Spectroscopic observation of matrix-isolated carbonic acid trapped from the gas phase.

Carbonic acid (H2CO3) is of fundamental importance, for example, for regulation of blood pH, in the acidification of the oceans, and in the dissolution of carbonates. This six-atom molecule commonly found in carbonated drinks in submicromolar concentrations has so far eluded most attempts at isolation and direct detection. Despite the widespread belief that it is a highly instable molecule, the pure solid could be prepared previously, and it is thought that solid carbonic acid exists in cirrus clouds on Earth and in astrophysical environments. Gas-phase carbonic acid was long thought to immediately decompose to water and carbon dioxide, and therefore to be nonexistent or detectable only as a trace component. We show herein that gas-phase carbonic acid is stable at temperatures above 200 K and describe the trapping of carbonic acid vapor in an inert matrix at 6 K. Spectroscopic analysis of this matrix reveals that carbonic acid vapor is composed of at least three species (Scheme 1): two monomeric conformers and the cyclic dimer (H2CO3)2; carbon dioxide and water are minor components. The molar ratio of the two monomers suggests that the cis-cis monomer is the most stable one, and the cis-trans monomer is less stable by about 4 kJmol , in accordance with theoretical predictions. The stability of gas-phase carbonic acid at temperatures above 200 K suggests that carbonic acid may sublime in astrophysical environments without decomposition, for example, on the poles of Mars or in the coma of comets such as Hale-Bopp, and, therefore, our infrared spectra represent a benchmark for possible identification of naturally occurring carbonic acid vapor.