| Fermi resonance(FR)is a phenomenon of energy transfer due to vibrational coupling,which existed widely within and between molecules.Generally,changes of local environment will cause varieties in molecular vibration,intermolecular binding mode,electron cloud distribution and so on.Moreover,such changes also have an effect on the energy transfer between the bimodal peaks of FR coupling.Since FR can accelerate the energy transfer between vibration modes,it affects the kinetic behavior of molecules and the rate of chemical reactions.Therefore,it is of great significance to understand the molecular vibration,electronic state coupling,molecular structure and properties by experimentally studying the regulation effect of local environment changes on FR,and deeply exploring its internal mechanism.At the same time,it has extremely important applications in spectrum line authentication and recognition,material science,biology and chemistry and other fields.Raman spectroscopy is a powerful tool for studying the structure of materials,with the advantages of simple sample preparation,nondestructive testing and high resolution.Raman spectroscopy can be applied to study the vibration and rotation of molecules and their interaction with electronic states.Based on this,by extending the original spectral signal to two-dimensional area,two-dimensional correlation Raman spectroscopy(2DCRS)can effectively improve spectral resolution,identify overlapping peaks and weak peaks,and establish correlation and temporal relationship between spectral changes brought through external disturbance,thus obtaining information that cannot be reflected in one-dimensional spectrum.In this paper,polar and non-polar Fermi resonance molecules were investigated by Raman spectroscopy and two-dimensional correlation analysis.The following innovative results are obtained by exploring the modulating effect of local environmental changes on the energy transfer between three typical Fermi doublets,acetonitrile,carbon tetrachloride,and pyridine,was explored respectively:(1)The FR of the AN-water mixture2?3+4 was studied using Raman spectroscopy.Based on the calculated FR parameters,it was found that the FR(2?3+4)was modulated by the hydrogen bonding network in water.There is a nonlinear modulation of the FR by the mixture with increasing AN concentration.At different mixture concentrations,there are different molecular bonding modes between AN and water molecules.The FR is related to the molecular bonding mode of AN-water and the percentage of different modes.In the temperature range of293?173 K,the FR parameters of 60%AN-water binary solutions show four stages:In the first stage(293-273 K),the energy transfer is caused by microscopic inhomogeneities and changes in the fundamental band(3 and4);In the second stage(263-233 K),phase separation leads to significant changes in peak position and area of2 mode,leading to changes of FR;In the third stage(223-233 K),there is a phase transition from liquid to solid?;In the fourth stage(213-203 K),AN changes from solid phase?to solid phase?.The combined analysis of experimental data and2DCRS method reveals the micro mechanism of FR bimodal energy transfer caused by hydrogen bond and temperature change.(2)The effect of N-methylpyrrolidone(NMP)on the FR(3?1+4)of CCl4was studied.In the range of NMP concentration from 0 to 100%,the electron cloud distribution of non-polar molecule CCl4 are changed by the perturbation of NMP,and the change of the interaction force between CCl4 molecules will modulate the FR.In the low temperature experiment of CCl4,the combination band(1+4)splitting occurs,which indicates that the structure symmetry and the molecular arrangement of CCl4 molecular changes,resulting in phase transition.The further analysis of 2DCRS shows that at low temperature,the splitting of combination band is affected by the effects of both fundamental band4 and FR.While3,1,4 are more sensitive to environmental changes,resulting in the energy transfer between Fermi double lines from combination band1+4 to3.(3)The FR effect(?1??12)of pyridine(Py)-water binary solution was studied.It is foundthe hydrogen bond(OHN)formed between Py-water plays a regulatory role on FR mainly in two aspects:1.The formation of hydrogen bonds leads to a more uniform distribution of electrons in pyridine.With the increasing concentration of water,the hydrogen bonds are more strengthened,and the degree of anharmonicity between Fermi double decreases,resulting in the decreased coupling coefficient W of FR;2.The different binding modes of Py and water will affect the proportion of different peaks at?1.So,the peak position of?1 in the ring breathing vibration mode is more susceptible to the modulation of hydrogen bond between Py and water,which leads to the change of FR.The splitting of the1 mode in the 90%Py-water binary solution was observed in low temperature.This phenomenon was caused by the change of phase separation and molecular binding.The phase transition of the solution system causes?12 changed accordingly,so the W of the solution increases.With the continuous cooling,the molecular vibration weakens,and the coupling coefficient W gradually decreases until the pyridine transforms into the solid phase.As the temperature decreases,the energy transfer of the Fermi doublet is regulated by the structure change of hydrogen bond and the phase transition of the solution.In this paper,solvent effect and temperature on three typical molecular FRs were studied,and the connection between vibrational modes and fundamental frequencies of the occurring FR was obtained in combination with 2DCRS.A clearer understanding of the mechanism of FR modulation by local environmental changes was established.This provides a strong support for the further development of the theoretical model of FR.At the same time,our results also clarified how the directional energy transfer between the FR coupling doublets occurs in a specific environment.It can provide a reference for the energy transfer phenomenon in the fields of biology,chemistry,geology,materials science,and astronomy research. |
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