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Author Topic: Are there any compounds of helium?  (Read 44284 times)

Offline Supercryptid

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Are there any compounds of helium?
« on: 15/12/2003 21:08:13 »
Helium is the first noble gas on the periodic table. Of the noble gases, it is the most inert and has the smallest atom. But might it be possible to form helium compounds? I believe all attempts so far have failed, but I have a couple of ideas:

According to the molecular orbital theory, a hydrogen atom should be able to form a stable bond with helium. Hydrogen has 1 electron and helium has 2. The resulting "hydrogen heliide" molecule would have 3 electrons; 2 of them in the bonding sigma orbital, and 1 of them in the antibonding sigma* orbital. This should give the molecule a bonding order of 0.5, which is only half a single bond.

Even though this bond would be very weak, it still has some strength and should allow hydrogen heliide (HHe) to exist as a stable molecule. Hydrogen heliide could probably exist as a single molecule, but putting many of those molecules together in the same container would be disasterous because of this equation:

2HHe -> H2 + 2He

If two HHe molecules collided with one another in the container with enough energy, the molecules would break apart and recombine, forming dihydrogen and helium. Dihydrogen has a bond order of 1.0, which is probably stronger than HHe's bond order of 0.5, so dihydrogen would be thermodynamically preferred.

Because of the very weak bond holding HHe together, it would probably take only a tiny bit of thermal energy to break it. Therefore, I assume that HHe might only be stable at VERY low temperatures.
_________

My next idea of a helium compound is borane heliide. A single borane molecule is electron deficiet, since it only has 6 valence electrons. It would like to get another electron pair so that it could complete it's octet. But look at helium; it has 2 electrons, which is just the number of electrons that the borane molecule needs!

The helium atom might then act as a Lewis base by sharing its electron pair with the borane, which is the Lewis acid. The helium atom would then be bonded to the central boron atom in borane by a coordinate bond. The molecule would then be tetrahedrally shaped, similar to methane in structure. All of the atoms in the molecule would have all the electrons that they need.

But, like HHe, we have a problem:

2BH3He -> B2H6 + 2He

If two of these borane heliide molecules interacted, then they might decompose into diborane and helium. Diborane is probably more thermodynamically stable than borane heliide, so BH3He would probably decompose rapidly at room temperature. Again, borane heliide would probably be stable at only very low temperatures.
_________

What do you think of my hypothetical helium compounds? Do you have any suggestions for a helium compound?
« Last Edit: 27/11/2008 21:03:45 by chris »


 

Offline Ylide

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Re: Are there any compounds of helium?
« Reply #1 on: 16/12/2003 05:31:35 »
Well, the only helium compound I've seen exists briefly as an intermediate.  Methane with 1 hydrogen substituted with tritium can undergo beta decay.  This forms an extra proton which increases the atomic number from 1 to 2, forming helium, and emitting an electron.  Helium then spontaneously decomposed from the molecule, leaving behind isolated methyl carbocation.  (which as organic chemists know is not a stable or easy-to-form molecule)

The moral of the story is, helium hates being locked up in a molecule so badly that it is more energetically favorable to form a methyl carbocation than it is for the molecule to remain as methyl helium.

As you mentioned above, very low temperatures might be able to accomplish this.  Some of the other inert gases can be caused to form molecules in very specific situations, so what you're speculating may just be possible.  Keep in mind that non-spontaneous reactions require an input of energy to make them happen.  Energy is bad for the unstable molecule you're proposing.  You'd need to calculate the free energy of the reaction to determine spontaneity before going any further, otherwise you're wasting your time.  (unless you come up with a really cool catalyst)





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Offline Supercryptid

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Re: Are there any compounds of helium?
« Reply #2 on: 16/12/2003 20:34:05 »
Yes, I do realize that methyl carbocations are notoriously unstable.

But the way that you describe it, it seems that a methyl radical should be formed instead of a methyl carbocation:

CH3T -> CH3. + He

When the tritium atom decays, it releases an electron from it's nucleus, so that that a neutron can decay into a proton. This forms helium-3. The electron that is emitted from the nucleus would be captured in the electron cloud of the helium atom, right? That would give the helium atom a net number of 3 electrons, which it doesn't like. The helium would break away from the carbon, leaving 1 of the electrons behind on the carbon and keeping 2 electrons to itself. That should produce a methyl radical.

Of course, the above situation can only be true if the emitted electron IS captured in the elctron cloud. If the elctron that is emitted by the tritium nucleus simply leaves the molecule for good, then you're right. I don't know that much about beta decay, so please correct me if I'm wrong.

But, if indeed a helium atom will not bond to an electron deficiet methyl carbocation, then my proposal for borane heliide is flawed. A methyl carbocation is isoelectronic with borane, so borane shouldn't be any better at capturing a helium atom.

Perhaps this is due to helium's super-high electronegativity (5.5)? That might be why it doesn't like to donate electrons to electron deficiet molecules.
 

