The trihydrogen cation or protonated molecular hydrogen (IUPAC name: hydrogenonium ion) is a cation (positive ion) with formula H+3, consisting of three hydrogen nuclei (protons) sharing two electrons. The trihydrogen cation is one of the most abundant ions in the universe. It is stable in the interstellar medium (ISM) due to the low temperature and low density of interstellar space. The role that H+3 plays in the gas-phase chemistry of the ISM is unparalleled by any other polyatomic ion. The trihydrogen cation is the simplest triatomic molecule, because its two electrons are the only valence electrons in the system. It is also the simplest example of a three-center two-electron bond system.
History H+3 was first discovered by J. J. Thomson in 1911. While using an early form of mass spectrometry to study the resultant species of plasma discharges, he discovered a large abundance of a polyatomic ion with a mass-to-charge ratio of 3. He stated that the only two possibilities were C4+ or H+3. Since the signal grew stronger in pure hydrogen gas, he correctly assigned the species as H+3. The formation pathway was discovered by Hogness & Lunn in 1925. They also used an early form of mass spectrometry to study hydrogen discharges. They found that as the pressure of hydrogen increased, the amount of H+3 increased linearly and the amount of H+2 decreased linearly. In addition, there was little H+ at any pressure. These data suggested the proton exchange formation pathway discussed below. In 1961, Martin et al. first suggested that H+3 may be present in interstellar space given the large amount of hydrogen in interstellar space and its reaction pathway was exothermic (~1.5 eV). This led to the suggestion of Watson and Herbst & Klemperer in 1973 that H+3 is responsible for the formation of many observed molecular ions. It was not until 1980 that the first spectrum of H+3 was discovered by Takeshi Oka, which was of the ν2 fundamental band (see #Spectroscopy) using a technique called frequency modulation detection. This started the search for extraterrestrial H+3. Emission lines were detected in the late 1980s and early 1990s in the ionospheres of Jupiter, Saturn, and Uranus. In the textbook by Bunker and Jensen Figure 1.1 reproduces part of the ν2 emission band from a region of auroral activity in the upper atmosphere of Jupiter,
and its Table 12.3 lists the transition wavenumbers of the lines in the band observed by Oka with their assignments. In 1996, H+3 was finally detected in the interstellar medium (ISM) by Geballe & Oka in two molecular interstellar clouds in the sightlines GL2136 and W33A. In 1998, H+3 was unexpectedly detected by McCall et al. in a diffuse interstellar cloud in the sightline Cygnus OB2#12. In 2006 Oka announced that H+3 was ubiquitous in interstellar medium, and that the Central Molecular Zone contained a million times the concentration of ISM generally.
Structure
The three hydrogen atoms in the molecule form an equilateral triangle, with a bond length of 0.90 Å on each side. The bonding among the atoms is a three-center two-electron bond, a delocalized resonance hybrid type of structure. The strength of the bond has been calculated to be around 4.5 eV (104 kcal/mol).
Isotopologues In theory, the cation has 10 isotopologues, resulting from the replacement of one or more protons by nuclei of the other hydrogen isotopes; namely, deuterium nuclei (deuterons, 2H+) or tritium nuclei (tritons, 3H+). Some of them have been detected in interstellar clouds. They differ in the atomic mass number A and the number of neutrons N:
H+3 = 1H+3 (A=3, N=0) (the common one). [DH2]+ = [2H1H2]+ (A=4, N=1) (deuterium dihydrogen cation). [D2H]+ = [2H21H]+ (A=5, N=2) (dideuterium hydrogen cation). D+3 = 2H+3 (A=6, N=3) (trideuterium cation). [TH2]+ = [3H1H2]+ (A=5, N=2) (tritium dihydrogen cation). [TDH]+ = [3H2H1H]+ (A=6, N=3) (tritium deuterium hydrogen cation). [TD2]+ = [3H2H2]+ (A=7, N=4) (tritium dideuterium cation). [T2H]+ = [3H21H]+ (A=7, N=4) (ditritium hydrogen cation). [T2D]+ = [3H22H]+ (A=8, N=5) (ditritium deuterium cation). T+3 = 3H+3 (A=9, N=6) (tritritium cation). The deuterium isotopologues have been implicated in the fractionation of deuterium in dense interstellar cloud cores.
Formation The main pathway for the production of H+3 is by the reaction of H+2 and H2.
H+2 + H2 → H+3 + H• The concentration of H+2 is what limits the rate of this reaction in nature - the only known natural source of it is via ionization of H2 by a cosmic ray in interstellar space:
H2 + cosmic ray → H+2 + e− + cosmic ray The cosmic ray has so much energy, it is almost unaffected by the relatively small energy transferred to the hydrogen when ionizing an H2 molecule. In interstellar clouds, cosmic rays leave behind a trail of H+2, and therefore H+3. In laboratories, H+3 is produced by the same mechanism in plasma discharge cells, with the discharge potential providing the energy to ionize the H2.
Destruction There are many destruction reactions for H+3. The dominant destruction pathway in dense interstellar clouds is by proton transfer with a neutral collision partner. The most likely candidate for a destructive collision partner is the second most abundant molecule in space, CO.
H+3 + CO → HCO+ + H2 The significant product of this reaction is HCO+, an important molecule for interstellar chemistry. Its strong dipole and high abundance make it easily detectable by radio astronomy. H+3 can also react with atomic oxygen to form OH+ and H2.
H+3 + O → OH+ + H2 OH+ then usually reacts with more H2 to create further hydrogenated molecules.
OH+ + H2 → OH+2 + H OH+2 + H2 → OH+3 + H At this point, the reaction between OH+3 and H2 is no longer exothermic in interstellar clouds. The most common destruction pathway for OH+3 is dissociative recombination, yielding four possible sets of products: H2O + H, OH + H2, OH + 2H, and O + H2 + H. While water is a possible product of this reaction, it is not a very efficient product. Different experiments have suggested that water is created anywhere from 5–33% of the time. Water formation on grains is still considered the primary source of water in the interstellar medium. The most common destruction pathway of H+3 in diffuse interstellar clouds is dissociative recombination. This reaction has multiple products. The major product is dissociation into three hydrogen atoms, which occurs roughly 75% of the time. The minor product is H2 and H, which occurs roughly 25% of the time.
… excerpt ends here. Continue reading the full article.
