Argonium

{{short description|Chemical compound}} {{Chembox | Watchedfields = | Name = | ImageFile = | ImageFile_Ref = | ImageSize = | ImageName = | ImageFile2 = Argonium-3D-vdW.png | ImageFile2_Ref = | ImageSize2 = | ImageName2 = | SystematicName = | OtherNames = Hydridoargon(1+)<br>argon hydride cation<ref>NIST Computational Chemistry Comparison and Benchmark Database, NIST Standard Reference Database Number 101. Release 19, April 2018, Editor: Russell D. Johnson III. http://cccbdb.nist.gov/</ref><br>protonated argon<ref>{{cite journal |arxiv=1607.00375|doi=10.3847/0004-637X/826/2/183|bibcode=2016ApJ...826..183N|title=The Chemistry of Interstellar Argonium and Other Probes of the Molecular Fraction in Diffuse Clouds|year=2016|last1=Neufeld|first1=David A.|last2=Wolfire|first2=Mark G.|journal=The Astrophysical Journal|volume=826|issue=2|pages=183|s2cid=118493563 |doi-access=free }}</ref> | IUPACName = Argonium ion | Section1 = {{Chembox Identifiers | CASNo = 12254-68-1 | CASNo_Ref = {{cascite|correct|CAS}} | ChemSpiderID = | ChemSpiderID_Ref = <!--{{chemspidercite|correct|chemspider}}--> | ChEBI = | ChEBI2 = | ChEBI2_Comment = | Gmelin = | SMILES = [ArH+] | StdInChI = 1S/ArH/h1H/q+1 | StdInChI_Ref = {{stdinchicite|correct|NIST}} | StdInChIKey = TVQSUVFYDVJWLI-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|correct|NIST}} }} | Section2 = {{Chembox Properties | Formula = {{Chem|ArH|+}} | Ar=1|H=1 | ConjugateBase = Argon | MolarMass = }} |Section8={{Chembox Related | OtherCompounds = Helium hydride ion, Neonium, Kryptonium, Xenonium }} }} '''Argonium''' (also called the '''argon hydride cation''', the '''hydridoargon(1+) ion''', or '''protonated argon'''; chemical formula ArH⁺) is a cation combining a proton and an argon atom. It can be made in an electric discharge, and was the first noble gas molecular ion to be found in interstellar space.<ref>{{cite news|last1=Quenqua|first1=Douglas|title=Noble Molecules Found in Space|url=https://www.nytimes.com/2013/12/17/science/space/noble-molecules-found-in-space.html?_r=0|access-date=26 September 2016|work=The New York Times|date=13 December 2013}}</ref>

==Properties== Argonium is isoelectronic with hydrogen chloride. Its dipole moment is 2.18&nbsp;D for the ground state.<ref name=extra/> The binding energy is 369&nbsp;kJ&nbsp;mol⁻¹<ref name="Schilke" /> (3.9&nbsp;eV<ref name="barlow" />). This is smaller than that of {{chem|H|3|+}} and many other protonated species, but more than that of {{chem|H|2|+}}.<ref name=Schilke/>

Rotationless radiative lifetimes of different vibrational states vary with isotope and become shorter for the more rapid high-energy vibrations: :{|class="wikitable" |+Lifetimes (ms)<ref>{{cite journal |title=Molecular Constants for the ¹Σ⁺ Ground State of the ArH⁺ Ion|author=Pavel Rosmus|journal=Theoretica Chimica Acta |volume=51|issue=4|pages=359–363|date=1979|doi=10.1007/BF00548944|s2cid=98475430}}</ref> !''v'' !!ArH⁺ !!ArD⁺ |- |1 ||2.28 ||9.09 |- |2 ||1.20 ||4.71 |- |3 ||0.85 ||3.27 |- |4 ||0.64 ||2.55 |- |5 ||0.46 ||2.11 |}

The force constant in the bond is calculated at 3.88 mdyne/Ų.<ref>{{cite journal|last1=Fortenberry|first1=Ryan C.|title=Quantum astrochemical spectroscopy|journal=International Journal of Quantum Chemistry|volume=117|issue=2|pages=81–91|date=June 2016|doi=10.1002/qua.25180|doi-access=free}}</ref>

==Reactions== The proton affinity of argon is {{val|369|ul=kJ/mol}}, less than that of dihydrogen.<ref name=Schilke/> This makes {{chem2|ArH+}} a superacid capable of exothermic protonation of {{chem2|H2}} and various neutral free radical species.

