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

• Published in: Journal of Raman Spectroscopy
• Main Authors: Mitterdorfer, Christian, Bernard, Jürgen, Klauser, Frederik, Winkel, Katrin, Kohl, Ingrid, Liedl, Klaus R., Grothe, Hinrich, Mayer, Erwin, Loerting, Thomas
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2011
• Subjects: Carbonic acid
• Online Access: http://dx.doi.org/10.1002/jrs.3001; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjrs.3001; http://onlinelibrary.wiley.com/wol1/doi/10.1002/jrs.3001/fullpdf

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. Copyright © 2011 John Wiley & Sons, Ltd.