Gas‐Phase Preparation of Carbonic Acid and Its Monomethyl Ester

Carbonic acid (H 2 CO 3 ), 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 H 2 CO 3 and its monomethyl ester (CH 3 OCO 2 H) through the gas‐phase pyrolysis of di‐ tert ‐butyl and tert ‐butyl methyl carbonate, respectively. H 2 CO 3 and CH 3 OCO 2 H 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 H 2 CO 3 , this is not true for the previously reported α‐polymorph. Instead, the vapor phase above α‐H 2 CO 3 corresponds to CH 3 OCO 2 H, which sheds new light on the research that has been conducted on molecular H 2 CO 3 over the last decades.