Editing Hydronium (section)

==Interstellar H<sub>3</sub>O<sup>+</sup>==

Hydronium is an abundant [[molecular ion]] in the [[interstellar medium]] and is found in diffuse<ref name=faure2003rce>{{Cite journal | doi = 10.1046/j.1365-8711.2003.06306.x|url=https://www.researchgate.net/publication/227780354| title = Rate coefficients for electron-impact rotational excitation of H<sub>3</sub><sup>+</sup> and H<sub>3</sub>O<sup>+</sup>| journal = [[Monthly Notices of the Royal Astronomical Society]]| volume = 340| issue = 2| pages = 468–472| year = 2003| last1 = Faure | first1 = A.| last2 = Tennyson | first2 = J.|bibcode=2003MNRAS.340..468F| doi-access = free}}</ref> and dense<ref name=hollis1986ilc>{{Cite journal | doi = 10.1038/322524a0| title =An interstellar line coincident with the P(2,l) transition of hydronium (H<sub>3</sub>O<sup>+</sup>)| journal = [[Nature (journal)|Nature]]| volume = 322| issue = 6079| pages = 524–526| year = 1986| last1 = Hollis | first1 = J. M.| last2 = Churchwell | first2 = E. B.| last3 = Herbst | first3 = E.| last4 = De Lucia | first4 = F. C.| bibcode =1986Natur.322..524H| s2cid =4346975}}</ref> molecular clouds as well as the plasma tails of comets.<ref name=rauer1997ica>{{cite journal |last1=Rauer |first1=H |year=1997 |title=Ion composition and solar wind interaction: Observations of comet C/1995 O1 (Hale-Bopp) |doi=10.1023/A:1006285300913 |journal=[[Earth, Moon, and Planets]] |volume=79 |pages=161–178 |bibcode=1997EM&P...79..161R|s2cid=119953549}}</ref> Interstellar sources of hydronium observations include the regions of Sagittarius B2, Orion OMC-1, Orion BN–IRc2, Orion KL, and the comet Hale–Bopp.

Interstellar hydronium is formed by a chain of reactions started by the ionization of {{chem2|H2}} into {{chem2|H2(+)}} by cosmic radiation.<ref name=vejbychristensen1997cbr>{{Cite journal | doi = 10.1086/304242| title =Complete Branching Ratios for the Dissociative Recombination of H<sub>2</sub>O<sup>+</sup>, H<sub>3</sub>O<sup>+</sup>, and CH<sub>3</sub><sup>+</sup>| journal = [[The Astrophysical Journal]]| volume = 483| issue =1| pages = 531–540| year = 1997| last1 = Vejby-Christensen | first1 = L.| last2 = Andersen | first2 = L. H.| last3 = Heber | first3 = O.| last4 = Kella | first4 = D.| last5 = Pedersen | first5 = H. B.| last6 = Schmidt | first6 = H. T.| last7 = Zajfman | first7 = D.| bibcode =1997ApJ...483..531V| doi-access = free}}</ref> {{H3O+}} can produce either {{chem2|OH(-)}} or {{H2O-nl}} through [[dissociative recombination]] reactions, which occur very quickly even at the low (≥10 K) temperatures of dense clouds.<ref name=neau2000drd>{{Cite journal | doi = 10.1063/1.481979| title = Dissociative recombination of D<sub>3</sub>O<sup>+</sup> and H<sub>3</sub>O<sup>+</sup>: Absolute cross sections and branching ratios| journal = [[The Journal of Chemical Physics]]| volume = 113| issue = 5| pages = 1762| year = 2000| last1 = Neau | first1 = A.| last2 = Al Khalili | first2 = A.| last3 = Rosén | first3 = S.| last4 = Le Padellec | first4 = A.| last5 = Derkatch | first5 = A. M.| last6 = Shi | first6 = W.| last7 = Vikor | first7 = L.| last8 = Larsson | first8 = M.| last9 = Semaniak | first9 = J.| last10 = Thomas | first10 = R.| last11 = Någård | first11 = M. B.| last12 = Andersson | first12 = K.| last13 = Danared | first13 = H.| last14 = Af Ugglas | first14 = M.| bibcode = 2000JChPh.113.1762N}}</ref> This leads to hydronium playing a very important role in interstellar ion-neutral chemistry.

