Investigating local field tuning Fermi resonance of CS₂ by Raman spectroscopy and DFT calculations

In the spectroscopic study of polyatomic molecules, Fermi resonance (FR) is a vibrational coupling and energy transfer phenomenon that widely exists intra- and intermolecular. In particular, the FR coupling between the fundamental mode ν₁ and the doubling mode 2ν₂ of the CS₂ molecule has attracted extensive research. In this work, we investigate the effect of local field on tuning the FR of CS₂. By analyzing the Raman spectra of CS₂ mixed with methanol and ethanol with different mole fractions, the results indicated that weak HBs interactions in binary solutions can be reflected by the linear frequency shift of the C-H bond vibrations (in methanol and ethanol) with different molar concentrations. Furthermore, the geometrical structure was optimized using DFT simulation, and the vibration analysis and interaction energy were carried out. The simulated Raman spectra are in good agreement with the experiments. In addition, high-pressure Raman spectra of CS₂ were obtained by diamond anvil cell technique (up to 9.19 GPa) and a pressure-induced phase transition was observed at 1.71 GPa. The results demonstrated that the pressure-induced polymerization phase transition of CS₂ molecules causes the close packing and more orderly arrangement of molecules, resulting in the enhancement of FR coupling. HB and high pressure tune the FR of the CS₂ molecule differently.