Last edited Mon Aug 4, 2025, 07:46 AM - Edit history (1)
1. Introduction
The chemistry of the early Universe is dominated by the elements hydrogen, helium, and lithium, the only ones to form in substantial quantities during Big Bang nucleosynthesis. All of the light elements started out fully ionized, until the temperature dropped far enough for electrons to start recombining with nuclei to form neutral atoms. Owing to its high first ionization potential of 24.6 eV, helium was the first element to become neutral and engage in chemical reactions (Galli & Palla 2013). The first molecules were formed in the radiative association process H+ + He → HeH+ + hν (Lepp et al. 2002), and subsequently other small molecular ions and neutral molecules were formed, among them H 2 + , H2, H 3 + , LiH, LiH+, and deuterated variants of those species (Glover & Savin 2009).
Helium is by far the most unreactive element known; there are no isolable compounds at all (excepting possibly clathrates, which don't involve "real" chemistry of their guest atoms). But that's precisely why He was the first neutral atom to form, which is what leads to the chemistry in the paper !
But I do wonder: what about HeHe+ ?? It seems to me that once He formed, surrounded by H+ and He+, it could well be the latter that would stick to neutral He more exergonically. After all, in MO terms, the overlap between two He 1s orbitals should be larger than between He 1s and H 1s, because of the energy difference between the latter; in addition the He 1s orbital is lower in energy than the H 1s to begin with. Thus the sigma bonding orbital should be much lower in energy, and the sigma* antibonding possibly higher, than in the He-H system. Dropping two electrons into that lower orbital forms a singly-bonded HeHe+2 molecular ion, which is a
local minimum, though endergonic with respect to two separated He+ ions, because of the very high Coulombic repulsion between the nuclei. Adding one more electron would weaken the net covalent bonding (by just over half) while adding more electron-nuclear attraction; secondarily, this should also increase the equilibrium bond length, reducing the nuclear-nuclear repulsion). Which effect would be greater, the former or the latter ? I hope the authors found it to be the former and thus HeHe+ did not get overlooked, and its omission would have been caught by referees.
I can't access the original publications, or their references, but I did find claims that the
"proton affinity" of He is ~46 kcal/mole (old units, I know) while the
bond strength of HeHe+ is ~55 kcal/mole, suggesting that He+ could easily displace H+ from HeH+. Maybe this is covered in the paper; as I said, I can't see the full text.
I also note that with He+ being much less common than H+, any He formed is far more likely to encounter H+ first. So one might expect HeH+ to be favored kinetically even if HeHe+ is favored thermodynamically. No idea what implications this has for the types of modeling they're doing.
ETA: Using only computational (
ab initio) results, the reaction
HeH+ & He+ ----> HeHe+ & H+
is predicted to be exergonic, by ~10 kcal/mole. Hmmmm.
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