| Fermi resonance (FR) is a phenomenon about anharmonic coupling and energy-transfer occurring extensively in intra-inter-molecules of similar vibrational mode and same symmetry. The study of FR is theoretically meaningful in the investigation of molecular vibrational states, the intercoupling of electronic states, as well as the molecular structures and properties, etc in Physics. Furthermore, it is also importantly useful in the identification and the assignment of spectral lines in Materials, Biology and Chemistry, etc such as the investigation of biomolecules, the textual research on the efficacy of anti-cancer drugs, the analysis on internal pressure inside inclusion in Geology, the impurity detection in crystals, as well as the investigations of acoustical and optical instruments, etc. Moreover, the investigation of FR under high pressure is closely concerned by researchers because of its important application in the study of molecular structures, properties and phase transition under high pressure. Nevertheless, the previous investigations of FR under high pressure focus little on polyatomic molecules, which were conducted mostly under much lower pressure. Moreover, to the best of our knowledge, previous investigations also focus little on FR occurred in binary solution under high pressure, especially on the comparison between the FR behaviors occurred respectively in binary solution and in neat liquid. Furthermore, the study on which two different FR phenomena occurred simultaneously in one molecule has not yet been reported before.This paper mainly researches the law of binary solution Fermi resonance under high pressure. The Raman spectra of binary solution (CCl4 and C6H6) and pure liquid have been measured up to pressures of 11 GPa. The results show that pressure effect on binary solution is different from pure liquid:mixing two liquid, owing to the changes of density of the solution, intermolecular distance decreases and interaction energy increases, the frequency shift (blue shift) of spectral bands increases, the frequency shift of binary liquid is faster than pure liquid frequency shift. Phase transitions (spectral bands splitting) change earlier and natural frequency differenceΔ0 increases with increasing pressure, the Fermi resonance bandsν1+ν6 andν8 of benzene andν1+ν4 andν3 of CCl4 disappear as the pressures decrease gradually, the spectral bands with different compressibility have different speed, CCl4 has smaller density, longer bond, smaller force constant and larder compressibility and is easy to compress, C6H6 has larger density, smaller bond, larger force constant, smaller compressibility and is hard to compress. The frequency shift of CCl4 is faster than benzene. In this paper we provide good reference on Raman bands assignment, certification under high pressure, also provide methods and ideas for study of the different environment of high pressure effect, intermolecular interaction and solvent effect.We analyze the law of pyridine Fermi resonance (ν1～ν12 andν1+ν6～ν8) in H2O solution under high pressure. The result shows that all the Raman spectral lines of C5H5N in H2O solution shift more markedly faster than their counterparts in neat C5H5N respectively. Also, the Raman activity of the fundamentalν1 of C5H5N in solution decreases more rapidly than in neat liquid. Meanwhile the FR behavior in aqueous solution varies much faster than that in neat liquid too, which means that the pressure effect in binary solution accelerated the shift of FR.The Raman spectra of neat pyridine under high pressure are measured. When the behaviors of Fermi resonance ofν1～ν12 andν1+ν6～ν8 with pressure being analyzed respectively, we find that the Raman activity of the fundamentalν1 decrease and disappeare eventually with pressure increasing, which induces weakening and disappearance in intensity of Fermi resonance betweenν1 andν12. Nevertheless, the variation of Raman intensity ofν1 has no effect on the presence and the behavior of Fermi resonance betweenν1+vν6 andν8 under high pressure, theν1 mode does influence the behavior of the FR betweenν1+ν6 andν8, those phenomena mentioned above are interpreted by group theory and Bertran theory in this article.
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