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In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, propertie ...
, hydronium (hydroxonium in traditional British English) is the common name for the
aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be r ...
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
, the type of
oxonium ion In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion (). Alkyloxonium Hydronium is one of a series of oxonium ions with the formula R''n''H3 ...
produced by
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, i ...
of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
s in solution give up a
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
(a positive
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...
ion, ) to the surrounding water molecules (). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous and conjugate base. Three main structures for the aqueous proton have garnered experimental support: The Eigen cation, which is a tetrahydrate, H3O+(H2O)3; the Zundel cation, which is a symmetric dihydrate, H+(H2O)2; and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.


Relation to pH

The
molar concentration Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of solu ...
of hydronium or ions determines a solution's pH according to \ce = -\log( ce\ce M) where M = mol/L. The concentration of
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
ions analogously determines a solution's pOH. The molecules in pure water auto-dissociate into aqueous protons and hydroxide ions in the following equilibrium: : In pure water, there is an equal number of hydroxide and ions, so it is a neutral solution. At , pure water has a pH of 7 and a pOH of 7 (this varies when the temperature changes: see
self-ionization of water The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen ...
). 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 In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the '' Nomenclature of O ...
, the hydronium ion should be referred to as ''oxonium''. ''Hydroxonium'' may also be used unambiguously to identify it. An
oxonium ion In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion (). Alkyloxonium Hydronium is one of a series of oxonium ions with the formula R''n''H3 ...
is any ion with a trivalent oxygen cation. For example, a protonated
hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
is an oxonium ion, but not a hydronium ion.


Structure

Since and N have the same number of electrons, is
isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in the ...
with
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous w ...
. As shown in the images above, has a
trigonal pyramidal molecular geometry In chemistry, a trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base, resembling a tetrahedron (not to be confused with the tetrahedral geometry). When all three atoms at the cor ...
with the oxygen atom at its apex. The
bond angle Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemical ...
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 molecule's symmetric top configuration is such that it belongs to the
point group In geometry, a point group is a mathematical group of symmetry operations ( isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and ever ...
. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are Δ''J'' = ±1 and Δ''K'' = 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

The hydrated proton is very acidic: at 25 °C, its p''K''a is approximately 0. The values commonly given for p''K''aaq(H3O+) 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. The disagreement comes from the ambiguity that to define p''K''a of H3O+ in water, H2O has to act simultaneously as a solute and the solvent. The IUPAC has not given an official definition of p''K''a that would resolve this ambiguity. Burgot has argued that H3O+(aq) + H2O (l) ⇄ H2O (aq) + H3O+ (aq) is simply not a thermodynamically well-defined process. For an estimate of p''K''aaq(H3O+), Burgot suggests taking the measured value p''K''aEtOH(H3O+) = 0.3, the p''K''a of H3O+ in ethanol, and applying the correlation equation p''K''aaq = p''K''aEtOH – 1.0 (± 0.3) to convert the ethanol p''K''a to an aqueous value, to give a value of p''K''aaq(H3O+) = –0.7 (± 0.3). On the other hand, Silverstein has shown that Ballinger and Long's experimental results support a pKa of 0.0 for the aqueous proton. Neils and Schaertel provide added arguments for a pKa of 0.0 The aqueous proton is 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 (aq) is the implicit standard used to judge the strength of an acid in water:
strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions. ...
s must be better proton donors than (aq), as otherwise a significant portion of acid will exist in a non-ionized state (i.e.: a weak acid). Unlike (aq) in neutral solutions that result from water's autodissociation, in acidic solutions, (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 A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
concentration of aqueous solution. Virtually all such free protons are quickly hydrated; acidity of an aqueous solution is therefore more accurately characterized by its concentration of (aq). In organic syntheses, such as acid catalyzed reactions, the hydronium ion () is used interchangeably with the ion; choosing one over the other has no significant effect on the mechanism of reaction.


