is likely UO2Ar5.


Beryllium oxide

When
beryllium Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...
atoms react with oxygen in a solid argon matrix (or beryllia is evaporated into the matrix) ArBeO is formed, and is observable by its infrared spectrum. The beryllia molecule is strongly polarised, and the argon atom is attracted to the beryllium atom. The bond strength of Ar−Be is calculated to be 6.7 kcal/mol (28 kJ/mol). The Ar−Be bond length is predicted to be 2.042 Å. The cyclic Be2O2 molecule can bind two argon atoms, or one argon along with another noble gas atom. Analogously, beryllium reacting with
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
and trapped in an argon matrix at 4 K forms ArBeS. It has a binding energy calculated to be 12.8 kcal/mol (54 kJ/mol). ArBeO2CO (beryllium carbonate) has been prepared (along with Ne, Kr and Xe adducts). The cyclic beryllium sulfite molecule can also coordinate an argon atom onto the beryllium atom in solid neon or argon matrix.


Carbonyl compounds

Group 6 element Group 6, numbered by IUPAC style, is a group of elements in the periodic table. Its members are chromium (Cr), molybdenum (Mo), tungsten (W), and seaborgium (Sg). These are all transition metals and chromium, molybdenum and tungsten are refract ...
s can form reactive penta
carbonyl In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...
s that can react with argon. These were actually argon compounds discovered in 1975, and were known before the discovery of HArF, but are usually overlooked.
Tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...
normally forms a hexacarbonyl, but when subject to ultraviolet radiation it breaks into a reactive pentacarbonyl. When this is condensed into a noble gas matrix the infrared and UV spectrum varies considerably depending on the noble gas used. This is because the noble gas present binds to the vacant position on the tungsten atom. Similar results also occur with
molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
and
chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...
. Argon is only very weakly bound to tungsten in ArW(CO)5. The Ar−W bondlength is predicted to be 2.852 Å. The same substance is produced for a brief time in supercritical argon at 21 °C. For ArCr(CO)5 the band maximum is at 533 nm (compared to 624 nm in
neon Neon is a chemical element; it has symbol Ne and atomic number 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with approximately two-thirds the density of ...
, and 518 nm in
krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...
). Forming 18-electron complexes, the shift in spectrum due to different matrices was much smaller, only around 5 nm. This clearly indicates the formation of a molecule using atoms from the matrix. Other carbonyls and complexed carbonyls also have reports of bonding to argon. These include Ru(CO)2(PMe3)2Ar, Ru(CO)2( dmpe)2Ar, ''η''6-C6H6Cr(CO)2Ar. Evidence also exists for ArHMn(CO)4, ArCH3Mn(CO)4, and ''fac''-(''η''2-dfepe)Cr(CO)3Ar. Other noble gas complexes have been studied by photolysis of carbonyls dissolved in liquid rare gas, possibly under pressure. These Kr or Xe complexes decay on the time scale of seconds, but argon does not seem to have been studied this way. The advantage of liquid noble gases is that the medium is completely transparent to
infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
radiation, which is needed to study the bond vibration in the solute. Attempts have been made to study carbonyl–argon adducts in the gas phase, but the interaction appears to be too weak to observe a spectrum. In the gas form, the absorption lines are broadened into bands because of rotation that happens freely in a gas. The argon adducts in liquids or gases are unstable as the molecules easily react with the other photolysis products, or
dimerize In chemistry, dimerization is the process of joining two identical or similar Molecular entity, molecular entities by Chemical bond, bonds. The resulting bonds can be either strong or weak. Many symmetrical chemical species are described as dim ...
, eliminating argon.


Coinage metal monohalides

The argon coinage metal monohalides were the first noble gas metal halides discovered, when the metal monohalide molecules were put through an argon jet. There were first found in Vancouver in 2000. ArMX with M = Cu, Ag or Au and X = F, Cl or Br have been prepared. The molecules are linear. In ArAuCl the Ar−Au bond is 2.47 Å, the stretching frequency is 198 cm−1 and the dissociation energy is 47 kJ/mol. ArAgBr also has been made. ArAgF has a dissociation energy of 21 kJ/mol. The Ar−Ag bond-length in these molecules is 2.6 Å. ArAgCl is isoelectronic with which is better known. The Ar−Cu bond length in these molecules is 2.25 Å.


