Argon compounds

Argonium (ArH+) is an ion combining a proton and an argon atom. It is found in interstellar space in diffuse atomic hydrogen gas where the fraction of molecular hydrogen H2 is in the range of 0.0001 to 0.001. Argonium is formed when H2+ reacts with Ar atoms: 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 Natural occurrence In the Crab Nebula, ArH+ occurs in several spots revealed by emission lines. 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 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. Larger charged argon clusters are also detectable in mass spectroscopy. The tetraargon cation is also linear. icosahedral 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. 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, it is initially electronically excited, but then energy is transferred to the whole cluster in the form of vibration. 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. ==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 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. :Formation: Ar + ν → Ar*; Ar* + H2 → ArH* + H The bond length in ArH* is calculated as 1.302 Å. The spectrum of argon monohydride, both ArH* and ArD*, 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 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-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 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. However it has not been detected . The force constant of the ArH bond in this is 1.895 mdyne/Å2 (). 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 the 11A′ state. 3Σ− is a triplet state 0.12 eV higher in energy, and 3A″ is a triplet state 0.18 eV higher. The Ar−O bond is predicted to be 1.684 Å long ==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 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 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, 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 microwave spectroscopy. 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. ==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+. ==ArnBO+==

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, irradiated by a 150 V electron beam. 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. 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. 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 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 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 (fcc) structure, as in solid argon. It is the surface energy that maintains an icosahedral shape, but for larger clusters internal pressure will attract the atoms into an fcc arrangement. Neutral argon clusters are transparent to visible light. 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 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 molecule X → B transitions. The spectrum of bromine is blue-shifted and spread out when it binds an argon atom. 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, I2Arn 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 molecule forming a monolayer around the H2O. Ar12·H2O is particularly stable, having an icosahedral 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 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. formyl radical (ArHCO), 7-azaindole, glyoxal, sodium chloride (ArNaCl), ArHCl, and cyclopentanone. : ==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. 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. 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 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 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 atoms into carbon monoxide. Uranium acts as a strong Lewis acid 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. 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 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. 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. The argon uranium dioxide molecule is likely UO2Ar5. ==Beryllium oxide==

When beryllium 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 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 elements can form reactive pentacarbonyls 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 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 and chromium. 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, and 518 nm in krypton). 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. Evidence also exists for ArHMn(CO)4, ArCH3Mn(CO)4, and fac-(η2-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 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, 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==