The hydronium cation, also known as hydroxonium is the positively charged polyatomic ion with
the chemical formula H 3O+. Hydronium, a type of oxonium ion, is
formed by the protonation of water (H 2O). This cation is often used to represent
the nature of the proton in aqueous solution, where the proton is highly solvated. The reality
is far more complicated, as a proton is bound to several molecules of water, such that other
descriptions such as H5O2+, H7O3+ and H9O4+ are increasingly accurate descriptions of
the environment of a proton in water. The ion H
3O+ has been detected in the gas phase.
Determination of pH It is the presence of hydronium ions relative
to hydroxide that determines a solution's pH. Water molecules auto-dissociate into hydronium
and hydroxide ions in the following equilibrium: 2 H
2O OH− + H 3O+
In pure water, there is an equal number of hydroxide and hydronium ions. At 25 °C and
atmospheric pressure their concentrations are approximately equal to 1.0×10−7 mol∙dm−3.
For these conditions, [H3O+] = 10−pH so pH = 7 is defined as neutral. A pH value
less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic
solution. Nomenclature
According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred
to as oxonium. Hydroxonium may also be used unambiguously to identify it. A draft IUPAC
proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry
contexts, respectively. An oxonium ion is any ion with a trivalent
oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium.
Structure Since O+ and N have the same number of electrons,
H 3O+ is isoelectronic with ammonia. As shown
in the images above, H 3O+ has a trigonal pyramid geometry with the
oxygen atom at its apex. The H-O-H bond angle is 113.6°, 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 H 3O+ molecule's symmetric top configuration
is such that it belongs to the C3v point group. Because of this symmetry and the fact that
it has a dipole moment, the rotational selection rules are ΔJ = ±1 and ΔK = 0. The 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.
Clusters including more than one water molecule have been detected. At high pressures, liquid
phase clusters can include up to 6 water molecules. Acids and acidity
Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons
do not exist in a free state: they are extremely reactive and are solvated by water. In general,
an acidic solute is the source of these hydrons; however, hydroniums exist even in pure water.
This special case of water reacting with water to produce hydronium ions is commonly known
as the self-ionization of water. The resulting hydronium ions are few and short-lived. pH
is a measure of the relative activity of hydronium and hydroxide ions in aqueous solutions. In
acidic solutions, hydronium is the more active, its excess proton being readily available
for reaction with basic species. Hydronium is very acidic: At 25 °C, its
pKa is −1.74. It is also the most acidic species that can exist in water(see leveling
effect): Any stronger acid will ionize and protonate a water molecule to form hydronium.
The acidity of hydronium is the implicit standard used to judge the strength of an acid in water:
strong acids must be better proton donors than hydronium, otherwise a significant portion
of acid will exist in a non-ionized state. Unlike hydronium in neutral solutions that
result from water's autodissociation, hydronium ions in acidic solutions are 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. We now know that virtually all such free protons quickly react with water to form
hydronium; acidity of an aqueous solution is therefore more accurately characterized
by its hydronium concentration. In organic syntheses, such as acid catalyzed reactions,
the hydronium ion (H 3O+) can be used interchangeably with the
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 H 3O+(H
2O) 6: 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 H
3O+(H 2O)
20 magic ion number structure might place the hydronium inside a dodecahedral cage.
However, more recent ab initio method molecular dynamics simulations have shown that, on average,
the hydrated proton resides on the surface of the H
3O+(H 2O)
20 cluster. Further, several disparate features of these simulations agree with their experimental
counterparts suggesting an alternative interpretation of the experimental results.
Two other well-known structures are the Zundel cations and Eigen cations. The Eigen solvation
structure has the hydronium ion at the center of an H
9O+ 4 complex in which the hydronium is strongly
hydrogen-bonded to three neighbouring water molecules. In the Zundel H
5O+ 2 complex the proton is shared equally by
two water molecules in a symmetric hydrogen bond. Recent work indicates that both of these
complexes represent ideal structures in a more general hydrogen bond network defect.
Isolation of the hydronium ion monomer in liquid phase was achieved in a nonaqueous,
low-nucleophilicity superacid solution (HF-SbF 5SO
2). The ion was characterized by high-resolution O-17 nuclear magnetic resonance.
A 2007 calculation of the enthalpies and free energies of the various hydrogen bonds around
the hydronium cation in liquid protonated water at room temperature and a study of the
proton-hopping mechanism using molecular dynamics showed that the hydrogen-bonds around the
hydronium ion 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 H 13O+
6 ion. The positive charge is thus delocalized over six water molecules.
