Electron Donor−Acceptor Interactions in Ethanol−CO2 Mixtures:  An Ab Initio Molecular Dynamics Study of Supercritical Carbon Dioxide

The nature of interactions between ethanol and carbon dioxide has been characterized using simulations via the Car−Parrinello molecular dynamics (CPMD) method. Optimized geometries and energetics of free-standing ethanol−CO2 clusters exhibit evidence for a relatively more stable electron donor−acceptor (EDA) complex between these two species rather than a hydrogen-bonded configuration. This fact has also been confirmed by the higher formation rate of the EDA complex in supercritical carbon dioxide−ethanol mixtures. The probability density distribution of CO2 molecules around ethanol in the supercritical state shows two high probability regions along the direction of the lone pairs on the oxygen atom of ethanol. The EDA interaction between ethanol and CO2 as well as that between CO2 molecules themselves leads to significant deviations from linearity in the geometry of the CO2 molecule. The vibrational spectra of carbon dioxide obtained from the atomic velocity correlation functions in the bulk system as well as from isolated complexes show splitting of the ν2 bending mode that arises largely from CO2−CO2 interactions, with ethanol contributing only marginally because of its low concentration in the present study. The stretching frequency of the hydroxyl group of ethanol is shifted to lower frequencies in the bulk mixture when compared to its gas-phase value, in agreement with experiments.