Transition metal oxides

In a solid argon matrix VO2 forms VO2Ar2, and VO4 forms VO4·Ar with binding energy calculated to be 12.8 and 5.0 kcal/mol (53 and 21 kJ/mol).
Scandium Scandium is a chemical element; it has Symbol (chemistry), symbol Sc and atomic number 21. It is a silvery-white metallic d-block, d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the lantha ...
in the form of ScO+ coordinates five argon atoms to yield . these argon atoms can be substituted by numbers of krypton or xenon atoms to yield even more mixed noble gas molecules. With
yttrium Yttrium is a chemical element; it has Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost a ...
, YO+ bonds six argon atoms, and these too can be substituted by varying numbers of krypton or xenon atoms. In the case of transition metal monoxides, ScO, TiO and VO do not form a molecule with one argon atom. However CrO, MnO, FeO, CoO and NiO can each coordinate one argon atom in a solid argon matrix. The metal monoxide molecules can be produced by laser ablation of the metal trioxide, followed by condensation on solid argon. ArCrO absorbs at 846.3 cm−1, ArMnO at 833.1, ArFeO at 872.8, ArCoO at 846.2, Ar58NiO at 825.7 and Ar60NiO at 822.8 cm−1. All these molecules are linear. There are also claims of argon forming coordination molecules in NbO2Ar2, NbO4Ar, TaO4Ar, VO2Ar2, VO4Ar, Rh(''η''2-O2)Ar2, Rh(''η''2-O2)2Ar2, Rh(''η''2-O2)2(''η''1-OO)Ar.
Tungsten trioxide Tungsten(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO3. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid . It is a light ...
, WO3, and tungsten dioxide mono-superoxide (η2-O2)WO2 can both coordinate argon in an argon matrix. The argon can be replaced by xenon or molecular oxygen to make xenon coordinated compounds or superoxides. For WO3Ar the binding energy is 9.4 kcal/mol and for (η2-O2)WO2 it is 8.1 kcal/mol.


Other transition metal compounds

ArNiN2 binds argon with 11.52 kcal/mol. The bending frequency of ArN2 is changed from 310.7 to 358.7 cm−1 when argon attaches to the nickel atom.


Other ions

Some other binary ions observed that contain argon include BaAr2+ and , VAr+, CrAr+, FeAr+, CoAr+, and NiAr+. Gold and silver cluster ions can bind argon. Known ions are , , , and . These have a triangular shaped metallic core with argon bound at the vertexes. ArF+ is also known to be formed in the reaction : + Ar → ArF+ + F and also :Ar+ + F2 → ArF+ + F. and also :SF + Ar → ArF+ + SF. The ions can be produced by ultraviolet light at 79.1 nm or less. The ionisation energy of fluorine is higher than that of argon, so breakup occurs thus: :ArF+ → Ar+ + F. The millimeter wave spectrum of ArF+ between 119.0232 and 505.3155 GHz has been measured to calculate molecular constants ''B''0 = , ''D''0 = 28.718 kHz. There is a possibility that a solid salt of ArF+ could be prepared with or anions. Excited or ionized argon atoms can react with molecular iodine gas to yield ArI+ Argon plasma is used as an ionisation source and carrier gas in
inductively coupled plasma mass spectrometry Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It i ...
. This plasma reacts with samples to produce monatomic ions, but also forms argon oxide (ArO+), and argon nitride (ArN+) cations, which can cause isobaric interference with detection and measurement of
iron-56 Iron-56 (56Fe) is the most common isotope of iron. About 91.754% of all iron is iron-56. Of all nuclides, iron-56 has the lowest mass per nucleon. With 8.8  MeV binding energy per nucleon, iron-56 is one of the most tightly bound nuclei. ...
(56Fe) and iron-54 (54Fe), respectively, in mass spectrometry. Platinum present in stainless steel can form platinum argide (PtAr+) which interferes with the detection of
uranium-234 Uranium-234 ( or U-234) is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million, or 1/18,000) of the raw uranium because its half ...
which can be used as a tracer in aquifers. Argon chloride cations can interfere with the detection of
arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...
as Ar35Cl+ has a mass-to-charge ratio almost identical to that of arsenic's one stable isotope, 75As. In these circumstances ArO+ may be removed by reaction with NH3. Alternatively electrothermal vaporization or using helium gas can avoid these interference problems. Argon can also form an anion with chlorine, ArCl, though this is not a problem for mass spectrometry applications as only cations are detected. The argon borynium ion, BAr+ is produced when BBr+ at energies between 9 and 11 eV reacts with argon atoms. 90% of the positive charge is on the argon atom. ArC+ ions can be formed when argon ions impact carbon monoxide with energies between 21 and 60 eV. However more C+ ions are formed, and when the energy is on the high side, O+ is higher. ArN+ can form when argon ions impact
dinitrogen Nitrogen is a chemical element; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh ...
with energies between 8.2 and 41.2 eV and peaking around 35 eV. However far more and N+ are produced. ArXe+ is held together with a strength of 1445 cm−1 when it is in the X electronic state, but 1013 cm−1 when it is in the B excited state. Metal–argon cations are called "argides". The argide ions produced during
mass spectroscopy Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
have higher intensity when the binding energy of the ion is higher. Transition elements have higher binding and ion flux intensity compared to main group elements. Argides can be formed in the plasma by excited argon atoms reacting with another element atom, or by an argon atom binding with another ion: :Ar+ + M → ArM+ + e; M+ + Ar → ArM+. Doubly charged cations, called superelectrophiles, are capable of reacting with argon. Ions produced include ArCF ArCH, ArBF and ArBF containing bonds between argon and carbon or boron. Doubly ionised acetylene HCCH2+ reacts inefficiently with argon to yield HCCAr2+. This product competes with the formation of Ar+ and argonium. The SiF ion reacts with argon to yield ArSiF. :


