I was wondering, do any allotropes of carbon dioxide exist, apart from the simple O=C=O molecule? For instance, could it exist with a silicon dioxide
structure? I did some quick calculations, and found that the conversion from the linear form to the silicon dioxide structure isn't that
endothermic:
Converting CO2 to the SiO2 form, for each molecule of CO2 requires breaking 2 C=O bonds, and forming 4 C-O bonds.
C=O bond energy = 728 kJ/mol
C-O bond energy = 351 kJ/mol
delta H=2x(C=O) - 4x(C-O)
= +52 kJ/mol, which isn't all that much, so it can't be much less stable than the linear form. However, I've never heard of carbon
dioxide existing in this form, so does it actually exist?JustMe - 11-3-2004 at 15:03
errr... aint allotropes just 4 elelments?Pyrovus - 13-3-2004 at 18:18
Probably, but it was the best word I could think of. Anyhow, it's the same concept, so I don't see why the word can't be used to
describe compounds.Proteios - 13-3-2004 at 23:09
sure i guess... and i never had a prob. understaning what you were going on about... but same chemical formula but different form (the current case)
is usually isomers.unionised - 14-3-2004 at 15:28
The last time I looked the various forms of SiO2 were refered to as "crystal modifications" or polymorphs. Not many of these forms seem to
be shown by CO2.vulture - 14-3-2004 at 15:45
Quote:
which isn't all that much, so it can't be much less stable than the linear form. However, I've never heard of carbon dioxide existing
in this form, so does it actually exist?
It would be more useful to calculate the entropy chance and the gibbs free energy from that.unionised - 14-3-2004 at 16:03
I do't know the value of the entropy change but I know that sticking lots of CO2 molecules together as a polymer will result in a lower entropy
system and tend to be favoured at low temperatures OTOH the reaction is endothermic so it will be favoured by high temperatures.
At best the pressure would need to be high enough to tend to offset the entropy term, at worst it isn't possible.
Pity really, would be interesting.Marvin - 14-3-2004 at 20:27
Pyrovus,
You are forgetting something though. While an 'additional' 52kj/mole might be easy for a molecule, this would be due to the order the atoms
are in, eg, N=O instead of C=O. If the silica structure existed endothermically it would be an exiplex, an *electronically* excited state.
Electronic states dont tend to last very long and if you could make a solid that was only solid becuase it was in an electronically excited state this
would be far more impressive than anything poly CO2/lattice CO2 would be capable of in terms of material properties. If nothing else your half an ev
difference would probably make the worlds most powerful IR laser for its weight.....
Given RTP stable CO2 has not been seen to form so far, but assuming for the moment it exists, you would need an immensly powerful trick working in
nature to keep it from forming under conditions used allready and yet find a method to make it. The kind of discovery that would win nobel prizes.unionised - 15-3-2004 at 15:30
I have found a few references to tetramethoxy methane (Bpt 113 c)
If that exists, then extrapolating to a diamond like structure for poly (CO2)doesn't seem too far fetched. Granted, the oxygens would prefer bent
rather than linear geometry.
Another possibility
Pyrovus - 26-3-2004 at 00:08
The SiO2 structure might be a bit ambitious, but what about smaller structures? For instance, perhaps cyclic polymers formed by the dehydration of
carbonic acid i.e. things like [-O-C(=O)-O-C(=O)-O-C(=O)-O-C(=O)-]n? Could this work?
