Ab Initio Study of CO2 Capture Mechanisms in Aqueous 2-Amino-2-methyl-1-propanol: Electronic and Steric Effects of Methyl Substituents on the Stability of Carbamate

Ab initio quantum chemical calculations, combined with the SMD solvation model, have been conducted to reveal CO2 absorption mechanisms and the origin of the reduced stability of carbamate in aqueous solution of 2-amino-2-methyl-1-propanol (AMP). The calculated Gibbs free energy change for all the elementary steps in the conversion of carbamate to bicarbonate reveals that the reduced thermodynamic stability of AMP carbamate is largely ascribable to the shift of the equilibrium from the carbamate to the zwitterion intermediate, from which CO2 can be released and successively converted to bicarbonate. This feature of AMP is an attribute of the electronic effects of the substitution by methyl groups specifically at the α-carbon. Furthermore, the steric effects of the methyl groups are investigated from a dynamical point of view that suggests a new scenario in which the methyl groups promote the release of CO2 from the zwitterion intermediate.