Polyatomic cations

Metal ions can also form with more than one argon atom, in a kind of argon metal cluster. Different sized metal ions at the centre of a cluster can fit different geometries of argons atoms around the ion. Argides with multiple argon atoms have been detected in mass spectrometry. These can have variable numbers of argon attached, but there are magic numbers, where the complex more commonly has a particular number, either four or six argon atoms. These can be studied by time of flight mass spectrometer analysis and by the photodissociation spectrum. Other study methods include Coulomb explosion analysis. Argon-tagging is a technique whereby argon atoms are weakly bound to a molecule under study. It results in a much lower temperature of the tagged molecules, with sharper infra-red absorption lines. The argon-tagged molecules can be disrupted by photons of a particular wavelength. Lithium ions add argon atoms to form clusters with more than a hundred argon atoms. The clusters Li+Ar4, and Li+Ar4 are particularly stable and common. Calculations show that the small clusters are all quite symmetrical. Li+Ar2 is linear, Li+Ar3 is flat and triangular shaped with D3h symmetry, Li+Ar4 is tetrahedral, Li+Ar5 could be a
square pyramid In geometry, a square pyramid is a Pyramid (geometry), pyramid with a square base and four triangles, having a total of five faces. If the Apex (geometry), apex of the pyramid is directly above the center of the square, it is a ''right square p ...
or
trigonal bipyramid A triangular bipyramid is a hexahedron with six triangular faces constructed by attaching two tetrahedra face-to-face. The same shape is also known as a triangular dipyramid or trigonal bipyramid. If these tetrahedra are regular, all faces of a t ...
shape. Li+Ar6 is an
octahedron In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
shape with Li at the centre. Li+Ar7 or slightly larger clusters have a core octahedron of argon atoms with one or more triangular faces capped by other argon atoms. The bonding is much weaker, which explains their greater scarcity. Sodium forms clusters with argon atoms with peaks at numbers of 8, 10, 16, 20, 23, 25 and 29, and also at the icosahedral numbers of 47, 50, 57, 60, 63, 77, 80, 116 and 147 argon atoms. This includes the
square antiprism In geometry, the square antiprism is the second in an infinite family of antiprisms formed by an even number, even-numbered sequence of triangle sides closed by two polygon caps. It is also known as an ''anticube''. If all its faces are regular ...
(8) and the capped square antiprism (10 atoms). In Ti+Ar1−n the argon atoms induce a mixing of the ground electronic state of 3d24s1 with 3d34s0. When a plasma of titanium in expanding argon gas is made via a laser, clusters from Ti+Ar up to Ti+Ar50 are formed. But Ti+Ar6 is much more common than all the others. In this the six argon atoms are arranged in an octahedron shape around the central titanium ion. For Ti+Ar2 DFT calculations predict it is linear, Ti+Ar3 is not even flat, and has one short and two longer Ti-Ar bonds. Ti+Ar4 is a distorted tetrahedron, with one longer Ti-Ar bond. Ti+Ar5 is an asymmetrical
trigonal bipyramid A triangular bipyramid is a hexahedron with six triangular faces constructed by attaching two tetrahedra face-to-face. The same shape is also known as a triangular dipyramid or trigonal bipyramid. If these tetrahedra are regular, all faces of a t ...
