Tuning The Fermi Resonance In Molecules By Pressure And Solvent

Fermi resonance（FR）is a vibration coupling and energy transfer phenomenon that is common in the infrared and Raman spectra of polyatomic molecules,and can be observed in various molecules,which plays an important role in spectral analysis.Fermi resonance phenomena are sensitive to external disturbances such as pressure,temperature and solvent,and are reflected in the spectrum as Fermi bimodal displacement,intensity signal changes and energy redistribution.Combining these phenomena,information such as molecular energy conversion and structural changes can be analyzed.The in-depth study of Fermi resonance model and the summary and grasp of the law are of great significance to the exploration of biological macromolecules,polymers and many other fields,and play an important role in materials,physics,chemistry,biology and many other disciplines.Raman scattering spectroscopy is an effective tool for non-destructive analysis of substances.Raman spectroscopy of molecules can provide more detailed information related to specific chemical bonds or functional groups,and has the advantages of short collection time,small sample size and simple sample preparation.In this paper,Raman spectroscopy combined with diamond anvil-high pressure experiment technology were used to explore the regulation of environmental changes on different types of Fermi resonance molecules:pyridine（fundamental and fundamental,fundamental and fundamental and combination）,carbon disulfide（fundamental and doubling）,α-hydroquinone（fundamental and fundamental）.And the following innovative results were obtained:（1）The Raman spectra of pyridine–ethanol binary solutions at different concentration fractions were obtained.The results indicated that C-H vibrational Raman peak of ethanol,and the Fermi resonance parameter showed a non-linear dependence on pyridine concentration,and that varying the concentration fraction of pyridine leads to the formation of intermolecular hydrogen bonds with different strengths,and that the electronic behaviour of the hydrogen bonding between pyridine and ethanol explains the emergence of a new Raman peak at 997 cm^-1,which was due to the presence of a polymer of hydrogen bonding.Secondly,the interaction mechanism of the pyridine-ethanol binary solution was analysed using two-dimensional correlation Raman spectroscopy,and the new peakν₁′generated by molecular polymerisation was more sensitive to environmental changes,suggesting that the hydrogen-bonded structure is preferentially responding to the pyridine to the concentration change.In addition,high-pressure Raman spectra of a 50%pyridine-ethanol binary solution at a pressure were measured（0.47-19.65 GPa）,pyridine mixed with ethanol was found to undergo a phase transition at a pressure of 6.35 GPa compared to 1.00 GPa for pure pyridine,suggesting that the intermolecular hydrogen bonding formed by the addition of ethanol makes the pyridine structure more stable.The results demonstrated that changes in the intensity ofν₁ did not affect the FR ofν₁+ν₆～ν₈,and the two Fermi resonance in the pyridine molecule are independent of each other.At 13.36 GPa,the disappearance of Raman peaksν₁’andν₁ were attributed to the weakening of hydrogen bonding and the adjustment of molecular symmetry by pressure during the phase transition.（2）The Raman spectra of CS₂ mixed with methanol and ethanol at different concentration fractions were investigated.The results indicated that there were weak hydrogen bonding interactions in the binary solution.In addition,the geometries were constructed and optimised with DFT calculations,and vibrational and interaction energy analyses were carried out,and the simulated Raman spectra were in agreement with the experimental results.Under the hydrogen bonding interaction,the transfer of energy from 2ν₂ toν₁,and the Fermi resonance coupling strength of CS₂ was weakened.Furthermore,in the in situ high-pressure Raman spectroscopy study,CS₂ underwent a pressure-induced polymerisation phase transition（0.38-9.19 GPa）.The results demonstrated that the pressure-induced polymerization phase transition of CS₂molecules causes the close packing and more orderly arrangement of molecules,resulting in the enhancement of FR coupling,which indicates the transfer of energy fromν₁ to 2ν₂.From the point of view of the strength of Fermi resonance coupling and the energy transfer,the hydrogen bonding and high pressure modulate the Fermi resonance of the CS₂ molecule in an opposite way.（3）The Raman and infrared spectra ofα-hydroquinone were studied at ambient pressure,and then the in situ high-pressure Raman spectra ofα-hydroquinone were investigated at 19.64 GPa.The results showed that two pressure-induced phase transitions occurred inα-hydroquinone at 3.61 and 12.46 GPa.At ambient pressure,there is no Fermi resonance between fundamental frequencies in theα-hydroquinone molecule.After the phase transition at 3.61 GPa,831 and 854 cm^-1 had the same symmetry,which satisfied the symmetry requirement of Fermi resonance.By calculating the Fermi resonance parameters,theΔ₀ tended to 0 after the phase transition,indicating that the two Raman peaks had similar vibrational energies,which Satisfied the energy requirement of the Fermi resonance.The intensity ratio R was consistent with the law that the intensity ratio of the Fermi bimodal peaks between fundamental frequencies under pressure decreases exponentially with the increase of pressure.The pressure-induced Fermi resonance phenomenon was generated between two fundamental frequency vibration modes（C-H bending vibration and C-C stretching vibration modes）.In this paper,the pressure and hydrogen bonding effects on the tuning of Fermi resonance of different types of molecules were investigated,and the tuning mechanism was elucidated,which provides theoretical references for exploring the physical mechanisms of molecular structure and vibrational changes under perturbation and establishing the Fermi resonance model in the fields of Fermi resonance modeling.