Higher-solvation deuteronium ions have been detected by mass spectrometry.
Solid hydronium salts For many strong acids, it is possible to form
crystals of their hydronium salt that are relatively stable. Sometimes these salts are
called acid monohydrates. As a rule, any acid with an ionization constant of 109 or higher
may do this. In general, acids whose ionization constant is below 109 cannot form stable H
3O+ salts. For example, hydrochloric acid has an ionization constant of 107, and mixtures
with water at all proportions are liquid at room temperature. However, perchloric acid
has an ionization constant of 1010, and, if liquid anhydrous perchloric acid and water
are combined in a 1:1 molar ratio, solid hydronium perchlorate forms.
The hydronium ion also forms stable compounds with the carborane superacid H(CB
11H(CH 3)
5B 6). X-ray crystallography shows a C3v symmetry
for the hydronium ion, with each proton interacting with a boron atom each from three carborane
anions 320 pm apart on average. The [H 3O][H(CB
11HCl)] 11 salt is also soluble in benzene. In crystals
grown from a benzene solution, the solvent co-crystallizes and a H
3O·(benzene)3 cation is completely separated from the anion. In the cation, three benzene
molecules surround hydronium, forming pi-cation interactions with the hydrogen atoms. The
closest approach of the anion at chlorine to the cation at oxygen is 348 pm.
There are also many examples of hydrated hydronium ions known, such as the H
5O+ 2 ion in HCl·2H
2O, the H 7O+
3 and H 9O+
4 ions both found in HBr·4H 2O.
Atmospheric H 3O+
The hydronium ion has been found in higher atmospheric layers.
Interstellar H 3O+
Motivation for study Hydronium is an abundant molecular ion in
the interstellar medium and is found in diffuse and dense molecular clouds as well as the
plasma tails of comets. 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
H 2 into H+
2 by cosmic radiation. H 3O+ can produce either OH− or H
2O through dissociative recombination reactions, which occur very quickly even at the low temperatures
of dense clouds. 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. However, H2O does not have many
favorable transitions for ground based observations. Although observations of HDO could potentially
be used for estimating H2O abundances, the ratio of HDO to H
2O is not known very accurately. Hydronium, on the other hand, has several
transitions that make it a superior candidate for detection and identification in a variety
of situations. This information has been used in conjunction with laboratory measurements
of the branching ratios of the various H 3O+ dissociative recombination reactions to
provide what are believed to be relatively accurate OH− and H2O abundances without
requiring direct observation of these species. Interstellar chemistry
H 3O+ is found in both diffuse and dense molecular
clouds. By applying the reaction rate constants corresponding to all of the currently available
characterized reactions involving H 3O+, 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 for each reaction at a given temperature can be determined. These
relative rates can be made in absolute rates by multiplying them by the [H
2] 2. By assuming T = 10 K for a dense cloud
and T = 50 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 T = 10 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 H 3O+'s chemical interactions under these conditions.
More about these reactions were observed and recorded regarding Hale-Bopp data. Finally,
it should also be noted that all three destruction mechanisms in the table below are classified
as dissociative recombination reactions. 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 k=α\alpha$, which is independent of temperature.
Since all three of these reactions produce either H2O 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 H3O+'s chemical interactions
under these conditions. Astronomical detections
As early as 1973 and before the first interstellar detection, chemical models of the interstellar
medium predicted that hydronium was an abundant molecular ion and that it played an important
role in ion-neutral chemistry. 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, which was followed by other, higher-resolution spectroscopy experiments.
Once several lines had been identified in the laboratory, the first interstellar detection
of H3O+ was made by two groups almost simultaneously in 1986. The first, published in June 1986,
reported observation of the JKvt = 11− – 21+ transition at 307192.41 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 32+ – 22− transition at 364797.427 MHz was observed in OMC-1 and Sgr B2. One year
later, the 30+ – 20− transition at 396272.412 MHz was observed in several regions, the clearest
of which was the W3 IRS 5 cloud. The first far-IR 43− – 33+ transition
at 69.524 µm was made in 1996 near Orion BN-IRc2. In 2001, three additional transitions
of H3O+ in were observed in the far infrared in Sgr B2; 21− – 11+ transition at 100.577 µm,
11− – 11+ at 181.054 µm and 20− – 10+ at 100.869 µm.
See also Hydron
Hydride Hydrogen anion
Proton affinity Standard hydrogen electrode
Ocean acidification References
External links ChemSpider = 109935
Journal of Chemical Physics