On the interaction between supercritical CO2 and epoxides combining infrared absorption spectroscopy and quantum chemistry calculations

The nature and strength of the interactions occurring between epoxides and CO2 have been investigated by combining infrared spectroscopy with quantum chemistry calculations. A series of infrared absorption experiments on four model epoxide molecules highly diluted in supercritical CO2 have been performed at constant temperature T = 40 degrees C for various CO2 pressures varying from 1 to 30 MPa. Then, we carried out a theoretical analysis based on quantum chemistry calculations using Density Functional Theory (B3PW91 and CAM-B3LYP) and ab initio (MP2) computational methods. A very good agreement between experimental and calculated vibrational frequency shifts of the epoxide ring vibrations group was obtained using the CAM-B3LYP functional, hence validating the calculated optimized geometries of the epoxide CO2 complexes. Whatever the epoxide considered, CO2 is found to be on average above the oxygen atom of the epoxy ring and interacts with the carbon atom of CO2 through a Lewis acid-Lewis base type of interaction. The substituents on the epoxide ring are found to influence the stability of the epoxide-CO2 complexes mainly because of the partial charge on the oxygen atom that is sensitive to the nature of the substituent.