Editing Hydronium

{{Short description|Aqueous cation H₃O⁺, the type of oxonium ion produced by protonation of water}}
{{about|the ion H<sub>3</sub>O<sup>+</sup>|the neutral compound H<sub>3</sub>O|trihydrogen oxide}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 417094923
| Name =
| ImageFile =
| ImageFileL1 = Hydroxonium-cation.svg
| ImageNameL1 = 3D diagram showing the pyramidal structure of the hydroxonium ion
| ImageFileR1 = Hydronium-3D-balls.png
| ImageNameR1 = Ball-and-stick model of the hydronium ion
| ImageFileL2 = Hydroxonium-3D-elpot.png
| ImageNameL2 = 3D electric potential surface of the hydroxonium cation
| ImageFileR2 = Hydronium.png
| ImageNameR2 = Van der Waals radius of Hydronium
| IUPACName = oxonium
| OtherNames = hydronium ion
| SystematicName =
| Section1 = {{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|??}}
| CASNo = 13968-08-6
| Gmelin = 141
| PubChem = 123332
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 29412
| SMILES = [OH3+]
| InChI = 1S/H2O/h1H2/p+1
| InChIKey = XLYOFNOQVPJJNP-UHFFFAOYSA-O
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 109935
 }}
| Section2 = {{Chembox Properties
| Formula = {{H3O+}}
| H=3|O=1
| Appearance =
| Density =
| MeltingPt =
| BoilingPt =
| Solubility =
| pKa = undefined
| ConjugateBase = [[Properties of water|Water]]
 }}
| Section3 = {{Chembox Hazards
| MainHazards =
| FlashPt =
| AutoignitionPt =
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| Section4 =
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}}
In [[chemistry]], '''hydronium''' ('''hydroxonium''' in traditional [[British English]]) is the [[cation]] {{chem2|[H3O]+}}, also written as {{H3O+}}, the type of [[oxonium ion]] produced by [[protonation]] of [[water]]. It is often viewed as the positive ion present when an [[Arrhenius acid]] is dissolved in water, as Arrhenius acid [[molecule]]s in [[Solution (chemistry)|solution]] give up a [[proton]] (a positive [[hydrogen]] ion, {{chem2|H+}}) to the surrounding water molecules ({{H2O}}). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous {{chem2|H+}} and conjugate base.

Three main structures for the aqueous proton have garnered experimental support:

* the Eigen cation, which is a tetrahydrate, H<sub>3</sub>O<sup>+</sup>(H<sub>2</sub>O)<sub>3</sub>
* the Zundel cation, which is a symmetric dihydrate, H<sup>+</sup>(H<sub>2</sub>O)<sub>2</sub>
* and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H<sup>+</sup>(H<sub>2</sub>O)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub><ref name="Reed2013">{{cite journal |last1=Reed |first1=C.A. |title=Myths about the proton. The nature of H+ in condensed media |journal=Acc. Chem. Res. |date=2013 |volume=46 |issue=11 |pages=2567–2575|doi=10.1021/ar400064q |pmid=23875729 |pmc=3833890}}</ref><ref name="Silverstein2014">{{cite journal |last1=Silverstein |first1=Todd P. |title=The aqueous proton is hydrated by more than one water molecule: Is the hydronium ion a useful conceit? |journal=J. Chem. Educ. |date=2014 |volume=91 |issue=4 |pages=608–610|doi=10.1021/ed400559t |bibcode=2014JChEd..91..608S}}</ref> 

Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form.<ref name="Thamer2015">{{cite journal |last1=Thamer |first1=M. |last2=DeMarco |first2=L. |last3=Ramesha |first3=K. |last4=Mandel |first4=A. |last5=Tokmakoff |first5=A. |title=Ultrafast 2D IR spectroscopy of the excess proton in liquid water |journal=Science |date=2015 |volume=350 |issue=6256 |pages=78–82|doi=10.1126/science.aab3908 |pmid=26430117 |bibcode=2015Sci...350...78T |s2cid=27074374 |doi-access=free}}</ref><ref>{{cite journal |last1=Daly Jr. |first1=C.A. |last2=Streacker |first2=L.M. |last3=Sun |first3=Y. |last4=Pattenaude |first4=S.R. |last5=Hassanali |first5=A.A. |last6=Petersen |first6=P.B.|display-authors=etal |title=Decomposition of the experimental Raman and IR spectra of acidic water into proton, special pair, and counterion contributions |journal=J. Phys. Chem. Lett. |date=2017 |volume=8 |issue=21 |pages=5246–5252|doi=10.1021/acs.jpclett.7b02435 |pmid=28976760}}</ref><ref name="Dahms2017">{{cite journal |last1=Dahms |first1=F. |last2=Fingerhut |first2=B.P. |last3=Nibbering |first3=E.T. |last4=Pines |first4=E. |last5=Elsaesser |first5=T. |title=Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy |journal=Science |date=2017 |volume=357 |issue=6350 |pages=491–495|doi=10.1126/science.aan5144 |pmid=28705988 |bibcode=2017Sci...357..491D |s2cid=40492001 |doi-access=free}}</ref><ref name="Fournier2018">{{cite journal |last1=Fournier |first1=J.A. |last2=Carpenter |first2=W.B. |last3=Lewis |first3=N.H. |last4=Tokmakoff |first4=A. |title=Broadband 2D IR spectroscopy reveals dominant asymmetric H5O2+ proton hydration structures in acid solutions |journal=Nature Chemistry |date=2018 |volume=10 |issue=9 |pages=932–937|doi=10.1038/s41557-018-0091-y |pmid=30061612 |bibcode=2018NatCh..10..932F |osti=1480907 |s2cid=51882732}}</ref>{{Non-primary source needed|date=March 2024}} For this reason, it has been suggested that wherever possible, the symbol H<sup>+</sup>(aq) should be used instead of the hydronium ion.<ref name="Silverstein2014" />

==Relation to pH==
The [[molar concentration]] of hydronium or {{chem2|H+}} ions determines a solution's [[pH]] according to

:pH = −log([{{chem2|H3O+}}]/M)

where M = mol/L. The concentration of [[hydroxide]] ions analogously determines a solution's [[pH#pOH|pOH]]. The molecules in pure water [[self-ionization of water|auto-dissociate]] into aqueous protons and hydroxide ions in the following equilibrium:

:{{chem2| H2O <-> OH-(aq) + H+(aq)}}

In pure water, there is an equal number of hydroxide and {{chem2|H+}} ions, so it is a neutral solution. At {{convert|25|C|F}}, pure water has a pH of 7 and a pOH of 7 (this varies when the temperature changes: see [[self-ionization of water]]). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.<ref name="usgs">{{cite web|url=https://www.usgs.gov/special-topic/water-science-school/science/ph-and-water?qt-science_center_objects=0#qt-science_center_objects|title=pH and Water|author=<!--No listed author-->|publisher=United States Geological Survey|access-date=9 November 2021}}</ref>

==Nomenclature==
According to [[IUPAC nomenclature of organic chemistry#Parent hydride cations|IUPAC nomenclature of organic chemistry]], the hydronium ion should be referred to as ''oxonium''.<ref>{{cite web |url=http://www.acdlabs.com/iupac/nomenclature/93/r93_583.htm |title=Table 17 Mononuclear parent onium ions |publisher=IUPAC}}</ref> ''Hydroxonium'' may also be used unambiguously to identify it.{{cn|date=January 2024}}

An [[oxonium ion]] is any cation containing a trivalent oxygen atom.

==Structure==
Since {{chem2|O+}} and N have the same number of electrons, {{H3O+}} is [[isoelectronic]] with [[ammonia]]. As shown in the images above, {{H3O+}} has a [[trigonal pyramidal molecular geometry]] with the oxygen atom at its apex. The {{chem2|H\sO\sH}} [[bond angle]] is approximately 113°,<ref>{{cite journal | title = Infrared spectroscopy of H<sub>3</sub>O<sup>+</sup>: the v<sub>1</sub> fundamental band | first1 = Jian | last1 = Tang |first2 = Takeshi | last2 = Oka | journal = [[Journal of Molecular Spectroscopy]] | volume = 196 | pages = 120–130 | year = 1999 | doi = 10.1006/jmsp.1999.7844 | pmid = 10361062 | issue = 1| bibcode = 1999JMoSp.196..120T}}</ref><ref>{{cite book| last1 = Bell | first1 = R. P. | page = 15 | edition = 2nd | title = The Proton in Chemistry | publisher = Cornell University Press | location = Ithaca | date = 1973 }}</ref> and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the {{H3O+}} molecule's [[symmetric top]] configuration is such that it belongs to the {{chem2|C_{3v}|}} [[Point groups in three dimensions|point group]]. Because of this symmetry and the fact that it has a dipole moment, the rotational [[selection rule]]s are Δ''J''&nbsp;=&nbsp;±1 and Δ''K''&nbsp;=&nbsp;0. The [[Transition dipole moment|transition dipole]] lies along the ''c''-axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane.

