Science

OKIsItJustMe

(21,642 posts) Mon Aug 4, 2025, 12:18 AM Aug 4

Chemistry at the Beginning: How Molecular Reactions Influenced the Formation of the First Stars

https://www.mpi-hd.mpg.de/mpi/en/public-relations/news/news-item/chemistry-at-the-beginning

Chemistry at the Beginning: How Molecular Reactions Influenced the Formation of the First Stars
28.07.2025

Researchers have discovered new insights into the reaction pathways of the first molecule in space.

The HeH⁺ ion was the first molecule to form in the early universe, before the first stars were born.
Researchers have investigated reactions of HeH⁺ with hydrogen atoms under conditions similar to those in the early universe for the first time, revealing significant discrepancies with previous theories.
The results significantly impact our understanding of the early chemistry of the universe and the processes that contributed to the formation of the first stars.

…

They found that, contrary to earlier predictions, the rate at which this reaction proceeds does not slow down with decreasing temperature, but remains almost constant. “Previous theories predicted a significant decrease in the reaction probability at low temperatures, but we were unable to verify this in either the experiment or new theoretical calculations by our colleagues,” explains Dr Holger Kreckel from the MPIK. ‘The reactions of HeH⁺ with neutral hydrogen and deuterium therefore appear to have been far more important for chemistry in the early universe than previously assumed,’ he continues. This observation is consistent with the findings of a group of theoretical physicists led by Yohann Scribano, who identified an error in the calculation of the potential surface used in all previous calculations for this reaction. The new calculations using the improved potential surface now align closely with the CSR experiment.

…

F. Grussie, J. Sahoo, Y. Scribano, D. Bossion, L. Berger, M. Grieser, L. W. Isberner, Á;. Kálosi, O. Novotný, D. Paul, A. Znotins, X. Urbain, H. Kreckel. Experimental confirmation of barrierless reactions between HeH⁺; and deuterium atoms suggests a lower abundance of the first molecules at very high redshifts. Astronomy & Astrophysics, 2025; 699: L12 DOI: 10.1051/0004-6361/202555316

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Chemistry at the Beginning: How Molecular Reactions Influenced the Formation of the First Stars (Original Post) OKIsItJustMe Aug 4 OP

First stars are important to understand. Frasier Balzov Aug 4 #1

How do you know that living matter has not always existed in some form? BWdem4life Aug 4 #3

Really important post. Thank you! byronius Aug 4 #2

You're welcome! OKIsItJustMe Aug 4 #5

Interesting. The idea that helium should be involved in the first chemistry in the Universe seems almost perverse. eppur_se_muova Aug 4 #4

Frasier Balzov

(4,499 posts)

1. First stars are important to understand.

Reply to OKIsItJustMe (Original post)

Mon Aug 4, 2025, 12:23 AM

Aug 4

I'm eager for more about first life. Abiogenesis. The emergence of life from non-living matter.

BWdem4life

(2,655 posts)

3. How do you know that living matter has not always existed in some form?

Reply to Frasier Balzov (Reply #1)

Mon Aug 4, 2025, 03:05 AM

Aug 4

byronius

(7,808 posts)

2. Really important post. Thank you!

Reply to OKIsItJustMe (Original post)

Mon Aug 4, 2025, 01:57 AM

Aug 4

OKIsItJustMe

(21,642 posts)

5. You're welcome!

Reply to byronius (Reply #2)

Mon Aug 4, 2025, 12:48 PM

Aug 4

(Obviously) I found it interesting as well!

eppur_se_muova

(39,765 posts)

4. Interesting. The idea that helium should be involved in the first chemistry in the Universe seems almost perverse.

Reply to OKIsItJustMe (Original post)

Mon Aug 4, 2025, 07:10 AM

Aug 4

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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