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

Abstract

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 °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.