Trihydrogen cation

The trihydrogen cation or protonated molecular hydrogen is a cation (positive ion) with formula H3+, 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 H3+ 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. 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. These 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 (see #Spectroscopy) using a technique called frequency modulation detection.