Substituent effect on the interaction of aromatic primary amines and diamines with supercritical CO2 from infrared spectroscopy and quantum calculations

The nature and the strength of the interactions occurring between aromatic primary amines and CO2 have been investigated by combining infrared spectroscopy with molecular modelling. A series of infrared absorption experiments on various aromatic mono- and diamines in supercritical CO2 have been performed at constant temperature (T = 40 °C) for various CO2 pressures varying from 6 to 30 MPa. Then, we carried out a theoretical analysis based on quantum calculations using both density functional (B3LYP) and ab initio (MP2) computational methods. Whatever the amine considered, CO2 is found to be on average above the nitrogen atom of the NH2group for which the donating lone pair interacts with the carbon atom of CO2. Several types of interactions have been identified, namely, electron donor–acceptor (EDA), hydrogen bonds as well as dispersion forces. Contrary to aliphatic amines, the dispersion interaction is significant in the aromatic amine–CO2 complexes because of the presence of the aromatic ring. The substituents are found to influence the stability and structure of the amine–CO2 complexes, directly by electrostatic and steric effects of the substituent, and indirectly through the change in the partial charge on the nitrogen atom. Finally, a good correlation has been put in evidence between the partial charge on the nitrogen atom and the EDA interaction occurring between the aromatic amines and CO2.