Argon compounds, the chemical compounds that contain the element argon, are rarely encountered due to the inertness of the argon atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and molecular ions containing argon have been made and also detected in space. One solid interstitial compound of argon, is stable at room temperature. was discovered by the CSIRO.
Argon ionises at 15.76 eV (1520 kJ mol-1), 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 forces. 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 to study stretching and bending movements, microwave spectroscopy and far infrared to study rotation, and also visible and ultraviolet spectroscopy to study different electronic configurations including excimers. Mass spectroscopy 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 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 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 is important in photolithography of silicon chips. These lasers make a strong ultraviolet emission at 193 nm.
Argonium
Argonium () 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, , is in the range of 0.0001 to 0.001.
Argonium is formed when reacts with Ar atoms:
and it is also produced from ions produced by cosmic rays and X-rays from neutral argon:
1.49 eV (144 kJ mol-1).
When encounters an electron, dissociative recombination can occur, but it is extremely slow for lower energy electrons, allowing to survive for a much longer time than many other similar protonated cations.
Artificial made from earthly Ar contains mostly the isotope Ar rather than the cosmically abundant Ar. Artificially it is made by an electric discharge through an argon-hydrogen mixture.
Natural occurrence In the Crab Nebula, 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 and ions. The column density of 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 is around .
Cluster argon cations The diargon cation, has a binding energy of 1.29 eV (124 kJ mol-1).
The triargon cation is linear, but has one Ar−Ar bond shorter than the other. Bond lengths are 2.47 and 2.73 ångströms. The dissociation energy to Ar and 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 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 , , and . 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 excimers. 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 + γ → Ar*; Ar* + → 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 will not form ArH this way.
In the van der Waals molecule of ArH, the bond length is calculated to be about 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 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 ) 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-type discharge tube, or other electric discharges. Yet another way is to create a beam of (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 . is predicted to be linear in the form Ar−H−H. The H−H distance is . The dissociation barrier is only mol-1, and readily loses a hydrogen atom to yield . The force constant of the ArH bond in this is 1.895 mdyne/Å2 ().
The argon trihydrogen cation has been observed in the laboratory. , 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 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 and to have a force constant of 2.988 mdyne/Å2 (). Such an argoxonium ion has also been empirically observed through the reaction of argon with interfacial water in microdroplets.
is a possible ionic molecule to detect in the lab, and in space, as the atoms that compose it are common. is predicted to be more weakly bound than , 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: . The dissociation yields , as this is a higher-energy state. The binding energy is 1.19 eV (115 kJ mol-1). The molecule is linear. The distance between two nitrogen atoms is . This distance is similar to that of neutral 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 at 2175 and at 2330 cm−1.
In the process of photodissociation, it is three times more likely to yield + compared to Ar + .
has been produced in a supersonic jet expansion of gas and detected by Fourier transform microwave spectroscopy. 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 .
The molecule is made by the following reaction:
.
Bis(dinitrogen) argon cation The argon ion can bond two molecules of dinitrogen () 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 (164 kJ mol-1) of energy is required to break this apart to and . The band origin of an infrared band due to antisymmetric vibration of the N=N bonds is at 2288.7272 cm−1 (27.379257 kJ mol-1). Compared to 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.
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.
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
.
forms four complexes with argon: ; two isomers of (one with equidistant Ar-B bonds and another with a short and long bond); and . 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.
(yields 0.435 eV, 42.0 kJ mol-1.)
Van der Waals molecules