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The Universe's First Molecule Found:


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https://www.nasa.gov/feature/the-universe-s-first-type-of-molecule-is-found-at-last

The first type of molecule that ever formed in the universe has been detected in space for the first time, after decades of searching. Scientists discovered its signature in our own galaxy using the world’s largest airborne observatory, NASA’s Stratospheric Observatory for Infrared Astronomy, or SOFIA, as the aircraft flew high above the Earth’s surface and pointed its sensitive instruments out into the cosmos.

When the universe was still very young, only a few kinds of atoms existed. Scientists believe that around 100,000 years after the big bang, helium and hydrogen combined to make a molecule called helium hydride for the first time. Helium hydride should be present in some parts of the modern universe, but it has never been detected in space — until now.

SOFIA found modern helium hydride in a planetary nebula, a remnant of what was once a Sun-like star. Located 3,000 light-years away near the constellation Cygnus, this planetary nebula, called NGC 7027, has conditions that allow this mystery molecule to form. The discovery serves as proof that helium hydride can, in fact, exist in space. This confirms a key part of our basic understanding of the chemistry of the early universe and how it evolved over billions of years into the complex chemistry of today. The results are published in this week’s issue of Nature.

 

more at link.................

 

the paper:

https://arxiv.org/ftp/arxiv/papers/1904/1904.09581.pdf

First astrophysical detection of the helium hydride ion (HeH+ )

During the dawn of chemistry1,2 when the temperature of the young Universe had fallen below ~4000 K, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With its higher ionization potentials, He++ (54.5 eV) and He+ (24.6 eV) combined first with free electrons to form the first neutral atom, prior to the recombination of hydrogen (13.6 eV). At that time, in this metal-free and low-density environment, neutral helium atoms formed the Universe’s first molecular bond in the helium hydride ion HeH+ , by radiative association with protons (He + H+ → HeH+ + hν). As recombination progressed, the destruction of HeH+ (HeH+ + H → He + H2 + ) created a first path to the formation of molecular hydrogen, marking the beginning of the Molecular Age. Despite its unquestioned importance for the evolution of the early Universe, the HeH+ molecule has so far escaped unequivocal detection in interstellar space. In the laboratory the ion was discovered as long ago as 19253 , but only in the late seventies was the possibility that HeH+ might exist in local astrophysical plasmas discussed4,5,6,7. In particular, the conditions in planetary nebulae were shown to be suitable for the production of potentially detectable HeH+ column densities: the hard radiation field from the central hot white dwarf creates overlapping Strömgren spheres, where HeH+ is predicted to form, primarily by radiative association of He+ and H. With the GREAT spectrometer8.9 on board SOFIA10 the HeH+ rotational ground-state transition at λ149.1 µm is now accessible. We report here its detection towards the planetary nebula NGC7027. The mere fact of its proven existence in nearby interstellar space constrains our understanding of the chemical networks controlling the formation of this very special molecular ion.

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2 hours ago, joigus said:

Very interesting. Thanks. My intuition is that the feeblest temperature background would split this tiny little weirdo of a molecule... :D 

I'm not so sure. This is an ion, HeH⁺, that is isoelectronic with H₂, i.e. with 2 electrons in a σ-bond formed by overlap of the 2 1s atomic orbitals. Though it will be strongly polar, due to the higher charge on the He nucleus (i.e. the 1s on He will be pulled in and won't overlap so well). I'm sure it is highly reactive: as a cation it will tend to pull electrons off whatever it comes into contact with, and it can easily form He by donating the proton to something.

What strikes me about it is that as, unlike H₂ it is polar, it will have a vibrational and rotational spectrum, so presumably can be detected in the IR and microwave regions of the spectrum.  

P.S. I see there is a Wiki article on it: https://en.wikipedia.org/wiki/Helium_hydride_ion#cite_note-Epa-23 according to which the bond strength is 178kJ/mol, about  40% that of H₂ so quite respectable. Also I notice they think it was a constituent of the primordial plasma, 280,000 yrs before the universe became transparent. So presumably it is not expected to fall apart thermally so easily. 

Edited by exchemist
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2 hours ago, exchemist said:

I'm not so sure. This is an ion, HeH⁺, that is isoelectronic with H₂, i.e. with 2 electrons in a σ-bond formed by overlap of the 2 1s atomic orbitals. Though it will be strongly polar, due to the higher charge on the He nucleus (i.e. the 1s on He will be pulled in and won't overlap so well). I'm sure it is highly reactive: as a cation it will tend to pull electrons off whatever it comes into contact with, and it can easily form He by donating the proton to something.

What strikes me about it is that as, unlike H₂ it is polar, it will have a vibrational and rotational spectrum, so presumably can be detected in the IR and microwave regions of the spectrum.  

P.S. I see there is a Wiki article on it: https://en.wikipedia.org/wiki/Helium_hydride_ion#cite_note-Epa-23 according to which the bond strength is 178kJ/mol, about  40% that of H₂ so quite respectable. Also I notice they think it was a constituent of the primordial plasma, 280,000 yrs before the universe became transparent. So presumably it is not expected to fall apart thermally so easily. 

Very nice account, very informative, and very clear. Thank you.

A simple calculation involving Boltzmann's constant gives you a temperature of a couple thousand Kelvin.

Dividing 178000 J by Avogadro's number, and further dividing that by Boltzman's constant produces about 2141 K to break the bonds regularly. So no wonder there's a lot of it about.

Breaking sigma bonds is not at all like breaking dipole-dipole hydrogen bonds, as in making water vapour...

Does that make sense?

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1 hour ago, joigus said:

Very nice account, very informative, and very clear. Thank you.

A simple calculation involving Boltzmann's constant gives you a temperature of a couple thousand Kelvin.

Dividing 178000 J by Avogadro's number, and further dividing that by Boltzman's constant produces about 2141 K to break the bonds regularly. So no wonder there's a lot of it about.

Breaking sigma bonds is not at all like breaking dipole-dipole hydrogen bonds, as in making water vapour...

Does that make sense?

True. H bonds typically have a bond strength of the order of 10% of covalent bonds, in water about 20kJ/mol. (Though they can range quite widely in strength in particular instances.)

But H-bonds are I think believed to be a bit more than purely electrostatic dipole attractions. At least, my understanding is that they have some directionality, associated with the "lone pairs" on the electronegative atom.  Whether it is covalent character, or just electrostatic attraction to electron density in the lone pairs, I'm not sure. As far as I know, they remain an object of theoretical study. 

Edited by exchemist
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