Gas-phase preparation of carbonic acid and its monomethyl ester

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.