Phase Transition Of Two Typical Heterocyclic Compounds Under High Pressure

Phase refers to the aggregation of homogeneous substances with identical physical and chemical properties and components without external force.When the external constraints,such as pressure,temperature and so on,exceed a certain threshold,the process of material changing from one phase to another is called phase transition[1].Phase change is the process of sudden change of material structure.Phase change is of great significance to the study of material structure change.Phase change of material may produce new phenomena or new substances,which can help people to study the internal structure of material.Pressure is one of the most important means of phase transformation.Under the action of pressure conditions,the structure of substances will change,which will affect the properties of substances.In this paper,heterocyclic compounds are the main research object.Emphasis is placed on the study of the changes of 2,2'-bipyridine and 1,4-dioxane under high pressure.The research method is to exert pressure on substances.We use diamond anvil(DAC)technology as a pressure method to study the structural changes of substances under high pressure.High pressure can change the molecular structure and interaction between molecules.It is an effective means to study molecules under extreme conditions.In this paper,the phase stability of 2,2'-bipyridine and 1,4-dioxane at high pressure was studied.2,2'-bipyridine and 1,4-dioxane are heterocyclic compounds,of which 2,2'-bipyridine is a nitrogen-containing heterocyclic compound and1,4-dioxane is an oxygen-containing heterocyclic compound.The Raman spectra of2,2'-bipyridine at high pressure were measured by in situ high pressure Raman spectroscopy.After data processing,we can draw a conclusion.The highest pressure of Raman spectroscopy of 2,2'-bipyridine is 15 GPa.By analyzing the theoretical simulation results of Raman spectroscopy combined with structure under highpressure,it is found that there is no abrupt change in Raman spectroscopy of2,2'-bipyridine at high pressure,indicating that no obvious phase transition occurs during high pressure.During the compression process,most Raman peaks tend to move in the direction of high wavenumber with the increase of pressure.From the Raman frequency-pressure relationship,all the vibration frequencies increase linearly with the pressure.Subsequently,the Raman spectra of 1,4-dioxane at high pressure were measured.It was found that the peak of Raman spectra shifted to high wavenumber with the increase of pressure by analyzing the Raman spectra of1,4-dioxane at high pressure.The phase transition occurs when the pressure is about0.5 GPa.The phase transition is from 1,4-dioxane liquid to solid.The phase transition occurs when the pressure is about 5 GPa and 9 GPa.We believe that this phenomenon may be related to the change of molecular structure.By studying the high pressure Raman spectra of two typical heterocyclic compounds,2,2'-bipyridine and 1,4-dioxane,we can know the change of the structure of the substance under high pressure and draw some conclusions.We can make a detailed study of other heterocyclic compounds by high pressure Raman spectroscopy,and further analyze whether different heterocyclic compounds will undergo phase transition under high pressure and the effect of material structure changes on phase stability.High-pressure Raman spectroscopy provides a great help for the study of substances.There are many directions for the study of heterocyclic compounds waiting for our study.