Spectroscopic detection of isolated carbonic acid.

Carbonic acid (cis-trans H(2)CO(3)) in the gas phase has been successfully produced in a supersonic jet using a pulsed discharge nozzle, and pure rotational transitions of this molecule have been observed by Fourier-transform microwave spectroscopy. Although the observed cis-trans conformer is not the global minimum structure, it is an important conformer as a starting point of its dissociation to CO(2) and H(2)O. Three deuterated isotopologues of the cis-trans conformer (cis-trans HDCO(3), cis-trans DHCO(3), and cis-trans D(2)CO(3)) have also been observed, yielding the r(0) structure of cis-trans H(2)CO(3). The present result is accurate enough to be used in radio astronomical observations.