*ArH⁺ + H₂ → Ar + {{chem|H|3|+}}<ref name=Schilke/> *ArH⁺ + C → Ar + CH⁺ *ArH⁺ + N → Ar + NH⁺ *ArH⁺ + O → Ar + OH⁺ *ArH⁺ + CO → Ar + COH⁺<ref name=Schilke/>

But the reverse reaction happens:

*Ar + {{chem|H|2|+}} → ArH⁺ + H.<ref name=Schilke/> *Ar + {{chem|H|3|+}} → *ArH⁺ + H₂<ref name=Schilke/>

Ar⁺ + H₂ has a cross section of 10⁻¹⁸&nbsp;m² for low energy. It has a steep drop off for energies over 100&nbsp;eV<ref name=Phelps>{{cite journal|title=Collisions of H⁺, {{chem|H|2|+}}, {{chem|H|3|+}}, ArH⁺, H⁻, H, and H₂ with Ar and of Ar⁺ and ArH⁺ with H₂ for Energies from 0.1&nbsp;eV to 10&nbsp;keV |first=A. V. |last=Phelps |journal=J. Phys. Chem. Ref. Data |volume=21 |issue=4 |date=1992|doi=10.1063/1.555917 }}</ref> Ar + {{chem|H|2|+}} has a cross sectional area of {{val|6e-19|u=m²}} for low energy {{chem|H|2|+}}, but when the energy exceeds 10&nbsp;eV yield reduces, and more Ar⁺ and H₂ is produced instead.<ref name=Phelps/>

Ar + {{chem|H|3|+}} has a maximum yield of ArH⁺ for energies between 0.75 and 1&nbsp;eV with a cross section of {{val|5e-20|u=m²}}. 0.6&nbsp;eV is needed to make the reaction proceed forward. Over 4&nbsp;eV more Ar⁺ and H starts to appear.<ref name=Phelps/>

Argonium is also produced from Ar⁺ ions produced by cosmic rays and X-rays from neutral argon.

*Ar⁺ + H₂ → *ArH⁺ + H<ref name=Schilke/> 1.49&nbsp;eV<ref name=barlow/>

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⁻ → Ar + H<ref name=Schilke/>

Because ionisation potential of argon atoms is lower than that of the hydrogen molecule (in contrast to that of helium or neon), the argon ion reacts with molecular hydrogen, but for helium and neon ions, they will strip an electron from a hydrogen molecule.<ref name=Schilke/>

*Ar⁺ + H₂ → ArH⁺ + H<ref name=Schilke/> *Ne⁺ + H₂ → Ne + H⁺ + H (dissociative charge transfer)<ref name=Schilke/> *He⁺ + H₂ → He + H⁺ + H<ref name=Schilke/>

==Spectrum== Artificial ArH⁺ made from earthly argon contains mostly the isotope ⁴⁰Ar rather than the cosmically abundant ³⁶Ar. Artificially it is made by an electric discharge through an argon–hydrogen mixture.<ref name=brown/> Brault and Davis were the first to detect the molecule using infrared spectroscopy to observe vibration–rotation bands.<ref name=brown/> {|class="wikitable" |colspan=2|Far infrared spectrum of ⁴⁰Ar¹H⁺<ref name=brown>{{cite journal|last1=Brown|first1=John M.|last2=Jennings|first2=D.A.|last3=Vanek|first3=M.|last4=Zink|first4=L.R.|last5=Evenson|first5=K.M.|title=The pure rotational spectrum of ArH+|journal=Journal of Molecular Spectroscopy|date=April 1988|volume=128|issue=2|pages=587–589|doi=10.1016/0022-2852(88)90173-7|bibcode=1988JMoSp.128..587B|url=https://zenodo.org/record/1253940}}</ref> |³⁶Ar |³⁸Ar<ref name=extra/> |- !Transition !colspan=3|observed frequency |- !''J'' !colspan=3|GHz |- |1←0 |615.8584 |617.525 |615.85815 |- |2←1 |1231.2712 |1234.602 |- |3←2 |1845.7937 |- |4←3 |2458.9819 |- |5←4 |3080.3921 |- |6←5 |3679.5835 |- |7←6 |4286.1150 |- |21←20 |12258.483 |- |22←21 |12774.366 |- |23←22 |13281.119 |}