Solvation

Researchers have yet to fully characterize the
solvation Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the ...
of hydronium ion in water, in part because many different meanings of solvation exist. A
freezing-point depression Freezing-point depression is a drop in the minimum temperature at which a substance freezes, caused when a smaller amount of another, non- volatile substance is added. Examples include adding salt into water (used in ice cream makers and for ...
study determined that the mean hydration ion in cold water is approximately : 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 magic ion number structure (called '' 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'' as in
nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies t ...
) might place the hydronium inside a
dodecahedral In geometry, a dodecahedron (Greek , from ''dōdeka'' "twelve" + ''hédra'' "base", "seat" or "face") or duodecahedron is any polyhedron with twelve flat faces. The most familiar dodecahedron is the regular dodecahedron with regular pentagon ...
cage. However, more recent ab initio method molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the 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 cation'' and the ''Eigen cation''. The Eigen solvation structure has the hydronium ion at the center of an complex in which the hydronium is strongly hydrogen-bonded to three neighbouring water molecules. In the Zundel 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 In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid ...
solution (). The ion was characterized by high resolution
nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
. 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 Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of the ...
showed that the hydrogen-bonds around the hydronium ion (formed with the three water
ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...
s in the first
solvation shell A solvation shell or solvation sheath is the solvent interface of any chemical compound or biomolecule that constitutes the solute. When the solvent is water it is called a hydration shell or hydration sphere. The number of solvent molecules s ...
of the hydronium) are quite strong compared to those of bulk water. A new model was proposed by Stoyanov based on
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...
in which the proton exists as an ion. The positive charge is thus delocalized over 6 water molecules.


Solid hydronium salts

For many
strong acids Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
, 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 or higher may do this. Acids whose ionization constants are below generally cannot form stable salts. For example,
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
has an ionization constant of , and mixtures with water at all proportions are liquid at room temperature. However,
perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous so ...
has an ionization constant of , and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, they react to form solid hydronium perchlorate (). The hydronium ion also forms stable compounds with the carborane superacid .
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
shows a
symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The salt is also soluble in
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen a ...
. In crystals grown from a benzene solution the solvent co-crystallizes and a 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 (non-bonding) 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 ion in , the and ions both found in .


Interstellar H3O+

Hydronium is an abundant
molecular ion Mass spectral interpretation is the method employed to identify the chemical formula, characteristic fragment patterns and possible fragment ions from the mass spectra. Mass spectra is a plot of relative abundance against mass-to-charge ratio. It i ...
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 into by cosmic radiation. can produce either or 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, 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 abundances, the ratio of HDO to 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 dissociative recombination reactions to provide what are believed to be relatively accurate and abundances without requiring direct observation of these species.


Interstellar chemistry

As mentioned previously, 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 , 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 cm3/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 . By assuming for a dense cloud and 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 . 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. 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 which is independent of temperature. Since all three of these reactions produce either or OH, these results reinforce the strong connection between their relative abundances and that of . 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. 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 ''J'' = 1 − 2 transition at 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 transitions. The first observations of each subsequent transition detection are given below in chronological order: In 1991, the 3 − 2 transition at was observed in OMC-1 and Sgr B2. One year later, the 3 − 2 transition at was observed in several regions, the clearest of which was the W3 IRS 5 cloud. The first far-IR 4 − 3 transition at 69.524 µm (4.3121 THz) was made in 1996 near Orion BN-IRc2. In 2001, three additional transitions of in were observed in the far infrared in Sgr B2; 2 − 1 transition at 100.577 µm (2.98073 THz), 1 − 1 at 181.054 µm (1.65582 THz) and 2 − 1 at 100.869 µm (2.9721 THz).


See also

*
Hydron Hydron has the following meanings: * Hydron (chemistry) In chemistry, the hydron, informally called proton, is the cationic form of atomic hydrogen, represented with the symbol . The general term "hydron", endorsed by the IUPAC, encompasses c ...
(hydrogen cation) *
Hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...
*
Hydrogen anion The hydrogen anion, H−, is a negative ion of hydrogen, that is, a hydrogen atom that has captured an extra electron. The hydrogen anion is an important constituent of the atmosphere of stars, such as the Sun. In chemistry, this ion is called ...
*
Hydrogen ion A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
* Grotthus mechanism * Trifluorooxonium * Law of dilution


References

{{reflist, 30em


External links


ChemSpider = 109935J Phys Chem infrared spectra of hydronium
Acids Oxonium compounds Water chemistry de:Oxonium