Protonated hydrogen

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Protonated hydrogen or the trihydrogenium cation H 3 + is an ionized molecule . It consists of three hydrogen nuclei and two electrons . The cation is the simplest triatomic molecule that has only two valence electrons . It is only stable at low temperatures and low pressures. The two electrons are equally bound to the three nuclei, making it the simplest example of a two-electron, three-center bond .

Occurrence

Protonated hydrogen is one of the most common ions in interstellar space ; in the interstellar medium it is stable because of the low temperatures and low density. In the gas phase chemistry of the interstellar medium, H 3 + plays a prominent role.

history

H 3 + was discovered by JJ Thomson in 1911. While studying plasmas with an early form of mass spectrometer , he was able to detect a larger number of particles with a mass-to-charge ratio of 3. In his opinion it should have been either H 3 + or C 4+ . Since C 4+ would be very unusual and since the signal was more pronounced in pure hydrogen, he settled on H 3 + .

In 1925, TR Hogness and EG Lunn discovered different reaction pathways for the formation of H 3 + . They too used a mass spectrometer to study hydrogen discharges. They found that the amount of H 3 + increased linearly with hydrogen pressure and the amount of H 2 + decreased. In addition, some H 3 + was found at each hydrogen pressure.

In 1961, DW Martin et al. suggest that H 3 + could be present in the interstellar medium. They justified this with the large amount of hydrogen present and with the fact that the reverse reaction path is exothermic (~ 1.5  eV ). This led Watson, Herbst and Klemperer in 1973 to the conjecture that H 3 + is responsible for the formation of many observed molecular ions.

The first spectrum of H 3 + was not recorded by Takeshi Oka until 1980 - more precisely: The ν 2 band was recorded using frequency modulation detection technology. Emission lines of the H 3 + were discovered in the late 1980s and 1990s in the ionosphere of the planets Jupiter , Saturn and Uranus .

In 1996, H 3 + was finally detected by Geballe and Oka in the interstellar medium in two gas clouds looking towards GL2136 and W33A, respectively. In 1998 H was 3 + also of McCall et al. discovered in a diffuse interstellar cloud looking towards Cyg OB2 # 12 . In 2006 Oka published that H 3 + can be found everywhere in interstellar space and that the central molecular zone ( CMZ ) contains a million times the concentration of H 3 + as the other interstellar medium.

structure

The structure of H 3 +
MO diagram of H 3 + .

The arrangement of the hydrogen nuclei corresponds to an equilateral triangle. The resonance structure of the molecule is a two-electron, three-center bond. The strength of the bond was calculated to be 4.5 eV. This molecule is a good example of how delocalization adds to the stability of a molecule.

education

The main pathway in the generation of H 3 + is the reaction of the dihydrogen cation H 2 + with H 2 .

H 2 + + H 2 → H 3 + + H

The H 3 + concentration is the rate-determining factor in this reaction. H 2 + is naturally generated in interstellar space by the ionization of H 2 under exposure to cosmic rays . The photons of cosmic rays have so much energy that only a fraction is needed to ionize an H 2 molecule. In interstellar clouds, cosmic rays leave a trace of H 2 + and therefore also of H 3 + . In the laboratory, H 3 + is generated in plasma discharge cells with a voltage at least equal to the ionization potential of H 2 by the same mechanism.

Decay

There are several decomposition reactions for H 3 + . The dominant path of decay in interstellar clouds is proton transfer through a neutral collision partner. The molecule in question is carbon monoxide (CO), the second most common molecule in space.

H 3 + + CO → HCO + + H 2

The main product of this reaction is HCO + , an important molecule in astrochemistry. Due to its great polarity and its great frequency, it can easily be detected using radio astronomy.

H 3 + can also react with atomic oxygen to form OH + and H 2 :

H 3 + + O → OH + + H 2

OH + then normally reacts with H 2 and consequently forms hydrogenated molecules.

OH + + H 2 → OH 2 + + H
OH 2 + + H 2 → OH 3 + + H

The possible subsequent reactions of OH 3 + and H 2 in interstellar space are not exothermic. The most common decomposition pathway of OH 3 + is dissociative recombination , which leads to four possible combinations of reaction products: H 2 O + H, OH + H 2 , OH + 2H, and O + H 2 + H. Although water is a possible one Reaction product, it is not very common.

H 3 + decays in gas clouds with 75% probability into three hydrogen atoms and only 25% probability into atomic and molecular hydrogen.

Ortho / Para-H 3 +

Collision between ortho-H 3 + and para-H 2 .

