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

Two different polymorphs of carbonic acid, a- and beta-H2CO3, 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 a-polymorph, compare it with its FT-IR spectrum and present band assignments in line with our work on the beta-polymorph [Angew. Chem. Int. Ed. 48 (2009) 26902694]. The Raman spectra also contain information in the wavenumber range similar to 90400 cm(-1), which was not accessible by FT-IR spectroscopy in the previous work. While the a-polymorph shows Raman and IR bands at similar positions over the whole accessible range, the rule of mutual exclusion is obeyed for the beta-polymorph. This suggests that there is a center of inversion in the basic building block of beta-H2CO3 whereas there is none in alpha-H2CO3. Thus, as the basic motif in the crystal structure we suggest the cyclic carbonic acid dimer containing a center of inversion in case of beta-H2CO3 and a catemer chain or a sheet-like structure based on carbonic acid dimers not containing a center of inversion in case of alpha-H2CO3. 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 beta-H2CO3 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 beta-oxalic acid (R=COOH) and produces an intense Raman active band at a very similar wavenumber. The absence of this band in a-H2CO3 confirms that the difference to beta-H2CO3 is found in the local coordination environment and/or monomer conformation rather than on the long range. Copyright (c) 2011 John Wiley & Sons, Ltd.