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.

Carbonic acid monohydrate

“…1. The vacuum thermolysis of carbonates led to the first reported synthesis of H 2 CO 3 , in the vapor phase; originally using ammonium bicarbonate (Terlouw et al 1987), the process was more recently accomplished with di-tert-butyl carbonate (Reisenauer et al 2014). 2.…”

“…Direct confirmation of the molecular composition has been confined to the gas phase syntheses, through neutralizationreionization mass spectrometry (Terlouw et al 1987) and microwave spectroscopy (Mori et al 2009, 2011); X-ray diffraction patterns were reported for an amorphous solid (Winkel et al 2007), however attempts to provide diffraction patterns of a crystallized sample were inconclusive (Mitterdorfer et al 2012;Bernard et al 2013). These limitations gave room to the belief, for two decades, that two different configurations of H 2 CO 3 (then designated as -H 2 CO 3 and -H 2 CO 3 ) had their own distinct amorphous, crystalline, and vapor phases preserved through sublimation and condensation (Hage et al 1995(Hage et al , 1996(Hage et al , 1998Bernard et al 2011Bernard et al , 2013; not until 2014 was it discovered that only one of these phases (-H 2 CO 3 ) pertained to H 2 CO 3 , while the putative -H 2 CO 3 actually consisted of the related monomethyl ester (Reisenauer et al 2014).…”

Novel Stable Compounds in the C-H-O Ternary System at High Pressure

“…The latter adopt an almost flat conformation in which hydrogens are in cis position with respect to the C=O bond. This type of HBs arrangement was shown to be the most energetically favorable at ambient conditions, ‘cis-cis’ being the most stable conformation of the isolated molecule28. The crystal structure of carbonic acid is significantly denser than water ice (2.147 vs 1.560 g/cm 3 at 1 GPa).…”

Stable solid and aqueous H2CO3 from CO2 and H2O at high pressure and high temperature

“…Until recently it was believed, that crystalline H 2 CO 3 could exist as two separate polymorphs, α - and β -H 2 CO 3 , which could even be resublimed while retaining their polymorphic structure 10 11 . However, by employing a novel strategy for the synthesis of solid H 2 CO 3 through gas-phase pyrolysis of alkyl carbonates, Reisenauer et al 12 could demonstrate that what was known as α -H 2 CO 3 is actually its monomethyl ester (CH 3 OCO 2 H) rather than the acid itself, leaving the β -polymorph as the only observed crystal structure of H 2 CO 3 to date. Yet, the exact crystal structure of β -H 2 CO 3 is still unknown.…”

“…The FT-IR and Raman spectra of solid H 2 CO 3 below 220 K at low pressure are now well established and several theoretical studies have reported on the structure and stability of H 2 CO 3 with respect to decomposition to CO 2 and H 2 O as well as on the relative stability of its isomers and on their vibrational characteristics 7 8 13 14 15 . Its molecular decomposition rate has been investigated both experimentally 16 and theoretically and it was found to be accelerated over 10 12 times at ambient conditions by as few as three water molecules to reach macroscopically observed decomposition rates 1 17 . Interestingly, increased thermal stability of solid H 2 CO 3 at elevated pressure has been predicted by ab initio molecular dynamic simulations 18 .…”