==Acids and acidity==
The hydrated proton is very acidic: at 25&nbsp;°C, its [[Acid dissociation constant|p''K''<sub>a</sub>]] is approximately 0.<ref>{{cite journal |last1=Meister |first1=Erich |last2=Willeke |first2=Martin |last3=Angst |first3=Werner |last4=Togni |first4=Antonio |last5=Walde |first5=Peter |date=2014 |title=Confusing Quantitative Descriptions of Brønsted-Lowry Acid-Base Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators |journal=[[Helv. Chim. Acta]] |volume=97 |issue=1 |pages=1–31 |doi=10.1002/hlca.201300321}}</ref> The values commonly given for p''K''<sub>a</sub><sup>aq</sup>(H<sub>3</sub>O<sup>+</sup>) are 0 or −1.74. The former uses the convention that the activity of the solvent in a dilute solution (in this case, water) is 1, while the latter uses the value of the concentration of water in the pure liquid of 55.5 M. Silverstein has shown that the latter value is thermodynamically unsupportable.<ref>{{cite journal |last1=Silverstein |first1=T.P. |last2=Heller |first2=S.T.|title=pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water? |journal=J. Chem. Educ. |date=2017 |volume=94 |issue=6 |pages=690–695|doi=10.1021/acs.jchemed.6b00623 |bibcode=2017JChEd..94..690S}}</ref> The disagreement comes from the ambiguity that to define p''K''<sub>a</sub> of H<sub>3</sub>O<sup>+</sup> in water, H<sub>2</sub>O has to act simultaneously as a solute and the solvent. The IUPAC has not given an official definition of p''K''<sub>a</sub> that would resolve this ambiguity. Burgot has argued that H<sub>3</sub>O<sup>+</sup>(aq) + H<sub>2</sub>O (l) ⇄ H<sub>2</sub>O (aq) + H<sub>3</sub>O<sup>+</sup> (aq) is simply not a thermodynamically well-defined process. For an estimate of p''K''<sub>a</sub><sup>aq</sup>(H<sub>3</sub>O<sup>+</sup>), Burgot suggests taking the measured value p''K''<sub>a</sub><sup>EtOH</sup>(H<sub>3</sub>O<sup>+</sup>) = 0.3, the p''K''<sub>a</sub> of H<sub>3</sub>O<sup>+</sup> in ethanol, and applying the correlation equation p''K''<sub>a</sub><sup>aq</sup> = p''K''<sub>a</sub><sup>EtOH</sup> − 1.0 (± 0.3) to convert the ethanol p''K''<sub>a</sub> to an aqueous value, to give a value of p''K''<sub>a</sub><sup>aq</sup>(H<sub>3</sub>O<sup>+</sup>) = −0.7 (± 0.3).<ref>{{Cite journal|last=Burgot|first=Jean-Louis|date=1998|title=PerspectiveNew point of view on the meaning and on the values of Ka○(H<sub>3</sub>O<sup>+</sup>, H<sub>2</sub>O) and Kb○(H<sub>2</sub>O, OH<sup>−</sup>) pairs in water|url=http://xlink.rsc.org/?DOI=a705491b|journal=The Analyst|volume=123|issue=2|pages=409–410|doi=10.1039/a705491b|doi-access=free|bibcode=1998Ana...123..409B }}</ref> On the other hand, Silverstein has shown that Ballinger and Long's experimental results <ref name="Ballinger1960">{{cite journal |last1=Ballinger |first1=P. |last2=Long |first2=F.A. |title=Acid Ionization Constants of Alcohols. II. Acidities of Some Substituted Methanols and Related Compounds |journal=J. Am. Chem. Soc. |date=1960 |volume=82 |issue=4 |pages=795–798|doi=10.1021/ja01489a008}}</ref> support a p''K''<sub>a</sub> of 0.0 for the aqueous proton.<ref>{{cite journal |last1=Silverstein |first1=T.P. |title=The aqueous proton is hydrated by more than one water molecule: Is the hydronium ion a useful conceit? |journal=J. Chem. Educ. |date=2014 |volume=91 |issue=4 |pages=608–610|doi=10.1021/ed400559t |bibcode=2014JChEd..91..608S}}</ref> Neils and Schaertel provide added arguments for a p''K''<sub>a</sub> of 0.0 <ref>{{cite web| title = What is the pKa of Water| url = http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Fundamentals/What_is_the_pKa_of_water%3F| publisher = [[University of California, Davis]]| date = 2015-08-09| access-date = 2022-04-03| archive-date = 2016-02-14| archive-url = https://web.archive.org/web/20160214222524/http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Fundamentals/What_is_the_pKa_of_water%3F| url-status = dead}}</ref>

