Editing Hydronium (section)

==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>