Astronomers are especially interested in determining the abundance of water in various interstellar climates due to its key role in the cooling of dense molecular gases through radiative processes.<ref name=neufeld1995tbd>{{Cite journal |doi=10.1086/192211 |title=Thermal Balance in Dense Molecular Clouds: Radiative Cooling Rates and Emission-Line Luminosities |journal=[[The Astrophysical Journal Supplement Series]] |volume=100 |pages=132 |year=1995 |last1=Neufeld |first1=D. A. |last2=Lepp |first2=S. |last3=Melnick |first3=G. J. |bibcode=1995ApJS..100..132N}}</ref> However, {{H2O-nl}} does not have many favorable transitions for ground-based observations.<ref name=wootten1986sih>{{Cite journal
 | pmid = 11542067|bibcode=1986A&A...166L..15W
| year = 1986
| last1 = Wootten
| first1 = A.
| title = A search for interstellar H<sub>3</sub>O<sup>+</sup>
| journal = [[Astronomy and Astrophysics]]
| volume = 166
| pages = L15–8
| last2 = Boulanger
| first2 = F.
| last3 = Bogey
| first3 = M.
| last4 = Combes
| first4 = F.
| last5 = Encrenaz
| first5 = P. J.
| last6 = Gerin
| first6 = M.
| last7 = Ziurys
| first7 = L. | author7-link = Lucy Ziurys
}}</ref> Although observations of HDO (the [[Heavy water|deuterated version of water]]<ref>{{GoldBookRef|title=heavy water|file=H02758}}</ref>) could potentially be used for estimating {{H2O-nl}} abundances, the ratio of HDO to {{H2O-nl}} is not known very accurately.<ref name=wootten1986sih />

Hydronium, on the other hand, has several transitions that make it a superior candidate for detection and identification in a variety of situations.<ref name=wootten1986sih /> This information has been used in conjunction with laboratory measurements of the branching ratios of the various {{H3O+}} dissociative recombination reactions<ref name=neau2000drd /> to provide what are believed to be relatively accurate {{chem2|OH(-)}} and {{H2O-nl}} abundances without requiring direct observation of these species.<ref name=herbst1977iou>{{cite journal |last1=Herbst |first1=E. |last2=Green |first2=S. |last3=Thaddeus |first3=P. |last4=Klemperer |first4=W. |year=1977 |title=Indirect observation of unobservable interstellar molecules |journal=[[The Astrophysical Journal]] |volume=215 |pages=503–510 |doi=10.1086/155381 |bibcode=1977ApJ...215..503H|hdl=2060/19770013020 |s2cid=121202097 |hdl-access=free}}</ref><ref name=phillips1992iha>{{Cite journal | doi = 10.1086/171945| title = Interstellar H<sub>3</sub>O<sup>+</sup> and its Relation to the O<sub>2</sub> and H<sub>2</sub>O- Abundances| journal = [[The Astrophysical Journal]]| volume = 399| pages = 533 | bibcode = 1992ApJ...399..533P| hdl = 1887/2260| year = 1992| last1 = Phillips | first1 = T. G.| last2 = Van Dishoeck | first2 = E. F. | last3 = Keene | first3 = J. | url = https://openaccess.leidenuniv.nl/bitstream/handle/1887/2260/352_044.pdf?sequence=1| hdl-access = free}}</ref>

