Hydronium

           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, [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. Note that [H₃O⁺]×[OH⁻], the ionic product of water, strongly increases with temperature so [H₃O⁺] 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.

Zundel cation

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, H₂O 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 H₂O 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 H₂O abundances without requiring direct observation of these species.