In chemistry, a hydronium ion is the cation H3O+, a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated
(bound to a solvent). The reality is far more complicated, and 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 H3O+ has been detected in the gas phase.
Determination of pH
It is the presence of hydronium ion relative to
hydroxide that determines a solution's
pH. Water molecules
auto-dissociate into hydronium and hydroxide ions in the following equilibrium:
- 2 H2O
OH− + H3O+
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, [H
3O
+] = 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.
Note that [H
3O
+]×[OH
−], the
ionic product of water, strongly increases with
temperature so [H
3O
+] is not equal to 10
−pH for temperatures other than 25 °C.
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,
H3O+ is
isoelectronic with
ammonia. As shown in the images above,
H3O+ has a trigonal pyramid geometry with the oxygen atom at its apex. The H-O-H bond angle is approximately 113°,
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 C
3v 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.
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. An
acidic
solute is generally the source of these hydrons; however, hydroniums
exist even in pure water. This special case of water reacting with water
to produce hydronium (and
hydroxide) 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 (assuming sufficient water for dissolution)(
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 (
H3O+) 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
H3O+(H2O)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
H3O+(H2O)20 magic ion number structure (called
magic
because of its increased stability with respect to hydration structures
involving a comparable number of water molecules) 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
H3O+(H2O)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
H9O+
4 complex in which the hydronium is strongly
hydrogen-bonded to three neighbouring water molecules.
In the Zundel
H5O+
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-SbF5SO2). 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 (formed with the three water
ligands 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
H13O+
6 ion. The positive charge is thus delocalized over six water molecules.
Motivation for study
Interstellar H3O+
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
H2 into
H+
2 by cosmic radiation.
H3O+ can produce either
OH− or
H2O through
dissociative recombination reactions, which occur very quickly even at the low (≥10 K) 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, H
2O does not have many favorable transitions for ground based observations.
Although observations of
HDO (the
deuterated version of water) could potentially be used for estimating H
2O abundances, the ratio of HDO to
H2O 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
H3O+ dissociative recombination reactions to provide what are believed to be relatively accurate
OH− and H
2O abundances without requiring direct observation of these species.
group) to the name
of the ring system left after the 
residue is replaced by two hydrogen atoms, preceded by a pair of
locants indicating the points of attachment of the carbonyl group and
the oxygen
atom of the lactone, respectively; the locant for the carbonyl group is
cited first, and, if there is a choice, is the lower locant. Multiplying
prefixes and pairs of locants separated by a colon
denote the precence of two or more lactone rings.
group after the name of the appropriate parent hydride preceded by a
pair of locants describing the points of attachment of the sulfonyl
group and oxygen atom, respectively; the
locant for the sulfonyl group is cited first and, if there is a choice,
is the lower locant. Multiplying prefixes and pairs of locants separated
by a colon are used to indicate two or more sultone
rings.
as part of a ring or ring system are called generically "lactams" and
their tautomers, 
, are "lactims". These compounds are named as heterocyclic compounds or in accordance with 
Tetrahydropyrrol-2-one
as a part of a ring are named as heterocycles or in accordance with
