Theoretical study of the structure of neutral, radical and anionic monoperoxo carbonic acid

Bicarbonate ion reacts with aqueous hydrogen peroxide, yielding monoperoxocarbonate (HCO4−, PCA−). This species is a better oxidizing agent than hydrogen peroxide itself and has been shown to lead to double bond epoxidation of alkenes under mild conditions. The structure of this anion as well as that of the parent monoperoxocarbonic acid (H2CO4, HPCA) are not known experimentally. The structures of the neutral, anionic and radical species derived from H2CO4 are reported in this paper employing DFT and MP2 methods, both in gas phase and simulating the bulk water solvent using the PCM method. Several conformers are found, of which the most stable one in gas phase has a planar, hydrogen-bonded structure similar to the one observed for peroxyformic acid (PFA). However, optimization of this structure in the bulk solvent leads to a non-planar conformation with a gauche disposition of the bonds in the COOH group. It is furthermore shown that the barriers separating the conformers are much smaller in solution, thus allowing easy interconversion.