Synthesis and Crystal Structure of Bulk Methanetetrol C(OH)4

Methanetetrol, C(OH)4 (orthocarbonic acid), is an elusive tethrahydroxymethane that, due to its rapid dissociation under ambient conditions, has never been synthesized in bulk. A recent study detected the formation of C(OH)4 molecules in the gas phase. Here, we show that C(OH)4 can be synthesized in a diamond anvil cell through the reaction between H2O and CO2 at 45(3) GPa and 50(3) GPa and temperatures <1200(200) K. The crystal structure of C(OH)4 was determined by single-crystal X-ray diffraction and corroborated by density-functional theory calculations together with Raman spectroscopy. It is composed of isolated [C(OH)4]-molecules linked by strong hydrogen bonds. Under high-pressure conditions, orthocarbonic and carbonic acids are denser than coexisting water ice phases, allowing them to segregate gravitationally and form stable reservoirs of oxidized carbon. These phases incorporate large amounts of carbon into their crystal structures as part of [CO4]4–- or [CO3]2–-building blocks. Because the amount of carbon that can be stored as CO2 in clathrates or filled-ices is significantly smaller, orthocarbonic and carbonic acids may provide a host mechanism for sequestering large carbon inventories in the icy mantles of water-rich exoplanets.