shape with one bond shorter. For clusters with seven or more argon atoms, the structure contains a Ti+Ar6 octahedton with triangular faces caped by more argon atoms. Cu+Ar2 is predicted to be linear. Cu+Ar3 is predicted to be planar T-shaped with an Ar-Cu-Ar angle of 93°. Cu+Ar4 is predicted to be rhombic planar (not square or tetrahedral). For alkali and alkaline earth metals the M+Ar4 cluster is tetrahedral. Cu+Ar5 is predicted to have a rhombic pyramid shape. Cu+Ar6 has a flattened octahedral shape. Cu+Ar7 is much less stable, and the seventh argon atom is outside an inner shell of six argon atoms. This is called capped octahedral. A complete second shell of argon atoms yields Cu+Ar34. Above this number a structural change takes place with an icosahedral arrangement with Cu+Ar55 and Cu+Ar146 having more stability. With a strontium ion Sr+ from two to eight argon atoms can form clusters. Sr+Ar2 has a triangle shape with ''C''2''v'' symmetry. Sr+Ar3 has a trigonal pyramid shape with ''C''3''v'' symmetry. Sr+Ar4 has two trigonal pyramids sharing a face and strontium at the common apex. It has a ''C''2''v'' symmetry. Sr+Ar6 has a pentagonal pyramid of argon atoms with the strontium atom ''below'' the base. Niobium tetraargide, Nb+Ar4 probably has the argon atoms arranged in a square around the niobium. Similarly for vanadium tetraargide, V+Ar4. The hexaargides, Co+Ar6 and Rh+Ar6 likely have octahedral argon arrangement. Indium monocation forms clusters with multiple argon, with magic numbers at 12, 18, 22, 25, 28, 45 and 54, and 70 argon atoms, which are numbers for icosahedral shapes. By zapping copper metal with a UV laser in an argon-carbon monoxide mixture, argon tagged copper carbonyl cations are formed. These ions can be studied by observing which wavelengths of infrared radiation cause the molecules to break up. These molecular ions include CuCO+Ar, Cu(CO)2+Ar, Cu(CO)3+Ar, Cu(CO)4+Ar which are respectively disrupted to lose argon, by infrared wavenumbers 2216, 2221, 2205 and 2194 cm−1 respectively. The argon binding energy is respectively 16.3, 1.01, 0.97 and 0.23 kcal/mol. The infrared absorption peak for Cu(CO)3+Ar is 2205 cm−1 compared to 2199 cm−1 for Cu(CO)3+. For Cu(CO)4+Ar the peak is at 2198 cm−1 compared to 2193 for Cu(CO)4+. For Cu(CO)2+Ar the peak is at 2221 cm−1 compared to 2218.3 for argon free, and for CuCO+Ar the peak is at 2216 cm−1 considerably different to 2240.6 cm−1 for CuCO+. Computationally predicted shapes for these molecular ions are linear for CuCO+Ar, slightly bent T-shaped for Cu(CO)2+Ar and a trigonal pyramid with argon at the top and a flat star like copper tricarbonyl forming the base. Ions studied by argon tagging include the hydrated proton H+(H2O)nAr with n=2 to 5, hydrated 18-crown-6 ether alkali metal ions, hydrated alkali metal ions, transition metal acetylene complexes, protonated ethylene, and IrO4+. Argon methyl cations, (or methyliumargon) ArxCH3+ are known for n=1 to 8. CH3+ is a Y shape, and when argon atoms are added they go above and below the plane of the Y. If more argon atoms are added they line up with the hydrogen atoms. Δ''H''0 for ArCH3+ is 11 kcal/mol, and for Ar2CH3+ it is 13.5 kcal/mol (for 2Ar + CH3+). Boroxyl ring cationic complexes with argon rB3O4sup>+, rB3O5sup>+, rB4O6sup>+ and rB5O7sup>+ were prepared ''via'' a laser vaporization at cryogenic temperatures and investigated by infrared gas phase spectroscopy. They were the first large stable gas phase complexes that feature strong dative bonding between argon and boron.


