Hydronium

There is no denying the fact that  hydronium is the common phenomena for the aqueous cation H₃O⁺ , the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H⁺) to the surrounding water molecules (H₂O).

Determination of pH

It is the amount of hydronium ions relative to hydroxide ions that determines a solution's pH. The molecules in pure water auto-dissociate (i.e.: react with each other) into hydronium and hydroxide ions in the following equilibrium:

2H2O ⇌ OH− + H3O, In pure water, there is an equal number of hydroxide and hydronium ions, so it is a neutral solution. At 25 °C, water has a pH 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.

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 ion.

Structure

Since O+ and N have the same number of electrons, H3O+  is microelectronic with ammonia. As shown in the images above, H₃O+  has a trigonometrical pyramidal molecular 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 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.

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.

The hydronium ion is very acidic: at 25 °C, its pKa is −1.7. It is the most acidic species that can exist in water (assuming sufficient water for dissolution): 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 (i.e.: a weak acid). Unlike hydronium in neutral solutions that result from water's auto dissociation, 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 salvation 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:[4] 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 – this is a similar usage of the word magic as in nuclear physics) might place the hydronium inside a tetrahedral 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.[6] Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

Zundelcation

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.[7] 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 non aqueous, low nucleophilicity super acid solution (HF-SbF­₅SO₂). The ion was characterized by high resolution 17 O 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[11] 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 6 water molecules.

Solid hydronium salts

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 109 or higher may do this. Acids whose ionization constant is below 109 generally cannot form stable H₃O+  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, they react to form solid hydronium perchlorate (H₃O⁺ •ClO₄).

The hydronium ion also forms stable compounds with the carborane superacid H(CB
11H(CH₃) 5Br6).[13] X-ray crystallography shows a C3v symmetry for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The [H3O][H(CB11HCl
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 (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 H
5O+2 ion in HCl·2H2O, the H₇O₃ and H9O+4 ions both found in HBr·4H2O.[14]

Interstellar H3O+

Motivation for study

Hydronium is an abundant molecular ion in the interstellar medium and is found in diffuse[15] and dense[16] molecular clouds as well as the plasma tails of comets.[17] 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.[18] H
3O+
 can produce either OH−
 or H
2O through dissociative recombination reactions, which occur very quickly even at the low (≥10 K) temperatures of dense clouds.[19] 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.[20] However, H2O does not have many favorable transitions for ground based observations.[21] Although observations of H_DO (the deuterated version of water[22]) 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.[21] This information has been used in conjunction with laboratory measurements of the branching ratios of the various H
3O+  dissociation recombination reactions[19] to provide what are believed to be relatively accurate OH−  and H2O abundances without requiring direct observation of these species.[23][24]

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 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 hydronium ions. 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 hydroponic ion's chemical interactions under these conditions. Finally, it should also be noted that all three destruction mechanisms in the table below are classified as dissociation recombination reactions.

Primary reaction pathways of H3O+  in the interstellar medium (specifically, dense clouds).orth 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 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 hydronium ion's chemical interactions under these conditions.