Gas‐Phase Preparation of Carbonic Acid and Its Monomethyl Ester

Abstract

Carbonic acid (H₂CO₃), an essential molecule of life (e.g., as bicarbonate buffer), has been well characterized in solution and in the solid state, but for a long time, it has eluded its spectral characterization in the gas phase owing to a lack of convenient preparation methods; microwave spectra were recorded only recently. Here we present a novel and general method for the preparation of H₂CO₃ and its monomethyl ester (CH₃OCO₂H) through the gas‐phase pyrolysis of di‐tert‐butyl and tert‐butyl methyl carbonate, respectively. H₂CO₃ and CH₃OCO₂H were trapped in noble‐gas matrices at 8 K, and their infrared spectra match those computed at high levels of theory [focal point analysis beyond CCSD(T)/cc‐pVQZ] very well. Whereas the spectra also perfectly agree with those of the vapor phase above the β‐polymorph of H₂CO₃, this is not true for the previously reported α‐polymorph. Instead, the vapor phase above α‐H₂CO₃ corresponds to CH₃OCO₂H, which sheds new light on the research that has been conducted on molecular H₂CO₃ over the last decades.

Das IR‐Spektrum der matrixisolierten Kohlensäure, die durch die Gasphasenpyrolyse von Boc₂O (Boc=tert‐Butoxycarbonyl) oder (tBuO)₂CO erhalten wurde, belegt zweifelsfrei, dass die Gasphase über ihrem α‐Polymorph nicht aus Kohlensäure besteht, sondern aus dem entsprechenden Monomethylester, der gleichermaßen ausgehend von MeOC(O)OtBu synthetisiert und erstmalig in der Gasphase identifiziert wurde.