The UV spectrum has two absorption points resulting in the ion breaking up. The 11.2&nbsp;eV conversion to the B¹Π state has a low dipole and so does not absorb much. A 15.8&nbsp;eV to a repulsive A¹Σ⁺ state is at a shorter wavelength than the Lyman limit, and so there are very few photons around to do this in space.<ref name=Schilke/>

==Natural occurrence== ArH⁺ occurs in interstellar diffuse atomic hydrogen gas. For argonium to form, the fraction of molecular hydrogen H₂ must be in the range 0.0001 to 0.001. Different molecular ions form in correlation with different concentrations of H₂. Argonium is detected by its absorption lines at 617.525&nbsp;GHz (''J'' = 1→0), and 1234.602&nbsp;GHz (''J'' = 2→1). These lines are due to the isotopolog ³⁶Ar¹H⁺ undergoing rotational transitions. The lines have been detected in the direction of the galactic centre SgrB2(M) and SgrB2(N), G34.26+0.15, {{not a typo|W31C}} (G10.62−0.39), W49(N), and W51e, however where absorption lines are observed, argonium is not likely to be in the microwave source, but instead in the gas in front of it.<ref name=Schilke>{{cite journal|last1=Schilke|first1=P.|last2=Neufeld|first2=D. A.|last3=Müller|first3=H. S. P.|last4=Comito|first4=C.|last5=Bergin|first5=E. A.|last6=Lis|first6=D. C.|last7=Gerin|first7=M.|last8=Black|first8=J. H.|last9=Wolfire|first9=M.|last10=Indriolo|first10=N.|last11=Pearson|first11=J. C.|last12=Menten|first12=K. M.|last13=Winkel|first13=B.|last14=Sánchez-Monge|first14=Á.|last15=Möller|first15=T.|last16=Godard|first16=B.|last17=Falgarone|first17=E.|title=Ubiquitous argonium (ArH⁺) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas|journal=Astronomy & Astrophysics|date=4 June 2014|volume=566|pages=A29|doi=10.1051/0004-6361/201423727|bibcode=2014A&A...566A..29S|arxiv = 1403.7902 |s2cid=44021593}}</ref> Emission lines are found in the Crab Nebula.<ref name=barlow>{{cite journal|last1=Barlow|first1=M. J.|last2=Swinyard|first2=B. M.|last3=Owen|first3=P. J.|last4=Cernicharo|first4=J.|last5=Gomez|first5=H. L.|last6=Ivison|first6=R. J.|last7=Krause|first7=O.|last8=Lim|first8=T. L.|last9=Matsuura|first9=M.|last10=Miller|first10=S.|last11=Olofsson|first11=G.|last12=Polehampton|first12=E. T.|title=Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula|journal=Science|date=12 December 2013|volume=342|issue=6164|pages=1343–1345|doi=10.1126/science.1243582|pmid=24337290|bibcode=2013Sci...342.1343B|arxiv = 1312.4843 |s2cid=37578581}}</ref>

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.<ref name=barlow/> The column density of ArH⁺ in the Crab Nebula is between 10¹² and 10¹³ atoms per square centimeter.<ref name=barlow/> Possible the energy required to excite the ions so that then can emit comes from collisions with electrons or hydrogen molecules.<ref name=barlow/> Towards the Milky Way centre the column density of ArH⁺ is around {{val|2e13|u=cm⁻²}}.<ref name=Schilke/>

