Tuning the Fermi resonance of pyridine using ethanol molecules

The Fermi resonance (FR) phenomenon is prevalent in infrared and Raman spectroscopy, and it can be observed in a variety of molecules. In particular, pyridine is a compound that has two Fermi doublets: ν₁ ∼ ν₁₂ and ν₁ + ν₆ ∼ ν₈. To analyze the effect of environmental changes on the FR, this study first investigated the Raman spectra of pyridine mixed with ethanol at different concentrations. Results indicated that the FR parameters exhibited a nonlinear dependence on the pyridine concentration fractions, and changing the concentration fraction of pyridine led to different hydrogen bond strengths. Second, the interaction mechanism of pyridine-ethanol binary solutions was analyzed by two-dimensional correlation Raman spectroscopy (2DCRS). In addition, high-pressure Raman spectra of a 50% pyridine-ethanol binary solution were measured up to a pressure of 19.65 GPa by a diamond anvil cell technique, and the phase transition of the binary solution occurred at 6.35 GPa. Finally, the impact of ethanol on the FR of pyridine was determined by deducing the FR parameters at different pressures. The turning point at 6.35 GPa was consistent with the Raman frequency-pressure relationships. The results demonstrated that changes in the intensity of ν₁ did not affect the FR of ν₁ + ν₆ ∼ ν₈, whereas the undisturbed frequency ν₁ still played a role in the FR. When the pressure was compressed to 13.36 GPa, the disappearance of the Raman peaks (ν₁ and ν₁') was attributed to the tuning of the molecular symmetry by pressure during the phase transition.