Offline Ylide

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Re: Are there any compounds of helium?
« Reply #3 on: 17/12/2003 02:22:54 »
Nope, it doesn't form a radical.  

When the beta particle is emitted, you get CH3-He+ + e-, after which the helium leaves with BOTH electrons in the He-C bond, leaving methyl carbocation.  I had never really seen the electronegativity for helium, but that makes perfect sense as to why it's so unreactive.  High electronegativity / high electron affinity = REALLY stable atom.

This leads me into a followup question that maybe you can answer:

Any idea why deuterium and tritium are less electronegative than hydrogen?  There are the same number of protons in each, so all I can figure is the extra neutrons are is shielding the protons slightly, causing less attractive force on any nearby electrons.



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Offline Supercryptid

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Re: Are there any compounds of helium?
« Reply #4 on: 17/12/2003 18:43:22 »
I've never heard of deuterium and tritium being less electronegative than protium (hydrogen). Your theory seems sensible, sense I can't think of any other reasons for the difference.
 

Offline phoenix

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Re: Are there any compounds of helium?
« Reply #5 on: 26/11/2008 00:19:47 »
Happened to come across the discussion about helium compounds.  Interesting. Noted helium won't even donate to CH3+ to make a stable molecule.  How about making tritiated fluoroform, CTF3, and see if helium has enough pity for an even less stable carbocation, as tritium decays to helium.

I've also run across an abstract about F+ ions liberated from Ca0.6Sr0.4F2/GaAs through electron stimulated desorption.  I don't know much about the technique or the conditions (which I surmise are vacuum), however it seems to me that since F+ should be more electronegative than He, in the presence of F+, maybe He might cave in and form a stable HeF+ system. 

Just some thoughts.

 

Offline moolots

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Are there any compounds of helium?
« Reply #6 on: 19/09/2011 13:55:33 »
hi i was wondering if you were able to create a new molecule with helium or hydrogen. Would there be any chance of it being even lighter then hydrogen
 

Offline peppercorn

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Are there any compounds of helium?
« Reply #7 on: 19/09/2011 14:55:50 »
hi i was wondering if you were able to create a new molecule with helium or hydrogen. Would there be any chance of it being even lighter then hydrogen

For a start have you any idea how Hydrogen atoms and Helium atoms bond?
Even if they could stick together doesn't it seem counter-intuitive that they would result in a molecule lighter than hydrogen?
 

Offline damocles

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Are there any compounds of helium?
« Reply #8 on: 20/09/2011 01:05:42 »
My colleague DJ Wilson has long been involved with high powered theoretical calculations for helium compounds and compound ions (Google "wilson helide" if you want the details, and insist on "helide" when Google tells you that you really wanted "halide"). Such "compounds" as there are involve very long and very weak bonds -- whether they "exist" really depends on where you want to draw the line between a chemical bond and a weak van der Waals type attraction.

The question of electronegativity differences between hydrogen-1 and its heavier isotopes deuterium and tritium has nothing to do with the neutron as such. It arises artificially because the heavier nuclei are associated with lower vibrational frequencies, and therefore deuterium and tritium compounds are situated deeper in their potential wells than the equivalent protium compounds -- effectively resulting in stronger bonds to these heavier isotopes. It is all to do with vibrational zero-point energy.
« Last Edit: 20/09/2011 01:10:04 by damocles »
 

Offline damocles

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Are there any compounds of helium?
« Reply #9 on: 20/09/2011 02:45:07 »
To clarify my last posting here: consider the equilibrium

H2+ + D2 reversible arrow D2+ + H2

There are just 3 electrons in this system. The more electronegative species will grab 2 of them. This is the simplest and most clear-cut situation to define the relative electronegativities of H and D.

Now the total electronic energy will be the same on both sides of this equation, because isotope substitution does not affect electronic factors (neutrons have zero charge). But vibrationally, the heavier atoms make for quite different characteristic frequencies:

H2 ≈ 4400 cm-1 ; D2 ≈ 4400/√2 ≈ 3100 cm-1
H2+ ≈ 2000 cm-1 ; D2+ ≈ 2000/√2 ≈ 1400 cm-1

So the vibrational energy of the lowest vibrational states on the left hand side will be ( 2000 + 3100 ) = 2550 cm-1,
while on the right hand side it will be ( 1400 + 4400) = 2900 cm-1

The products on the left hand side will therefore have lower energy than those on the right, the equilibrium will be dominated by the left, and deuterium will appear to be more electronegative than protium.

 This is mainly for the benefit of the chemists who read here; I hope I have put it clearly enough that others will be able to understand.
« Last Edit: 20/09/2011 02:47:30 by damocles »
 

Offline Bored chemist

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Are there any compounds of helium?
« Reply #10 on: 20/09/2011 19:07:25 »
It depends what you call a compound
http://physics.unifr.ch/fr/page/189/
 

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Are there any compounds of helium?
« Reply #10 on: 20/09/2011 19:07:25 »

 

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