Matrix Isolation Studies of Carbonic Acid—The Vapor Phase above the β-Polymorph

Abstract

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 (C2v and Cs). In contrast to the earlier study on α-H2CO3, we do not find evidence for centrosymmetric (C2h) 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-H2CO3.

The hydration structure of aqueous carbonic acid from X-ray absorption spectroscopy

“…As described in a very recent review [7], carbonic acid has been well studied in both the gas phase [8][9][10] and in cryogenic matrixes [11][12][13][14][15], but there have been very few successful spectroscopic studies of the aqueous acid, and none of its electronic structure [16][17][18]. Carbonic acid is intrinsically unstable upon contact with even a single water molecule, reacting via a proton chain mechanism to rapidly form aqueous bicarbonate, carbonate, and hydrated protons [19][20][21], which comprises the reversible mechanism for dissolution of CO 2 gas.…”

Alpha‐Carbonic Acid Revisited: Carbonic Acid Monomethyl Ester as a Solid and its Conformational Isomerism in the Gas Phase

“…UV irradiation and subsequent analysis by using difference spectra (between experiments prior to and after irradiation) was performed to aid the assignment, similar to the case of matrix isolated carbonic acid . UV irradiation causes isomerization, specifically from monomer structure I to structure II.…”

“…[11] The previously reported polymorphic transition from bto a-H 2 CO 3 [10a] has to be reinterpreted as a methylation of b-H 2 CO 3 in acidic methanolic solution. The matrix spectra of H 2 CO 3 isolated from 'a-carbonic acid' [12,19] are reinterpreted as well. This conclusion could not have been made without the isotopic labeling studies and the comparison with high-level calculations for a comprehensive set of molecules.…”

“…Note that detailed discussions of b-H 2 CO 3 and CAEE, including also matrix isolation, can be found in our earlier work. [12,15,19] Chem. Eur.…”

“…In Figure 8 b, only the band at 1242 cm À1 remains unexplained. Figure S6 (in the Supporting Information) shows a direct comparison of the FTIR spectra of matrix isolation experiments of CAME (a), carbonic acid (CA, b), [19] and carbonic acid monoethyl ester (CAEE, c). [15] All three spectra are based on an identical preparation as described in the Experimental Section.…”