Antiphon said:
Borek, the wikipedia entry for Carbonic acid has a table entry for solubility in water that reads "exists only in solution". Does this mean you can't make a pure quantity of it or that it isn't done? I'm trying to reconcile this solubility with what you had said about it being quite stable when water is not present.
Well, having read the paper (and re-done a few of the calcs, see above).. the case is that yes, it's stable in the sense that a
single H
2CO
3 molecule in vacuum is very stable. They claim a '0.18 million year half-life' which the wikipedia article cites. However, given the methods used, that number is give-or-take an order of magnitude. It's still very stable either way though.
However, even a single water molecule will catalyze the decomposition dramatically (as they showed), so you'd have a chain reaction and any significant amount of carbonic acid would likely decompose rapidly.
They don't seem to have taken into account the possibility that H
2CO
3 decomposition might be able to occur autocatalytically (coordinated proton transfer in the dimer). I suspect this can happen as well. (and I'm running a calculation on the side to see if I'm right.*)
Anyway, carbonic acid
doesn't exist only in solution (http://www.sciencemag.org/cgi/content/abstract/279/5355/1332" ). It's just damn hard to keep it from decomposing. Not because it's an unstable compound, but because it's decomposition is catalyzed by its own decomposition products (and possibly by itself).
*Edit/update: Yes, I was correct. H
2CO
3 can catalyze its own decomposition, and in fact does so better than one or two water molecules do. I'm going to have to take a look around if this is published yet. I might get a small paper out of this...