The Role of Negative Hyperconjugation in Decomposition of Bicarbonate and Organic Carbonate Anions

Abstract

Intrinsic electronic effects of the bicarbonate anion and its organic derivatives have been examined computationally. Decomposition pathways of methyl hydrogen carbonate and a series of methyl aryl/alkyl carbonate diesters, via the bicarbonate anion and aryl/alkyl carbonate anions, respectively, to final products CO2 and the hydroxide anion or aryloxide/alkoxide anions, have been calculated at the MP2(full)/6-311+G(2d,p) level of theory in the gas phase as well as with the IEFPCM solvation model. The optimized geometries of the bicarbonate anion and its derivatives have revealed that the O2C−OH/Ar/R bond is noticeably elongated. In addition, NBO analysis has indicated that the transfer of electron density from the carboxylate moiety into the antibonding σ* orbital of the O2C−OH/Ar/R bond occurs, weakening the bond noticeably. In aromatic carbonates, the anionic charge is further delocalized to the aromatic ring, additionally reducing the negative charge in the carboxylate moiety and weakening the O2C−OAr bond. These indicators show that the negative (inverse) hyperconjugation is operative in these anions, determining their thermodynamic and kinetic stability toward both the recombination with a cation and further decomposition.