Raman Spectroscopic Study Of Typical Cyclic Anhydrides Under High Pressure

Pressure is a basic thermodynamic parameter and an effective method to control the structure of matter.It can adjust the atomic spacing,change the arrangement of atoms,and trigger the structural phase transition of matter.The pressure can also change the energy band structure of the material,and make the valence band and conduction band of the insulator overlap,thus making it become a conductor.Under high pressure,substances will exhibit new structures,new properties and new physical laws that do not exist under normal pressure.High pressure science has been closely combined with chemical production,and it is an important method to study the structure or electronic phase change of materials and synthesize new materials.Cyclic anhydride is an important derivative of carboxylic acid.It is the raw material of a variety of chemical products and plays an important role in medical,biological,chemical and other fields.In recent years,the copolymerization between it and epoxides has attracted much attention.The pressure can reduce the chemical reaction barrier,improve the reaction rate and productivity,and study the structural change of cyclic anhydride under high pressure,which is conducive to exploring new chemical reaction paths,reducing the reaction cost,and broadening the application prospects of cyclic anhydride.In this paper,two typical cyclic anhydrides,phthalic anhydride and pyromellitic dianhydride,were selected.The high pressure Raman spectra of these two samples were measured using diamond anvil technology and in-situ Raman spectroscopy measurement technology,and their structural phase transitions were studied.And the anharmonic coupling between the fundamental modes was explored.Phthalic anhydride（C₈H₄O₃）is formed by phthalic acid after dehydration,which plays an irreplaceable role in the chemical industry and is an intermediate in the production of a variety of important chemical products.In this paper,we performed a high-pressure Raman study of phthalic anhydride up to 16 GPa.The results show that most of the Raman bands move towards high wave number with the increase of pressure.At about 1.9 GPa and 6.6 GPa,the slope of the frequency-pressure relationship has significantly changed.We believe that phthalic anhydride has two possible first-order phase transitions at these pressure points.We observe that with the increase of pressure,the relative strength of the two basic vibration modes with wave numbers of 773 cm^-1and 801 cm^-1change significantly.We infer that the pressure-induced phase transition acquires the requirements of Fermi resonance.Fermi resonance occurs between the two fundamental modes,and energy transfer occurs.We discuss the relationship between the four Fermi resonance parameters and pressure changes.The pyromellitic dianhydride(C₁₀H₂O₆)is a compound formed by the dehydration of pyromellitic acid.It is similar to phthalic anhydride in structure.Both are composed of benzene ring and carbonyl group,but the benzene ring of pyromellitic dianhydride has two more carbonyl groups than phthalic anhydride.Therefore,in order to explore the effect of the structural difference caused by carbonyl on the structural change of pyromellitic dianhydride and phthalic anhydride under high pressure,we measured the Raman spectra of pyromellitic dianhydride up to 16 GPa.We found that most of the Raman bands have a good linear relationship with pressure,and the wave number increases linearly with the increase of pressure.Under the effect of pressure,the relative strength of the 1837 cm^-1and 1865 cm^-1Raman bands changed significantly.At about 8.0 GPa,we observed the disappearance of Raman bands and the emergence of new Raman bands,and the frequency-pressure relationship has an obvious inflection point.We believe that pyromellitic dianhydride may undergo a first-order phase transition at 8.0 GPa.Compared with the phase transitions of phthalic anhydride at 1.9 GPa and 6.6 GPa,the phase transition of pyromellitic dianhydride is delayed,and pyromellitic dianhydride only undergoes a phase transition.We infer that four symmetrical carbonyl structures ensure its structural stability under high pressure.In addition,we also discuss the different response of several Raman bands to pressure before and after the phase transition.