Carbonic Acid:  From Polyamorphism to Polymorphism

Abstract

Layers of glassy methanolic (aqueous) solutions of KHCO3 and HCl were deposited sequentially at 78 K on a CsI window, and their reaction on heating in vacuo in steps from 78 to 230 K was followed by Fourier transform infrared (FTIR) spectroscopy. After removal of solvent and excess HCl, IR spectra revealed formation of two distinct states of amorphous carbonic acid (H2CO3), depending on whether KHCO3 and HCl had been dissolved in methanol or in water, and of their phase transition to either crystalline alpha- or beta-H2CO3. The main spectral features in the IR spectra of alpha- and beta-H2CO3 are observable already in those of the two amorphous H2CO3 forms. This indicates that H-bond connectivity or conformational state in the two crystalline phases is on the whole already developed in the two amorphous forms. The amorphous nature of the precursors to the two crystalline polymorphs is confirmed using powder X-ray diffraction. These diffractograms also show that alpha- and beta-amorphous H2CO3 are two distinct structural states. The variety of structural motifs found within a few kJ/mol in a computational search for possible crystal structures provides a plausible rationalization for (a) the observation of more than one amorphous form and (b) the retention of the motif observed in the amorphous form in the corresponding crystalline form. The polyamorphism inferred for carbonic acid from our FTIR spectroscopic and powder X-ray diffraction studies is special since two different crystalline states are linked to two distinct amorphous states. We surmise that the two amorphous states of H2CO3 are connected by a first-order-like phase transition.

Aqueous Carbonic Acid (H2CO3)

“…This transition takes place within hours at~200 K and within minutes at~220 K and was followed by Raman [28] and IR spectroscopy and powder X-ray diffraction. [29] The basic structural motif found in the two polymorphs of carbonic acid has already developed in the two amorphous forms. From spectroscopic considerations the building block in the b-polymorph and in the amorphous precursor is a centrosymmetric unit such as the cyclic carbonic acid dimer.…”

The formation and stability of carbonic acid on outer Solar System bodies

“…1 b and 1 c), some peaks shift, some bands narrow, and some splittings are observed. As an example, the broad, weak band near 2555 cm -1 (3.914 µm) sharpens considerably and moves to 2612 cm-1 (3.828 µm) on warming to 240 K. Additional details on the shifts and intensity changes of 142CO3 features can be found in Winkel et al (2007).…”

Raman‐spektroskopische Studie der Phasenumwandlung von amorpher in kristalline β‐Kohlensäure

“…[18][19][20][21][22][23][24] Fourier-Transformation(FT)-IR-spektroskopische Studien belegen die Bildung unterschiedlicher Polymorphe. Ein Polymorph (b-H 2 CO 3 ) entsteht bei hochenergetischer Bestrahlung [10][11][12][13][14][15][16][17] oder bei Protonierung in gefrierkonzentrierten wässrigen Lösungen, [19,20,22,24] das andere (a-H 2 CO 3 ) bei Protonierung in methanolischer Lösung. [18,19,[21][22][23][24] b-H 2 CO 3 kann durch Behandlung mit Methanol/HCl in a-H 2 CO 3 überführt werden.…”

“…Ein Polymorph (b-H 2 CO 3 ) entsteht bei hochenergetischer Bestrahlung [10][11][12][13][14][15][16][17] oder bei Protonierung in gefrierkonzentrierten wässrigen Lösungen, [19,20,22,24] das andere (a-H 2 CO 3 ) bei Protonierung in methanolischer Lösung. [18,19,[21][22][23][24] b-H 2 CO 3 kann durch Behandlung mit Methanol/HCl in a-H 2 CO 3 überführt werden. [19] Bisher wurden diese beiden Polymorphe nur IR-spektroskopisch charakterisiert.…”

“…[36] enthält mehrere Beispiele für ein zufälliges Zusammenfallen von Raman-und IR-Banden in cyclischen Dimeren kristalliner Carbonsäuren. Das Raman-Spektrum von a-H 2 CO 3 wiederum enthält mehrere Raman-Banden, die mit IRBanden zusammenfallen, [18,19,[21][22][23][24] stimmt. [37] Kristalline Kohlensäure als Dicarbonsäure kann intermolekulare Wasserstoffbrückenbindungen zwischen dem OH-Protonendonor und dem C = O-Protonenakzeptor bilden, wobei entweder Dimere oder Catemere entstehen.…”