The most common molecule in dense interstellar clouds is H 2 . If an H 3 + molecule collides with H 2 , there is no stoichiometric yield. However, proton transfer can take place which changes the spin of the two molecules, depending on the nuclear spin of the proton. Two different spin configurations are possible, ortho and para. Ortho-H 3 + has three protons with parallel spins, resulting in a total spin of 3/2. Para-H 3 + has two protons with a parallel spin and one proton with an anti-parallel spin, so that the total spin is 1/2. Similarly, H 2 also has two spin states, ortho and para, where ortho-H 2 has a total nuclear spin of 1 and para-H 2 has a total nuclear spin of 0. When ortho-H 3 + and para-H 2 collide, the transferred proton changes the overall nuclear spin of the molecule, so that one then obtains a para-H 3 + and an ortho-H 2 .

Spectroscopy

The spectroscopy of H 3 + is a challenge. The pure rotation spectrum is pretty weak. Ultraviolet light is too energetic and would split the molecule. H 3 + can be seen in the rotational vibration spectrum (IR) . The oscillation ν 2 of H 3 + can be seen as an asymmetrical band, since the molecule has a weak dipole moment. Since Oka's spectrum was recorded, over 900 absorption lines in the infrared range have been measured. H 3 + emission lines have been measured while observing the atmosphere of the planet Jupiter. The H 3 + emission lines are the lines that could not be assigned when measuring molecular hydrogen.

Astronomical detection

H 3 + was discovered in two types of celestial bodies: at Jupiter's moons and in interstellar clouds. It was discovered in Jupiter's moons in the planet's ionosphere, in the region where high-energy solar radiation ionizes particles in the atmosphere. Since there is a high proportion of hydrogen in this part of the atmosphere , solar radiation can produce a considerable amount of H 3 + there . This means: in a broadband radiation source like the sun, a lot of H 3 + can be raised to higher energy levels and then relax through spontaneous or stimulated emissions.

Planetary atmospheres

The detection of H 3 + was first published by Drossart in 1989, who discovered the ion in the ionosphere of Jupiter. He found a total of 23 H 3 + lines with a density of 1.39 × 10 9 / cm 2 . With the help of these lines he was able to determine the temperature of H 3 + at about 1100 ° C, which is comparable to the temperatures that were determined for the emission lines of H 2 . In 1993, H 2 was discovered by Gaballe near Saturn and by Trafton on the planet Pluto.

Molecular interstellar clouds

H 3 + was only detected in interstellar space in 1996, when Geballe & Oka demonstrated the detection of H 3 + in two gas clouds looking towards GL2136 and W33A. Both sources had temperatures of H 3 + of about 35 K (-238 ° C) and a density of about 10 14 / cm 2 . Since then, H 3 + has been detected in numerous other cloud lines of sight , such as AFGL 2136, Mon R2 IRS 3, GCS 3-2, GC IRS 3, and in LkHα 101.

Diffuse interstellar clouds

Surprisingly, three H 3 + lines were found by McCall in 1998 in a diffuse cloud line of sight from Cyg OB2 No. 12 discovered. Prior to 1998, the density of H 2 was probably too low to produce a detectable amount of H 3 + . McCall detected a temperature of approximately 27 K (-246 ° C) and a column density of ~ 10 14 / cm 2 , the same density that Geballe & Oka discovered. Since then, H 3 + has been detected in many other diffuse cloud lines of sight, such as GCS 3-2, GC IRS 3, and ζ Persei.

Steady state model predictions

To determine the path length of H 3 + in these clouds, Oka used a steady-state model to determine the density and the proportion of H 3 + in diffuse and dense clouds. Diffuse and dense clouds have the same formation mechanism for H 3 + but different decay mechanisms. In dense clouds, the protein transfer to CO is the dominant pathway for degradation; this corresponds to the predicted density of 10 −4 cm −3 in dense clouds.

n (H 3 + ) = ( ζ / k CO ) [ n (H 2 ) / n (CO)] ≈ 10 −4 / cm 3
n (H 3 + ) = ( ζ / k e ) [ n (H 2 ) / n (C + )] ≈ 10 −6 / cm 3

In diffuse clouds, the dominant mechanism of decay is dissociative recombination. This agrees with the density of 10 −6 / cm 3 in diffuse clouds. Since the density for diffuse and dense clouds are roughly estimated to be of the same order of magnitude, diffuse clouds must be 100 times the path length of the dense clouds. Therefore one can determine the relative density of these clouds with an H 3 + sample.