The aqueous proton is [[Leveling effect|the most acidic species that can exist in water]] (assuming sufficient water for dissolution): any stronger acid will ionize and yield a hydrated proton. The acidity of {{chem2|H+}}(aq) is the implicit standard used to judge the strength of an acid in water: [[strong acid]]s must be better proton donors than {{chem2|H+}}(aq), as otherwise a significant portion of acid will exist in a non-ionized state (i.e.: a weak acid). Unlike {{chem2|H+}}(aq) in neutral solutions that result from water's autodissociation, in acidic solutions, {{chem2|H+}}(aq) is long-lasting and concentrated, in proportion to the strength of the dissolved acid.

pH was originally conceived to be a measure of the [[hydrogen ion]] concentration of aqueous solution.<ref>{{cite journal |last1=Sorensen |first1=S. P. L. |date=1909 |title=Ueber die Messung und die Bedeutung der Wasserstoffionenkonzentration bei enzymatischen Prozessen |journal=[[Biochemische Zeitschrift]] |language=de |volume=21 |pages=131–304}}</ref> Virtually all such free protons are quickly hydrated; acidity of an aqueous solution is therefore more accurately characterized by its concentration of {{chem2|H+}}(aq). In organic syntheses, such as acid catalyzed reactions, the hydronium ion ({{H3O+}}) is used interchangeably with the {{chem2|H+}} ion; choosing one over the other has no significant effect on the mechanism of reaction.

==Solvation==
Researchers have yet to fully characterize the [[solvation]] of hydronium ion in water, in part because many different meanings of solvation exist. A [[freezing-point depression]] study determined that the mean hydration ion in cold water is approximately {{chem2|H3O+(H2O)6}}:<ref>{{cite journal |last1=Zavitsas |first1=A. A. |date=2001 |title=Properties of water solutions of electrolytes and nonelectrolytes |journal=[[The Journal of Physical Chemistry B]] |volume=105 |issue=32 |pages=7805–7815 |doi=10.1021/jp011053l}}</ref> on average, each hydronium ion is solvated by 6 water molecules which are unable to solvate other solute molecules.

Some hydration structures are quite large: the {{chem2|H3O+(H2O)20}} magic ion number structure (called ''[[Magic number (chemistry)|magic number]]'' because of its increased stability with respect to hydration structures involving a comparable number of water molecules – this is a similar usage of the term ''[[Magic number (physics)|magic number]]'' as in [[magic number (physics)|nuclear physics]]) might place the hydronium inside a [[dodecahedron|dodecahedral]] cage.<ref>{{cite journal |last1=Hulthe |first1=G. |last2=Stenhagen |first2=G. |last3=Wennerström |first3=O. |last4=Ottosson |first4=C-H. |date=1997 |title=Water cluster studied by electrospray mass spectrometry |doi=10.1016/S0021-9673(97)00486-X |journal=[[Journal of Chromatography A]] |volume=512 |pages=155–165}}</ref> However, more recent [[Ab initio quantum chemistry methods|ab initio method]] molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the {{chem2|H3O+(H2O)20}} cluster.<ref>{{cite journal |last1=Iyengar |first1=S. S. |last2=Petersen |first2=M. K. |last3=Burnham |first3=C. J. |last4=Day |first4=T. J. F. |last5=Voth |first5=G. A. |last6=Voth |first6=G. A. |date=2005 |title=The Properties of Ion-Water Clusters. I. The Protonated 21-Water Cluster |pmid=16164293 |url=http://www.indiana.edu/~ssiweb/papers/21-mer.pdf |doi=10.1063/1.2007628 |journal=[[The Journal of Chemical Physics]] |volume=123 |issue=8 |page=084309 |bibcode=2005JChPh.123h4309I}}</ref> Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