===Interstellar chemistry===
As mentioned previously, {{H3O+}} is found in both diffuse and dense molecular clouds. By applying the [[reaction rate]] constants (''α'', ''β'', and ''γ'') corresponding to all of the currently available characterized reactions involving {{H3O+}}, it is possible to calculate ''k''(''T'') for each of these reactions. By multiplying these ''k''(''T'') by the relative abundances of the products, the relative rates (in cm<sup>3</sup>/s) for each reaction at a given temperature can be determined. These relative rates can be made in absolute rates by multiplying them by the {{chem2|[H2]^{2}|}}.<ref name=udfa>{{cite web|title = H<sub>3</sub>O<sup>+</sup> formation reactions|url = http://udfa.ajmarkwick.net/index.php?species=41|work = The UMIST Database for Astrochemistry}}</ref> By assuming {{math|1=''T''&nbsp;=&nbsp;10&nbsp;K}} for a dense cloud and {{math|1=''T''&nbsp;=&nbsp;50&nbsp;K}} for a diffuse cloud, the results indicate that most dominant formation and destruction mechanisms were the same for both cases. It should be mentioned that the relative abundances used in these calculations correspond to TMC-1, a dense molecular cloud, and that the calculated relative rates are therefore expected to be more accurate at {{math|1=''T''&nbsp;=&nbsp;10&nbsp;K}}. The three fastest formation and destruction mechanisms are listed in the table below, along with their relative rates. Note that the rates of these six reactions are such that they make up approximately 99% of hydronium ion's chemical interactions under these conditions.<ref name=rauer1997ica /> All three destruction mechanisms in the table below are classified as [[dissociative recombination]] reactions.<ref>{{Cite web|title=Dissociative recombination {{!}} physics|url=https://www.britannica.com/science/dissociative-recombination|access-date=2021-09-30|website=Encyclopedia Britannica|language=en}}</ref>
[[Image:PrimaryH3OPathways.png|thumb|Primary reaction pathways of {{H3O+}} in the interstellar medium (specifically, dense clouds).]]

{|border="1" cellpadding="5" cellspacing="0" align="center" class="wikitable"
|-
! rowspan="2" | Reaction
! rowspan="2" |Type
! colspan="2" | Relative rate (cm<sup>3</sup>/s)
|-
! at 10 K
! at 50 K
|-
|{{chem2|H2 + H2O+ -> H3O+ + H}}
|Formation
|2.97{{e|-22}}
|2.97{{e|-22}}
|-
|{{chem2|H2O + HCO+ -> CO + H3O+}}
|Formation
|4.52{{e|-23}}
|4.52{{e|-23}}
|-
|{{chem2|H3+ + H2O -> H3O+ + H2}}
|Formation
|3.75{{e|-23}}
|3.75{{e|-23}}
|-
|{{chem2|H3O+ + e(-) -> OH + H + H}}
|Destruction
|2.27{{e|-22}}
|1.02{{e|-22}}
|-
|{{chem2|H3O+ + e(-) -> H2O + H}}
|Destruction
|9.52{{e|-23}}
|4.26{{e|-23}}
|-
|{{chem2|H3O+ + e(-) -> OH + H2}}
|Destruction
|5.31{{e|-23}}
|2.37{{e|-23}}
|}
It is also worth noting that the relative rates for the formation reactions in the table above are the same for a given reaction at both temperatures. This is due to the reaction rate constants for these reactions having ''β'' and ''γ'' constants of 0, resulting in {{math|1=''k''&nbsp;=&nbsp;''α''}} which is independent of temperature.

Since all three of these reactions produce either {{H2O-nl}} or OH, these results reinforce the strong connection between their relative abundances and that of {{H3O+}}. The rates of these six reactions are such that they make up approximately 99% of hydronium ion's chemical interactions under these conditions.

===Astronomical detections===
As early as 1973 and before the first interstellar detection, chemical models of the interstellar medium (the first corresponding to a dense cloud) predicted that hydronium was an abundant molecular ion and that it played an important role in ion-neutral chemistry.<ref name=herbst1973fad>{{cite journal |last1=Herbst |first1=E. |last2=Klemperer |first2=W. |year=1973 |title=The formation and depletion of molecules in dense interstellar clouds |journal=[[The Astrophysical Journal]] |volume=185 |page=505 |doi=10.1086/152436 |bibcode=1973ApJ...185..505H|doi-access=free}}</ref> However, before an astronomical search could be underway there was still the matter of determining hydronium's spectroscopic features in the gas phase, which at this point were unknown. The first studies of these characteristics came in 1977,<ref name=schwarz1977gpi>{{cite journal |last1=Schwarz |first1=H.A. |year=1977 |title=Gas phase infrared spectra of oxonium hydrate ions from 2 to 5&nbsp;μm |journal=[[Journal of Chemical Physics]] |volume=67 |issue=12 |page=5525 |doi=10.1063/1.434748 |bibcode=1977JChPh..67.5525S}}</ref> which was followed by other, higher resolution spectroscopy experiments. Once several lines had been identified in the laboratory, the first interstellar detection of H<sub>3</sub>O<sup>+</sup> was made by two groups almost simultaneously in 1986.<ref name=hollis1986ilc /><ref name=wootten1986sih /> The first, published in June 1986, reported observation of the ''J''{{su|b=''K''|p=vt}}&nbsp;=&nbsp;1{{su|b=1|p=−}}&nbsp;−&nbsp;2{{su|b=1|p=+}} transition at {{val|307192.41|u=MHz}} in [[OMC-1]] and [[Sgr B2]]. The second, published in August, reported observation of the same transition toward the [[Orion-KL]] nebula.

