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The trihydrogen cation, also known as protonated molecular hydrogen or H3+, 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 H3+ plays in the gas-phase chemistry of the ISM is unparalleled by any other molecular ion. The cation is also the simplest triatomic molecule, since 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== H3+ 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 H3+. Since C4+ would be very unlikely and the signal grew stronger in pure hydrogen gas, he correctly assigned the species as H3+. 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 H3+ increased linearly and the amount of H2+ 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 H3+ 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 H3+ is responsible for the formation of many observed molecular ions. It was not until 1980 that the first spectrum of H3+ 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 H3+. Emission lines were detected in the late 1980s and early 1990s in the ionospheres of Jupiter, Saturn, and Uranus. In 1996, H3+ was finally detected in the interstellar medium (ISM) by Geballe & Oka in two molecular interstellar clouds in the sightlines GL2136 and W33A. In 1998, H3+ was unexpectedly detected by McCall ''et al.'' in a diffuse interstellar cloud in the sightline Cygnus OB2#12. In 2006 Oka announced that H3+ was ubiquitous in interstellar medium, and that the Central Molecular Zone contained a million times the concentration of ISM generally.〔PNAS, 2006〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Trihydrogen cation」の詳細全文を読む スポンサード リンク
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