Understanding the formation mechanism of crystalline hydrated polymorphs of carbonic acid from CO2 clathrate hydrate

Carbon dioxide (CO2) and water (H2O) icy mixtures, together with a few other simple molecules, have a prominent role in astrochemical processes. The formation of the solid 1 : 1 CO2 : H2O adduct, carbonic acid (H2CO3), has been long studied because of its importance in the biological and geochemical domains. Our recent discovery of a novel, highly reproducible synthetic path to obtain crystalline hydrated H2CO3 polymorphs from low temperature compression of CO2 clathrate hydrate highlighted the role of pressure in the chemistry of crystal ices. Herein, we report an extensive study about the reaction mechanism leading to this synthesis, unveiling a multi-step temperature-governed process that involves the formation of carbonic acid molecules starting from about 200 K and their organisation in a crystalline lattice at temperatures close to 270 K. In addition, the essential role played by the clathrate in the formation of crystalline carbonic acids is clearly highlighted, while the entire process meticulously mimics what can occur during the subduction of icy materials.