Insights into the interactions of CO2 with amines: a DFT benchmark study

The interaction between CO2 and 1,2-diaminoethane was computed using pure and hybrid density functionals. The CAM-B3LYP and wB97X-D functionals using a triple-zeta basis set that includes diffuse and polarization functions are the best functionals for calculating the relative energies of the zwitterion intermediate compared to a coupled-cluster with a single, double and non-iterative triple excitation (CCSDM) approach extrapolated to a complete basis set limit. With the two functionals and the triple-xi basis set, the zwitterion is 1.70 kcal mol(-1) less stable than the reactants, and close to 1.63 kcal mori computed using the CCSDM approach. The inclusion of vibrational and thermal corrections and of entropic effects increases the relative energy of the zwitterion to 14.7 kcal mol(-1). Bending of the CO2 geometry increases its acidity due to a 1.09 eV reduction in the LUMO energy. Calculation of the CO2 interaction energy with a set of amines revealed that the interaction energies show a high correlation with their basicities, with the stronger bases stabilizing the zwitterion. For the most basic amine computed (3,4,6,7,8,9-hexahydro-2H-pyrimido[1,2-a]pyrimidine), the Gibbs free energy of the zwitterion is 15.8 kcal mol(-1) lower than the reactants. Therefore, for this highly basic amine, the zwitterion may have a longer lifetime, in contrast to 2-aminoethanol (MEA), where it is only a transient species.