Astronomers have spotted the universe’s first molecule
sciencemag.org
sciencemag.org
Here is another link of a publication that didn't break the press release: http://www.astronomy.com/news/2019/04/astronomers-find-oldes...
HeH has a binding energy of 25.1 kJ/mole [1]. H2 has a dissociation energy of 104 kcal/mole [2]. Expressing these energies in eV per molecule, it’s ~0.26eV for HeH and ~ 4.5eV for H2. So H2 would seem to be stable at higher temperatures and should form first in a cooling gas mixture whose particle kinetic energies are declining.
Or is “binding energy” different from “dissociation energy”?
Would like to understand a little of the thermodynamics that allow HeH rather than H2 to be the primordial molecule.
[1] https://en.wikipedia.org/wiki/Helium_hydride_ion [2] https://en.wikipedia.org/wiki/Bond-dissociation_energy
Binding energy is a bit more vague in my opinion. For atoms it means disassembling the atom into free electrons and a nucleus. In chemistry it is the average energy required to break all bonds in a molecule.
Source: Chemist and wiki
But TFA says that H+ reacted with neutral He. That gives you HeH+, which is what they detected. And I see that the ionization energies of He are 2372.3 kJ/mol for the first electron, and 5250.5 kJ/mol for the second. But the ionization energy of H is just 1312.0 kJ/mol. So neutral He would have formed long before neutral H, and He plus H+ was the first possible combination.
Also, HeH+ is an extremely strong acid, so it would grab an electron from just about anything else that it encountered. And neutral HeH is not stable.
So at a temperature cool enough for neutral He to exist and still hot enough for [H+] ions, they were pretty much the only massive things around that could react with each other.
If an [HeH+] caught a hot electron, it would fall apart into He and H, with no further need to share electrons. A [H+] could glom onto that neutral H to make [H2+] (which also forms when cosmic rays blast through an H2 cloud). If an [H2+] caught another electron, the H2 and another [H2+] will react to make [H3+] and H. So it turns out [H3+] is a pretty common hot cloud molecule, too.
Plain old H2 doesn't exist by itself until the temperature drops enough for those positive ions to catch electrons and stop being an acidic plasma.
I can't find the information now, but the non-charged version is probably not stable (or the bound is extremely weak).
The title of the research paper is: Astrophysical detection of the helium hydride ion HeH+
Not quite. The paper makes it clear that what was actually observed was HeH+:
> Here we report observations, based on advances in terahertz spectroscopy8,9 and a high-altitude observatory10, of the rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027.
In any case, the scarcity of signatures outside violent environments indicates that no primordial HeH compounds survived, and these are recent re-occurrences.
So, "the first type of compound". Less punchy, I guess.
Only with respect to use (properties).
They still all have different histories (space/time lines).
If they indeed have found the first molecule, that would be a huge thing (and a huge mystery, of how they've managed to (a) find it, (b) recognize it as such, since all are interchangeable).
Or are they really talking about a first compound? And if they are, why not say so?
Using the phys.org link, because it's more substantive.