The trihydrogen cation or protonated
molecular hydrogen is a cation (positive ion) with formula H+, 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 molecular 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.
H+ was first discovered by J. J. Thomson in 1911. While studying the resultant species of plasma discharges, he discovered something very odd. Using an early form of mass spectrometry, he discovered a large abundance of a molecular ion with a mass-to-charge ratio of 3. He stated that the only two possibilities were C4+ or H+. Since C4+ would be very unlikely and the signal grew stronger in pure hydrogen gas, he correctly assigned the species as H+.
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+ increased linearly and the amount of H+ decreased linearly. In addition, there was little H+ at any pressure. This 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+ is responsible for the formation of many observed molecular ions.
It was not until 1980 that the first spectrum of H+ was discovered by Takeshi Oka, which was of the ν2 fundamental band using a technique called frequency modulation detection. This started the search for interstellar H+ Emission lines were detected in the late 1980s and early 1990s in the ionospheres of Jupiter, Saturn, and Uranus.
In 1996, H+ 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 2006Oka
announced that H+ was ubiquitous in interstellar
medium, and that the Central Molecular Zone contained a million times the
concentration of ISM generally.
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 molecular 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.
Scientific History
H+ was first discovered by J. J. Thomson in 1911. While studying the resultant species of plasma discharges, he discovered something very odd. Using an early form of mass spectrometry, he discovered a large abundance of a molecular ion with a mass-to-charge ratio of 3. He stated that the only two possibilities were C4+ or H+. Since C4+ would be very unlikely and the signal grew stronger in pure hydrogen gas, he correctly assigned the species as H+.
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+ increased linearly and the amount of H+ decreased linearly. In addition, there was little H+ at any pressure. This 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+ is responsible for the formation of many observed molecular ions.
It was not until 1980 that the first spectrum of H+ was discovered by Takeshi Oka, which was of the ν2 fundamental band using a technique called frequency modulation detection. This started the search for interstellar H+ Emission lines were detected in the late 1980s and early 1990s in the ionospheres of Jupiter, Saturn, and Uranus.
In 1996, H+ 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
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).
Astronomical Detection
H+ has been
detected in two types of celestial environments: Jovian planets and
interstellar clouds. In Jovian planets, it has been detected in the planet's
ionospheres, the region where the Sun's high energy radiation ionizes the
particles in the atmosphere. Since there is a high level of H2 in these atmospheres, this radiation can
produce a significant amount of H+. Also, with a
broadband source like the Sun, there is plenty of radiation to pump the H+ to higher energy states from which it can relax by
stimulated and spontaneous emission.
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