Spectroscopic detection of the most stable carbonic acid, cis-cis H2CO3.

Carbonic acid had not been detected by any spectroscopic means for a long period. Recently, we have reported the detection of its second most stable conformer, cis-trans H(2)CO(3), as the first spectroscopic detection of the isolated carbonic acid molecule. In the present work, the most stable conformer of carbonic acid, cis-cis 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 a Fourier-transform microwave spectrometer. In addition to cis-cis H(2)CO(3), its deuterated isotopologue, cis-cis D2CO3, has been observed, yielding the r(0) structure of the cis-cis conformer. Furthermore, hyperfine constants of the deuterated cis-trans conformers were also determined. The two structures for the stable isolated carbonic acid molecule, those of the cis-cis and cis-trans conformers, are considered to provide basic information for the understanding of chemical reactions involving carbonic acid The present result is accurate enough to be used in radio astronomical observations, where the ortho∕para ratio of cis-cis H(2)CO(3) may be used as an important probe of interstellar chemistry.