Local structural order in carbonic acid polymorphs: Raman and FT‐IR spectroscopy

Abstract

Two different polymorphs of carbonic acid, α-and β-H 2 CO 3 , were identified and characterized using infrared spectroscopy (FT-IR) previously. Our attempts to determine the crystal structures of these two polymorphs using powder and thin-film X-ray diffraction techniques have failed so far. Here, we report the Raman spectrum of the α-polymorph, compare it with its FT-IR spectrum and present band assignments in line with our work on the β-polymorph [Angew. Chem. Int. Ed. 48 (2009) 2690-2694]. The Raman spectra also contain information in the wavenumber range ∼90-400 cm −1 , which was not accessible by FT-IR spectroscopy in the previous work. While the α-polymorph shows Raman and IR bands at similar positions over the whole accessible range, the rule of mutual exclusion is obeyed for the β-polymorph. This suggests that there is a center of inversion in the basic building block of β-H 2 CO 3 whereas there is none in α-H 2 CO 3 . Thus, as the basic motif in the crystal structure we suggest the cyclic carbonic acid dimer containing a center of inversion in case of β-H 2 CO 3 and a catemer chain or a sheet-like structure based on carbonic acid dimers not containing a center of inversion in case of α-H 2 CO 3 . This hypothesis is strengthened when comparing Raman active lattice modes at <400 cm −1 with the calculated Raman spectra for different dimers. In particular, the intense band at 192 cm −1 in β-H 2 CO 3 can be explained by the inter-dimer stretching mode of the centrosymmetric RC(OHO) 2 CR entity with R OH. The same entity can be found in gas-phase formic acid (R H) and in β-oxalic acid (R COOH) and produces an intense Raman active band at a very similar wavenumber. The absence of this band in α-H 2 CO 3 confirms that the difference to β-H 2 CO 3 is found in the local coordination environment and/or monomer conformation rather than on the long range.

http://omicsonline.org/open-access/hybrid-membranes-prepared-by-the-solgel-process-and-based-on-silica-polyvinyl-acetate-for-methane-enrichment-from-biogas-2155-9589.1000128.php?aid=24675

“…From Figure 5, the diffusion of gases follows the next order: H 2 diffuses first, starting with According to our Raman observations, in the literature has been reported the presence of a very strong band around 2976 cm -1 for the stretching of OH groups and also a band between 641 and 684 wavenumber assigned to δ(COO) groups [32]. Mitterdorfer et al [33] mention that some di and polycarboxilicc acids show the existence of some small Raman bands in the region between 1607 and 1782 cm -1 , corresponding to the assimetric stretching of the C=O group; as a consequence of the connection of the OH group to the carboxylic group or directly to a carbon atom. This can be observed for both the alfa form (1630 cm -1 ) and the beta form (1608 cm -1 ).…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VII

“…studied the first‐order Raman scattering of the MAX phases Ta 4 AlC 3 , Nb 4 AlC 3 , Ti 4 AlN 3 , and Ta 2 AlC . Mitterdorfer and co‐workers studied local structural order in carbonic acid polymorphs using Raman and FTIR spectroscopy . Presser et al .…”

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

“…15 It has thus been suggested that β-carbonic acid may be found on the Martian surface, on interstellar grains, on comets, especially in the Oort cloud, or on Jupiter’s icy satellites Europa, Ganymede, and Callisto. 4,16−19 …”

“…24 The local symmetries of these polymorphs were inferred by testing the validity of the mutual exclusion principle from Raman and IR spectroscopic data. 19 The monomers are arbitrarily depicted in the cis–cis conformation ( C 2 v ). In the crystal field of each of the two polymorphs also the cis–trans conformation ( C s ) could be the more stable one.…”