Two isotopologs of argonium ³⁶ArH⁺ and ³⁸ArH⁺ are known to be in a distant unnamed galaxy with a redshift of ''z'' = 0.88582 (7.5&nbsp;billion light years away) which is on the line of sight to the blazar PKS 1830−211.<ref name=extra>{{cite journal|last1=Müller|first1=Holger S. P.|last2=Muller|first2=Sébastien|last3=Schilke|first3=Peter|last4=Bergin|first4=Edwin A.|last5=Black|first5=John H.|last6=Gerin|first6=Maryvonne|last7=Lis|first7=Dariusz C.|last8=Neufeld|first8=David A.|last9=Suri|first9=Sümeyye|title=Detection of extragalactic argonium, ArH⁺, toward PKS 1830−211|journal=Astronomy & Astrophysics|date=7 October 2015|volume=582|pages=L4|doi=10.1051/0004-6361/201527254|bibcode=2015A&A...582L...4M|arxiv = 1509.06917 |s2cid=10017142}}</ref>

Electron neutralization and destruction of argonium outcompletes the formation rate in space if the H₂ concentration is below 1 in 10⁻⁴.<ref name=Neufeld>{{Cite journal|arxiv=1607.00375|title=The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds|journal=The Astrophysical Journal|volume=826|issue=2|pages=183|date=1 July 2016|author1=David A. Neufeld|author2= Mark G. Wolfire|doi=10.3847/0004-637X/826/2/183|bibcode = 2016ApJ...826..183N |s2cid=118493563 |doi-access=free }}</ref>

==History== Using the McMath solar Fourier transform spectrometer at Kitt Peak National Observatory, James W. Brault and Sumner P. Davis observed ArH⁺ vibration-rotation infrared lines for the first time.<ref>{{cite journal|last1=Brault|first1=James W|last2=Davis|first2=Sumner P|title=Fundamental Vibration-Rotation Bands and Molecular Constants for the ArH⁺ Ground State (¹Σ⁺ )|journal=Physica Scripta|date=1 February 1982|volume=25|issue=2|pages=268–271|doi=10.1088/0031-8949/25/2/004|bibcode=1982PhyS...25..268B|s2cid=250825672 }}</ref> J. W. C. Johns also observed the infrared spectrum.<ref>{{cite journal|last1=Johns|first1=J.W.C.|title=Spectra of the protonated rare gases|journal=Journal of Molecular Spectroscopy|date=July 1984|volume=106|issue=1|pages=124–133|doi=10.1016/0022-2852(84)90087-0|bibcode=1984JMoSp.106..124J}}</ref>

==Use== Argon facilitates the reaction of tritium (T₂) with double bonds in fatty acids by forming an ArT⁺ (tritium argonium) intermediate.<ref>{{cite journal|last1=Peng|first1=C. T.|title=Mechanism of Addition of Tritium to Oleate by Exposure to Tritium Gas|journal=The Journal of Physical Chemistry|date=April 1966|volume=70|issue=4|pages=1297–1304|doi=10.1021/j100876a053|pmid=5916501}}</ref> When gold is sputtered with an argon-hydrogen plasma, the actual displacement of gold is done by ArH⁺.<ref>{{cite journal|last1=Jiménez-Redondo|first1=Miguel|last2=Cueto|first2=Maite|last3=Doménech|first3=José Luis|last4=Tanarro|first4=Isabel|last5=Herrero|first5=Víctor J.|title=Ion kinetics in Ar/H₂ cold plasmas: the relevance of ArH⁺|journal=RSC Advances|date=3 November 2014|volume=4|issue=107|pages=62030–62041|doi=10.1039/C4RA13102A|pmid=26702354|pmc=4685740|bibcode=2014RSCAd...462030J|issn=2046-2069|url=https://digital.csic.es/bitstream/10261/107548/3/2014%20Jimenez-Redondo%20Ar%2bH2%20kinetics.pdf}}</ref>

==References== {{Reflist|30em}}

{{Noble gas compounds}} {{Molecules detected in outer space}}

Category:Argon compounds Category:Cations