{{Anchor|Zundel cation}}
[[Image:Zundel-cation.JPG|thumb|Zundel cation]]
Two other well-known structures are the ''Zundel cation'' and the ''Eigen cation''. The Eigen solvation structure has the hydronium ion at the center of an {{chem2|H9O4(+)}} complex in which the hydronium is strongly [[hydrogen bond|hydrogen-bonded]] to three neighbouring water molecules.<ref>{{cite journal|last1=Zundel|first1=G. |last2= Metzger|first2=H. |date=1968|title= Energiebänder der tunnelnden Überschuß-Protonen in flüssigen Säuren. Eine IR-spektroskopische Untersuchung der Natur der Gruppierungen H502+ |journal=[[Zeitschrift für Physikalische Chemie]]|volume=58|issue= 5_6 |pages=225–245|doi=10.1524/zpch.1968.58.5_6.225|s2cid=101048854}}</ref> In the Zundel {{chem2|H5O2(+)}} complex the proton is shared equally by two water molecules in a [[symmetric hydrogen bond]].<ref>{{cite journal |last1=Wicke |first1=E. |last2=Eigen |first2=M. |last3=Ackermann |first3=Th |year=1954 |title=Über den Zustand des Protons (Hydroniumions) in wäßriger Lösung |journal=[[Zeitschrift für Physikalische Chemie]] |volume=1 |issue=5_6 |pages=340–364 |doi=10.1524/zpch.1954.1.5_6.340}}</ref> A work in 1999 indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect.<ref>{{cite journal |last1=Marx |first1=D. |last2=Tuckerman |first2=M. E. |last3=Hutter |first3=J. |last4=Parrinello |first4=M. |year=1999 |title=The nature of the hydrated excess proton in water |doi=10.1038/17579 |journal=[[Nature (journal)|Nature]] |volume=397 |issue=6720 |pages=601–604 |bibcode=1999Natur.397..601M|s2cid=204991299}}</ref>

Isolation of the hydronium ion monomer in liquid phase was achieved in a nonaqueous, low nucleophilicity [[superacid]] solution ({{chem2|HF(-)SbF5SO2}}). The ion was characterized by high resolution {{chem2|^{17}O}} [[nuclear magnetic resonance]].<ref>{{Cite journal | doi = 10.1021/ja00508a040| title = Water and related systems. 1. The hydronium ion (H<sub>3</sub>O<sup>+</sup>). Preparation and characterization by high resolution oxygen-17 nuclear magnetic resonance| journal = [[Journal of the American Chemical Society]]| volume = 101| issue = 14| pages = 3959–3960| year = 1979| last1 = Mateescu | first1 = G. D. | last2 = Benedikt | first2 = G. M.}}</ref>

A 2007 calculation of the [[enthalpy|enthalpies]] and [[Thermodynamic free energy|free energies]] of the various hydrogen bonds around the hydronium cation in liquid protonated water<ref>{{Cite journal |doi=10.1021/jp068960g |pmid=17388314 |url=http://vintage.fh.huji.ac.il/~agmon/Fullpaper/JPCA111-2253.pdf |title=Structure and Energetics of the Hydronium Hydration Shells |journal=[[The Journal of Physical Chemistry A]] |volume=111 |issue=12 |pages=2253–6 |year=2007 |last1=Markovitch |first1=O. |last2=Agmon |first2=N. |bibcode=2007JPCA..111.2253M |citeseerx=10.1.1.76.9448 |access-date=2018-08-30 |archive-date=2018-08-31 |archive-url=https://web.archive.org/web/20180831002422/http://vintage.fh.huji.ac.il/~agmon/Fullpaper/JPCA111-2253.pdf |url-status=dead }}</ref> at [[room temperature]] and a study of the [[proton hopping]] mechanism using [[molecular dynamics]] showed that the hydrogen-bonds around the hydronium ion (formed with the three water [[ligand]]s in the first [[solvation shell]] of the hydronium) are quite strong compared to those of bulk water.

