Matrix Isolation Studies of Carbonic AcidThe Vapor Phase above the β‑Polymorph

Twenty years ago two different polymorphs of carbonic acid, α- and β-H₂CO₃, were isolated as thin, crystalline films. They were characterized by infrared and, of late, by Raman spectroscopy. Determination of the crystal structure of these two polymorphs, using cryopowder and thin film X-ray diffraction techniques, has failed so far. Recently, we succeeded in sublimating α-H₂CO₃ and trapping the vapor phase in a noble gas matrix, which was analyzed by infrared spectroscopy. In the same way we have now investigated the β-polymorph. Unlike α-H₂CO₃, β-H₂CO₃ was regarded to decompose upon sublimation. Still, we have succeeded in isolation of undecomposed carbonic acid in the matrix and recondensation after removal of the matrix here. This possibility of sublimation and recondensation cycles of β-H₂CO₃ adds a new aspect to the chemistry of carbonic acid in astrophysical environments, especially because there is a direct way of β-H₂CO₃ formation in space, but none for α-H₂CO₃. Assignments of the FTIR spectra of the isolated molecules unambiguously reveal two different carbonic acid monomer conformers (<i>C</i>_2<i>v</i> and <i>C</i>_<i>s</i>). In contrast to the earlier study on α-H₂CO₃, we do not find evidence for centrosymmetric (<i>C</i>_2<i>h</i>) carbonic acid dimers here. This suggests that two monomers are entropically favored at the sublimation temperature of 250 K for β-H₂CO₃, whereas they are not at the sublimation temperature of 210 K for α-H₂CO<sub>3