Microscopic Investigation of Ethylene Carbonate Interface: A Molecular Dynamics and Vibrational Spectroscopic Study

Ethylene carbonate (EC) liquid and its vapor-liquid interface were investigated using a combination of molecular dynamics (MD) simulation and vibrational IR, Raman and sum frequency generation (SFG) spectroscopies. The MD simulation was performed with a flexible and polarizable model of the EC molecule newly developed for the computation of vibrational spectra. The internal vibration of the model was described on the basis of the harmonic couplings of vibrational modes, including the anharmonicity and Fermi resonance coupling of C＝O stretching. The polarizable model was represented by the charge response kernel (CRK), which is based on ab initio molecular orbital calculations and can be readily applied to other systems. The flexible and polarizable model can also accurately reproduce the structural and thermodynamic properties of EC liquid. Meanwhile, a comprehensive set of vibrational spectra of EC liquid, including the IR and Raman spectra of the bulk liquid as well as the SFG spectra of the liquid interface, were experimentally measured and reported. The set of experimental vibrational spectra provided valuable information for validating the model, and the MD simulation using the model comprehensively elucidates the observed vibrational IR, Raman, and SFG spectra of EC liquid. Further MD analysis of the interface region revealed that EC molecules tend to orientate themselves with the C＝O bond parallel to the interface. The MD simulation explains the positive Im$ \chi ^{(2)}$](ssp) band of the C＝O stretching region in the SFG spectrum in terms of the preferential orientation of EC molecules at the interface. This work also elucidates the distinct lineshapes of the C＝O stretching band in the IR, Raman, and SFG spectra. The lineshapes of the C＝O band are split by the Fermi resonance of the C＝O fundamental and the overtone of skeletal stretching. The Fermi resonance of C＝O stretching was fully analyzed using the empirical potential parameter shift analysis (EPSA) method. The apparently different lineshapes of the C＝O stretching band in the IR, Raman, and SFG spectra were attributed to the frequency shift of the C＝O fundamental in different solvation environments in the bulk liquid and at the interface. This work proposes a systematic procedure for investigating the interface structure and SFG spectra, including general modeling procedure based on ab initio calculations, validation of the model using available experimental data, and simultaneous analysis of molecular orientation and SFG spectra through MD trajectories. The proposed procedure provides microscopic information on the EC interface in this study, and can be further applied to investigate other interface systems, such as liquid-liquid and solid-liquid interfaces.