According to this page http://en.wikipedia.org/wiki/H2CO3 this compound is hard to isolate if at all.
It seems in water it dissociates into water and CO2 and can nearly only exist in HCO3- and CO3 2- anions.
The half life of pure H2CO3 is 180.000 years at room temperature! Water catalyses the decomposition, which is further driven by autocatalysis. 1 or 2
molecules of water are enough to start the reaction. That's why it's hard to isolate. But it can be isolated in a pure state. And the molecule H2CO3
also exists in the rain, tap water, and so on (1% of the CO2 exists as H2CO3)...