"Strange" Brønsted acids


H2+, H3+, HeH+, LiH2+, CH5+, NeH+ ...


All these species are stable but very reactive molecular ions. But hardly any of them is known to elementary chemistry texts. That is a sad state and I tell you why:
1) H2+ is the simplest molecule, hence the prototype for understanding chemical bonding. Its complete theoretical description, starting 1927 by Øjivind Burrau (after first steps of H. Urey, W. Pauli and others), marked the birth and childhood of theoretical chemistry. It is the only molecule for which the Schrödinger equation (using the Born-Oppenheimer principle, see Tutorials) can be and is exactly solved, hence, results are in complete agreement with experiment (we owe the rigorous theory of H2+ to Edward Teller, 1930). "H2+ plays a role of importance [for the theory of molecules] equal to that of the H atom in the problem of the electronic structure of complex atoms", cited from Eyring, Walter & Kimball: Quantum Chemistry (1944, p.201). Therefore, the theory of H2+ was and still is more important than the approximate description of H2 in the same year by Heitler & London (which is to H2+ like He to the H atom). Although the mathematical skill of most freshmen will not be sufficient for mastering the finer details of the quantum-mechanical solution, the pictorial presentation of the results can easily be comprehended.- H2+ is formed by cosmic ray ionization of H2 and therefore very abundant in interstellar space. It protonates the all abundant but very weak base H2 + H2+ -> H3+ + H; it is well known to any mass-spectroscopist;
2) H3+ is the first triatomic species and of key importance during the birth of our universe. It is a potent infrared radiator and was/is responsible for the cooling down in the early phases of star formation by radiating heat away into the universe; it is also present in vast amounts on Jupiter and the other gas planets; physical chemists and astrophysicists investigate the details of its energy states, electromagnetic interactions, and chemical reactions as a very active field of research.
3) HeH+ is the strongest Brønsted acid known ("pKa"(i.e. protonation function Ho) ~ -63, compare to concentrated sulfuric acid with pKa(Ho) -12); HeH+ is also very abundant in the cosmos but harder to detect than H3+, because it has only a very small dipole moment and so does barely radiate in the infrared (does it belong to dark matter?);
4) LiH can easily be protonated to LiH2+ which is well known to plasma physicists;
5) CH5+ frightened orthodox chemists when it was discovered in 1950; they could'nt believe that the dogmatic bonding number 4 of carbon can be cracked. It is often used in mass-spectrometry for chemical ionization by proton transfer; CH5+ can protonate e.g. any hydrocarbon, rendering it very reactive and thus paved the way to epochal new reaction paths for the transformation of hydrocarbons;
6) NeH+ is the next protonated noble gas atom, easily formed by HeH+ + Ne -> He + NeH+; continue by cyclical substitution to ArH+, KrH+, XeH+... In fact there are known protonation products of most atoms. This is easily understandable through Kimball's model and really quite obvious: He has the highest ionization potential, hence the hardest valence shell in the Periodic System. If He can be protonated in the gas phase e.g. by H+ + He (+ collision partner M) -> HeH+, then the softer valence shells of all other atoms should allow protonation by HeH+, and do so.

None of these molecular ions, and several 1000 more, can be sold nor bought anywhere. Is this the reason they are not known to chemistry teachers? Chemistry seems to be the only science selecting their elementary teaching objects by commercial importance! It's about time to change this. Why is it so important to learn details about the industrial production of sulfuric acid or polyethylene, since long perfectly optimized by chemical engineers? Because these products top about the highest yearly tonnage worldwide? Are there no better criteria for the importance to learn a bit of chemistry? E.g. the impact of volcanic SO2, forming sulfuric acid, on the ozone layer in the arctic upper stratosphere? Let's find them in the following samples:
HTML version
  • H2+
  • H3+
  • HeH+
  • LiH2+
  • CH5+
  • NeH+
  • Wolfram Notebook
  • H2+
  • H3+
  • HeH+
  • LiH2+
  • CH5+
  • NeH+