Dications

Dications with argon are known for the coinage metals. Known dications include CuArn2+ and AgArn2+ for n=1-8, with a peak occurrence of CuAr42+, or AgAr42+, and AuArn2+ n=3–7. In addition to the four argon atoms, the six argon atoms clusters have enhanced concentration. The stability of the ions with two positive charges is unexpected as the ionization energy of argon is lower than the second ionization energy of the metal atom. So the positive second charge on the metal atom should move to the argon, ionizing it, and then forming a highly repulsive molecule that undergoes a Coulomb explosion. However these molecules appear to be kinetically stable, and to transfer the charge to an argon atom, they have to pass through a higher energy state. The clusters with four argon atoms are expected to be square planar, and those with six, to be octahedral distorted by the
Jahn–Teller effect The Jahn–Teller effect (JT effect or JTE) is an important mechanism of spontaneous symmetry breaking in molecular and solid-state systems which has far-reaching consequences in different fields, and is responsible for a variety of phenomena in sp ...
.


Polyatomic anions

Examples of anions containing strong bonds with noble gases are extremely rare: generally nucleophilic nature of anions results in their inability to bind to noble gases with their negative
electron affinity The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy This differs by si ...
. However, the 2017 discovery of " superelectrophilic anions", gas phase fragmentation products of ''closo''-
dodecaborate The dodecaborate(12) anion, 12H12sup>2−, is a borane with an icosahedral arrangement of 12 boron atoms, with each boron atom being attached to a hydrogen atom. Its symmetry is classified by the molecular point group Ih. Synthesis and re ...
s, led to the observation of stable anionic compounds containing a boron-noble gas bond with significant degree of covalent interaction. The most reactive superelectrophilic anion 12(CN)11sup>−, fragmentation product of cyanated cluster 12(CN)12sup>2-, was reported to bind argon spontaneously at room temperature.


Solid compounds

Armand Gautier noticed that rock contained argon (and also nitrogen) that was liberated when the rock was dissolved in acid however how the argon was combined in rock was ignored by the scientific community.


Fullerene solvates

Solid
buckminsterfullerene Buckminsterfullerene is a type of fullerene with the formula . It has a cage-like fused-ring structure ( truncated icosahedron) made of twenty hexagons and twelve pentagons, and resembles a football. Each of its 60 carbon atoms is bonded to i ...
has small spaces between the C60 balls. Under 200 MPa pressure and 200 °C heat for 12 hours, argon can be intercalated into the solid to form crystalline Ar1C60. Once this cools down it is stable at standard conditions for months. Argon atoms occupy octahedral interstitial sites. The crystalline lattice size is almost unchanged at room temperature, but is slightly larger than pure C60 below 265 K. However argon does stop the buckyballs spinning below 250 K, a lower temperature than in pure C60. Solid C70 fullerene will also absorb argon under pressure of 200 MPa and at a temperature of 200 °C. C70·Ar has argon in octahedral sites and has the rock salt structure, with cubic crystals in which the lattice parameter is 15.001 Å. This compares to the pure C70 lattice parameter of 14.964 Å, so the argon forces the crystals to expand slightly. The C70 ellipsoidal balls rotate freely in the solid, they are not locked into position by extra argon atoms filling the holes. Argon gradually escapes over a couple of days when the solid is stored at standard conditions, so that C70·Ar is less stable than C60·Ar. This is likely to be due to the shape and internal rotation allowing channels through which Ar atoms can move. When
fullerene A fullerene is an allotropes of carbon, allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may ...
s are dissolved and crystallized from
toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...
, solids may form with toluene included as part of the crystal. However, if this crystallization is performed under a high pressure argon atmosphere, toluene is not included, being replaced by argon. The argon is then removed from the resultant crystal by heating to produce unsolvated solid fullerene.


Clathrate

Argon forms a clathrate with
hydroquinone Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''para' ...
(HOC6H4OH)3•Ar. When crystallised from benzene under a pressure of 20 atmospheres of argon, a well defined structure containing argon results. An argon-
phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...
clathrate 4C6H5OH•Ar is also known. It has a binding energy of 40 kJ/mol. Other substituted phenols can also crystallise with argon. The argon water clathrate is described in the Aqueous argon section.


Argon difluoride

Argon difluoride, ArF2, is predicted to be stable at pressures over 57 GPa. It should be an electrical insulator.


Ne2Ar and Ar2Ne

At around 4 K there are two phases where neon and argon are mixed as a solid: Ne2Ar and Ar2Ne. With Kr, solid argon forms a disorganized mixture.