Individual evidence

  1. ^ Thomson, JJ: Rays of Positive Electricity . In: Proceedings of the Royal Society A . 89, No. 607, 1913, pp. 1-20. bibcode : 1913RSPSA..89 .... 1T . doi : 10.1098 / rspa.1913.0057 .
  2. ^ Hogness, TR, Lunn, EG: The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis . In: Physical Review volume = 26 . 1925, pp. 44-55. doi : 10.1103 / PhysRev.26.44 .
  3. ↑ Martin, DW, McDaniel, EW, Meeks, ML: On the Possible Occurrence of H 3 + in Interstellar Space . In: Astrophysical Journal . 134, 1961, p. 1012. doi : 10.1086 / 147232 .
  4. ^ Watson, WD: The Rate of Formation of Interstellar Molecules by Ion-Molecule Reactions . In: Astrophysical Journal . 183, 1973, p. L17. doi : 10.1086 / 181242 .
  5. ↑ Herbst, E., Klemperer, W .: The Formation and Depletion of Molecules in Dense Interstellar Clouds . In: Astrophysical Journal . 185, 1973, p. 505. doi : 10.1086 / 152436 .
  6. ↑ a b Oka, T .: Observation of the Infrared Spectrum of H 3 + . In: Physical Review Letters . 45, No. 7, 1980, pp. 531-534. doi : 10.1103 / PhysRevLett.45.531 .
  7. ↑ a b Drossart, P .: Detection of H 3 + on Jupiter . In: Nature . 340, No. 6234, 1989, p. 539. doi : 10.1038 / 340539a0 .
  8. ↑ a b Geballe, T .: Detection of H 3 + Infrared Emission Lines in Saturn . In: Astrophysical Journal . 408, No. 2, 1993, p. L109. doi : 10.1086 / 186843 .
  9. ↑ a b Trafton, LM: Detection of H 3 + from Uranus . In: Astrophysical Journal . 405, 1993, p. 761. doi : 10.1086 / 172404 .
  10. ↑ a b Geballe, TR, Oka, T .: Detection of H 3 + in Interstellar Space . In: Nature . 384, No. 6607, 1996, pp. 334-335. doi : 10.1038 / 384334a0 . PMID 8934516 .
  11. ^ A b McCall, BJ: Detection of H 3 + in the Diffuse Interstellar Medium Toward Cygnus OB2 No. 12 . In: Science . 279, No. 5358, 1998, pp. 1910-1913. doi : 10.1126 / science.279.5358.1910 .
  12. ↑ a b T. Oka: Interstellar H 3 + . In: PNAS . 103, No. 33, 2006, pp. 12235-12242. bibcode : 2006PNAS..10312235O . doi : 10.1073 / pnas.0601242103 . PMID 16894171 . PMC 1567864 (free full text).
  13. ^ McCall, BJ: Dissociative Recombination of Rotationally Cold H 3 + . In: Physical Review A . 70, No. 5, 2004, p. 052716. doi : 10.1103 / PhysRevA.70.052716 .
  14. ↑ a b c Herbst, E .: The Astrochemistry of H 3 + . In: Philosophical Transactions of the Royal Society A . 358, No. 1774, 2000, pp. 2523-2534. doi : 10.1098 / rsta.2000.0665 .
  15. ^ Watson, JKG: Forbidden rotational spectra of polyatomic molecules . In: Journal of Molecular Spectroscopy . 40, No. 3, 1971, pp. 546-544. bibcode : 1971JMoSp..40..536W . doi : 10.1016 / 0022-2852 (71) 90255-4 .
  16. ↑ a b B. J. McCall: Observations of H 3 + in Dense Molecular Clouds . In: Astrophysical Journal . 522, 1999, pp. 338-348. bibcode : 1999ApJ ... 522..338M . doi : 10.1086 / 307637 .
  17. ↑ a b c d Goto, M .: Absorption Line Survey of H 3 + toward the Galactic Center Sources I. GCS 3-2 and GC IRS3 . In: Astron. Soc. Japan . 54, 2002, p. 951.
  18. ↑ SD Brittain: Interstellar H 3 + Line Absorption toward LkHα 101 . In: Astrophysical Journal . 606, No. 2, 2004, pp. 911-916. bibcode : 2004ApJ ... 606..911B . doi : 10.1086 / 383024 .
  19. ^ BJ McCall: An Enhanced Cosmic-ray Flux towards ζ Persei Inferred from a Laboratory Study of the H 3 + -e - Recombination Rate . In: Nature . 422, No. 6931, 2003, pp. 500-2. arxiv : astro-ph / 0302106 . bibcode : 2003Natur.422..500M . doi : 10.1038 / nature01498 . PMID 12673244 .