Raman investigation of the CO2 complex formation in CO2-acetone mixtures

Polarized and depolarized Raman spectra of CO2-acetone mixtures have been measured along the isotherm 313 K as a function of CO2 concentration (0.1-0.9 molar fractions in CO2) by varying the pressure from 0.2 up to 8 MPa. Upon CO2 addition, a new band appears at about 655 cm(-1) and is assigned to the lower frequency nu 2(1) component of the bending mode after degeneracy removal due to the formation of a 1:1 electron donor acceptor (EDA) CO2 complex. The equilibrium constant associated with the complex formation was estimated and found close to those of contact charge transfer complexes. The main modifications of the Fermi dyad of CO2 in the mixtures compared with that of pure CO2 at equivalent density have been assessed. The band-shape analysis revealed that each dyad component is described by two Lorentzian profiles, showing that a tagged CO2 molecule probes two kinds of environment in its first shell of neighbors. The first one involves nonspecific interactions of CO2 with surrounding acetone whereas the second is assigned to the signature of 'transient' CO2 complexes formed with acetone. An upper bound life time of the complex has been estimated to be 8 ps. In addition, a broad band has been detected between the Fermi dyad peaks at about 1320 cm(-1) and its origin interpreted as a further evidence of the CO2-acetone heterodimer formation. Finally, the values of the equilibrium concentration of the heterodimer versus the total concentration of CO2 deduced from the analysis of the nu 2(1) band and from the Fermi dyad have been compared, and the difference is interpreted as due to a lack of theoretical approach of Fermi resonance transitions associated with species existing in different environments.