These first detections have been followed by observations of a number of additional {{H3O+}} transitions. The first observations of each subsequent transition detection are given below in chronological order:

In 1991, the 3{{su|b=2|p=+}}&nbsp;−&nbsp;2{{su|b=2|p=−}} transition at {{val|364797.427|u=MHz}} was observed in OMC-1 and Sgr B2.<ref name=wootten1991myx>{{Cite journal |doi=10.1086/186178 |title=Detection of interstellar H<sub>3</sub>O<sup>+</sup> – A confirming line |journal=[[The Astrophysical Journal]] |volume=380 |pages=L79 |year=1991 |last1=Wootten |first1=A. |last2=Turner |first2=B. E. |last3=Mangum |first3=J. G. |last4=Bogey |first4=M. |last5=Boulanger |first5=F. |last6=Combes |first6=F. |last7=Encrenaz |first7=P. J. |last8=Gerin |first8=M. |bibcode=1991ApJ...380L..79W}}</ref> One year later, the 3{{su|b=0|p=+}}&nbsp;−&nbsp;2{{su|b=0|p=−}} transition at {{val|396272.412|u=MHz}} was observed in several regions, the clearest of which was the W3&nbsp;IRS&nbsp;5 cloud.<ref name=phillips1992iha />

The first far-IR 4{{su|b=3|p=−}}&nbsp;−&nbsp;3{{su|b=3|p=+}} transition at 69.524&nbsp;μm (4.3121&nbsp;THz) was made in 1996 near [[Becklin–Neugebauer Object|Orion BN]]-IRc2.<ref name=timmermann1996pdm>{{Cite journal | doi = 10.1086/310055| title = Possible discovery of the 70&nbsp;μm {H<sub>3</sub>O<sup>+</sup>} 4{{su|b=3|p=−}}&nbsp;−&nbsp;3{{su|b=3|p=+}} transition in Orion BN-IRc2| journal = [[The Astrophysical Journal]]| volume = 463| issue = 2| pages = L109| year = 1996| last1 = Timmermann | first1 = R. | last2 = Nikola | first2 = T. | last3 = Poglitsch | first3 = A. | last4 = Geis | first4 = N. | last5 = Stacey | first5 = G. J. | last6 = Townes | first6 = C. H. | bibcode = 1996ApJ...463L.109T| doi-access = free}}</ref> In 2001, three additional transitions of {{H3O+}} in were observed in the far infrared in Sgr&nbsp;B2; 2{{su|b=1|p=−}}&nbsp;−&nbsp;1{{su|b=1|p=+}} transition at 100.577&nbsp;μm (2.98073 THz), 1{{su|b=1|p=−}}&nbsp;−&nbsp;1{{su|b=1|p=+}} at 181.054&nbsp;μm (1.65582 THz) and 2{{su|b=0|p=−}}&nbsp;−&nbsp;1{{su|b=0|p=+}} at 100.869&nbsp;μm (2.9721 THz).<ref name=goicoechea2001fid>{{Cite journal |doi=10.1086/321712 |title=Far-infrared detection of H<sub>3</sub>O<sup>+</sup> in Sagittarius B2 |journal=[[The Astrophysical Journal]] |volume=554 |issue=2 |pages=L213 |year=2001 |last1=Goicoechea |first1=J. R. |last2=Cernicharo |first2=J. |bibcode=2001ApJ...554L.213G|doi-access=free|hdl=10261/192309 |hdl-access=free }}</ref>