ArH4

Under high pressure stoichiometric solids are formed with hydrogen and oxygen: Ar(H2)2 and Ar(O2)3. Ar(H2)2 crystallises in the hexagonal C14 MgZn2
Laves phase Laves phases are intermetallic phase (matter), phases that have composition AB2 and are named for Fritz Laves who first described them. The phases are classified on the basis of geometry alone. While the problem of Close-packing of equal spheres ...
. It is stable to at least 200 GPa, but is predicted to change at 250 GPa to an AlB2 structure. At even higher pressures the hydrogen molecules should break up followed by metallization.


ArO and ArO6

Oxygen and argon under pressure at room temperature form several different alloys with different crystal structures. Argon atoms and oxygen molecules are similar in size, so that a greater range of miscibility occurs compared to other gas mixtures. Solid argon can dissolve up to 5% oxygen without changing structure. Below 50% oxygen a
hexagonal close packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occ ...
phase exists. This is stable from about 3GPa to 8.5 GPa. Typical formula is ArO. With more oxygen between 5.5 and 7 GPa, a cubic ''Pm''3''n'' structure exists, but under higher pressure it changes to a ''I''2''d'' space group form. With more than 8.5 GPa these alloys separate to solid argon and ε-oxygen. The cubic structure has a unit cell edge of 5.7828 Å at 6.9 GPa. The representative formula is Ar(O2)3.


ArHe2

Using
density-functional theory Density functional theory (DFT) is a computational quantum mechanics, quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground sta ...
ArHe2 is predicted to exist with the MgCu2 Laves phase structure at high pressures below 13.8 GPa. Above 13.8 GPa it transforms to AlB2 structure.


Ar-TON

Under pressure argon inserts into
zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...
. Argon has an atomic radius of 1.8 Å, so it can insert into pores if they are big enough. Each unit cell of the TON zeolite can contain up to 5 atoms of argon, compared to 12 of neon. Argon infused TON zeolite (Ar-TON) is more compressible than Ne-TON as the unoccupied pores become elliptical under increased pressure. When Ar-TON is brought to atmospheric pressure, the argon only desorbs slowly, so that some remains in the solid without external pressure for a day.


Nickel argide

At 140 GPa and 1500K nickel and argon form an alloy, NiAr. NiAr is stable at room temperature and a pressure as low as 99 GPa. It has a
face-centred cubic In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties o ...
(fcc) structure. The compound is metallic. Each nickel atom loses 0.2 electrons to an argon atom which is thereby an oxidant. This contrasts with Ni3Xe, in which nickel is the oxidant. The volume of the ArNi compound is 5% less than that of the separate elements at these pressures. If this compound exists in the core of the Earth it could explain why only half the
argon-40 Argon (Ar) has 26 known isotopes, from Ar to Ar, of which three are stable (Ar, Ar, and Ar). On Earth, Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are Ar with a half-life of 268 years, Ar with a half-life of 32.9 ye ...
that should be produced during the radioactive decay that produces
geothermal heating Geothermal heating is the direct use of geothermal energy for some heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ o ...
seems to exist on the Earth.


Organoargon chemistry

Organoargon chemistry describes the synthesis and properties of
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s containing a
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 ...
to
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
chemical bond A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...
. Very few such compounds are known. The reaction of acetylene dications with argon produced in 2008. Reaction of the dication with argon produced : this reaction is unique to argon among the noble gases. The compound FArCCH has been theoretically studied and is predicted to be stable. FArCCF might also be stable enough to synthesise and detect, but probably not FArCCArF. Calculations in 2015 suggest that FArCCH and FArCH3 are stable, but not FArCN. should be kinetically stable, as is also expected of the krypton and xenon (but not helium) analogues. HArC4H (for which the krypton analogue is known) and HArC6H have also been predicted as stable. FArCO+ and ClArCO+ should be metastable and might be possible to characterise under cryogenic conditions. Calculations suggest that HArCCF and HCCArF should be stable, and that HNgCCF molecules should be more stable than HNgCCH (Ng = Ar, Kr, Xe); the corresponding krypton species have been experimentally produced, but not the argon species despite an experimental attempt. HCCNgCN and HCCNgNC (Ng = Ar, Kr, Xe) are likewise computed to be stable, but experimental searches for them have failed.


References


External links

* * * {{Chemical compounds by element Chemical compounds by element Compounds