A new model was proposed by Stoyanov based on [[infrared spectroscopy]] in which the proton exists as an {{chem2|H13O6+}} ion. The positive charge is thus delocalized over 6 water molecules.<ref>{{Cite journal | doi = 10.1021/ja9101826|pmc=2946644|pmid=20078058| title = The Structure of the Hydrogen Ion ({{chem|H|aq|+}}) in Water| journal = [[Journal of the American Chemical Society]]| volume = 132| issue = 5| pages = 1484–1485| date = January 15, 2010| last1 = Stoyanov | first1 = Evgenii S. | last2 = Stoyanova | first2 = Irina V. | last3 = Reed | first3 = Christopher A.}}</ref>

==Solid hydronium salts==
{{See also|Hydronium perchlorate}}
For many [[strong acids]], it is possible to form crystals of their hydronium salt that are relatively stable. These salts are sometimes called ''acid monohydrates''. As a rule, any acid with an [[ionization constant]] of {{10^|9}} or higher may do this. Acids whose ionization constants are below {{10^|9}} generally cannot form stable {{H3O+}} salts. For example, [[nitric acid]] has an ionization constant of {{10^|1.4}}, and mixtures with water at all proportions are liquid at room temperature. However, [[perchloric acid]] has an ionization constant of {{10^|10}}, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, they react to form solid [[hydronium perchlorate]] ({{chem2|H3O+*ClO4(-)}}).{{citation needed|date=November 2021}}

The hydronium ion also forms stable compounds with the [[carborane superacid]] {{chem2|H(CB11H(CH3)5Br6)}}.<ref>
{{Cite journal|doi=10.1021/ja0551335 | pmid=16464096| url = https://www.researchgate.net/publication/7310527| title = The Nature of the H<sub>3</sub>O<sup>+</sup> Hydronium Ion in Benzene and Chlorinated Hydrocarbon Solvents. Conditions of Existence and Reinterpretation of Infrared Data| journal = [[Journal of the American Chemical Society]]| volume = 128| issue = 6| pages = 1948–58| year = 2006| last1 = Stoyanov | first1 = Evgenii S. | last2 = Kim | first2 = Kee-Chan | last3 = Reed | first3 = Christopher A. | s2cid=33834275}}</ref> [[X-ray crystallography]] shows a {{chem2|C_{3v}|}} [[point group|symmetry]] for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 [[picometer|pm]] apart on average. The {{chem2|[H3O] [H(CB11HCl11)]}} salt is also soluble in [[benzene]]. In crystals grown from a benzene solution the solvent co-crystallizes and a {{chem2|H3O*(C6H6)3}} cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming [[Cation–π interaction|pi-cation interactions]] with the hydrogen atoms. The closest (non-bonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.

There are also many known examples of salts containing hydrated hydronium ions, such as the {{chem2|H5O2(+)}} ion in {{chem2|HCl*2H2O}}, the {{chem2|H7O3(+)}} and {{chem2|H9O4(+)}} ions both found in {{chem2|HBr*4H2O}}.<ref>{{Greenwood&Earnshaw}}</ref>

[[Sulfuric acid]] is also known to form a hydronium salt {{chem2|H3O(+)HSO4(-)}} at temperatures below {{convert|8.49|C|F}}.<ref>I. Taesler and I. Olavsson (1968). "Hydrogen bond studies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Crystallogr. B24, 299-304. https://doi.org/10.1107/S056774086800227X</ref>

==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>

==See also==
* [[Hydron (chemistry)|Hydron]] (hydrogen cation)
* [[Hydride]]
* [[Hydrogen anion]]
* [[Hydrogen ion]]
* [[Grotthus mechanism]]
* [[Trifluorooxonium]]
* [[Law of dilution]]

==References==
{{reflist|30em}}

==External links==
* [https://pubs.acs.org/doi/abs/10.1021/j100372a014?journalCode=jpchax J Phys Chem infrared spectra of hydronium]

[[Category:Acids]]
[[Category:Oxonium compounds]]
[[Category:Water chemistry]]