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

Abstract

In this work, earlier studies reporting α‐H2CO3 are revised. The cryo‐technique pioneered by Hage, Hallbrucker, and Mayer (HHM) is adapted to supposedly prepare carbonic acid from KHCO3. In methanolic solution, methylation of the salt is found, which upon acidification transforms to the monomethyl ester of carbonic acid (CAME, HO‐CO‐OCH3). Infrared spectroscopy data both of the solid at 210 K and of the evaporated molecules trapped and isolated in argon matrix at 10 K are presented. The interpretation of the observed bands on the basis of carbonic acid [as suggested originally by HHM in their publications from 1993–1997 and taken over by Winkel et al., J. Am. Chem. Soc. 2007 and Bernard et al., Angew. Chem. Int. Ed. 2011] is inferior compared with the interpretation on the basis of CAME. The assignment relies on isotope substitution experiments, including deuteration of the OH‐ and CH3‐ groups as well as 12C and 13C isotope exchange and on variation of the solvents in both preparation steps. The interpretation of the single molecule spectroscopy experiments is aided by a comprehensive calculation of high‐level ab initio frequencies for gas‐phase molecules and clusters in the harmonic approximation. This analysis provides evidence for the existence of not only single CAME molecules but also CAME dimers and water complexes in the argon matrix. Furthermore, different conformational CAME isomers are identified, where conformational isomerism is triggered in experiments through UV irradiation. In contrast to earlier studies, this analysis allows explanation of almost every single band of the complex spectra in the range between 4000 and 600 cm−1.

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

“…However, at 10−20 K, solid H 2 CO 3 can also be formed through reactions 5−7 involving newly formed CO ice that does not desorb from the ice at temperatures below 30 K (Acharyya et al 2007). As pointed out by Oba et al (2010), this reaction pathway leads to the formation of a different form of H 2 CO 3 , namely the γ-polymorph (Köck et al 2020). The H 2 CO 3 synthesized at 20 K in our experiments is likely a combination of βand γ-polymorph ice.…”

“…α-H 2 CO 3 presents weaker absorption bands in the O-H stretch regions than β-H 2 CO 3 , which has its strongest absorption band around 2600 cm −1 . However, more recently, the synthesis of α-H 2 CO 3 in methanolic solutions was reassigned to the monomethyl ester of carbonic acid (CAME, HO-CO-OCH 3 , Reisenauer et al 2014;Köck et al 2020), leaving the β-polymorph as the only confirmed form of H 2 CO 3 to date. It is known that H 2 CO 3 synthesized through energetic irradiation of CO 2 :H 2 O ice mixtures produces amorphous β-H 2 CO 3 that crystallises if heated to temperatures above 220 K (Hage et al 1995(Hage et al , 1996bBernard et al 2013).…”

“…The structure of H 2 CO 3 obtained in this way differs from that of β-H 2 CO 3 and it is closer, but not identical, to what was once considered to be the structure of α-H 2 CO 3 . Hence, a γ-polymorph might have been produced in the work by Oba et al (2010) from the reaction of CO ice with OH radicals (Köck et al 2020) CO + OH → trans-HOCO…”

Linear and Helical Carbonic Acid Clusters

“…9−11 However, recent work with more advanced experimental techniques has shown that αcarbonic acid is actually the monomethyl ester of carbonic acid and is not a polymorph negating the need for any further analysis of the α phase. 12 Even so, efforts to describe βcarbonic acid computationally have been successful in correlating with experiments for infrared and Raman spectra of a linear octamer. 13 Attempts to construct a crystal structure through molecular dynamic methods suggest that the crystals with "sheet-like hydrogen bonding topologies" are among the most stable.…”

Increase of Radiative Forcing through Midinfrared Absorption by Stable CO2 Dimers?

“…We performed infrared spectroscopy measurements of mixtures of gases in frozen neon matrixes, i.e., matrix-isolation infrared (MI-IR) spectroscopy. Former studies 17 , 18 have shown that the composition of an atmosphere trapped in a frozen matrix resembles the gas phase composition. Thus, MI-IR spectroscopy is a suitable analytical technique for investigating short-lived and unstable species from the gas phase.…”