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Raman Spectroscopic Study Of Pyridine And Its Derivatives Under High Pressure

Posted on:2024-04-26Degree:MasterType:Thesis
Country:ChinaCandidate:Y Q ChenFull Text:PDF
GTID:2531307064481724Subject:Optics
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Heterocyclic compounds are organic compounds with heterocyclic structures in a molecule.In addition to the carbon atom(C),they also contain non-carbon atoms such as nitrogen atom(N),oxygen atom(O),or sulfur atom(S)to form a ring.Among them,non-carbon atoms such as N,O,and S are collectively called heterocyclic atoms.For the number of heteroatoms in a hybrid ring,there can be one or more;There can be one or many kinds of heteroatoms.The vast majority of spices,dyes,and pharmaceutical pesticides in life are synthesized by heterocyclic compounds or heterocyclic compounds as precursors.Heterocyclic compounds also play the role of sensitizers and stabilizers in industrial production.Heterocyclic compounds have such wide applications that they cannot be separated from their unique structure.The structure of a substance determines its properties,so a more in-depth analysis of the structure of typical heterocyclic compounds is necessary.Pressure,as a basic thermodynamic parameter,can effectively regulate the internal structure and intermolecular interactions of substances without external impurities.In the process of exerting pressure on a substance,the volume of the substance decreases at a macro level.At the microscopic level,the shorter distance between molecules,the shorter bond length,and the more overlapping electron orbit in the substance result in the structural phase transition of the substance.At the same time,the external pressure can also reduce the chemical reaction barrier between substances,thus inducing the occurrence of reactions that are difficult or impossible to complete under normal pressure.Therefore,high pressure science provides effective ideas and means for exploring the structural phase transition of existing materials and the synthesis of new materials.In this paper,two typical heterocyclic compounds,nicotinic acid,and pyridine are used as starting points,and high-pressure in situ Raman spectroscopy is used as the characterization method.Diamond is used to providing an extremely high-pressure environment for the anvil unit.The structural stability and phase change of the two compounds under extreme conditions are studied effectively.Niacin,a six-membered heterocyclic compound with carboxyl(-COOH)as its ring substitution group,is one of the 13 vitamins necessary for the human body.It also plays an important role in repairing the biological genetic material(DNA)and inhibiting the proliferation of cancer cells.The atmospheric pressure crystal structure of nicotinic acid is composed of a one-dimensional infinite chain.One cell contains two antiparallel dimer units and is connected by O-H N and C-H O hydrogen bonds.In order to further study the structure of nicotinic acid,the high-pressure Raman spectroscopy of nicotinic acid was analyzed.It was found that nicotinic acid underwent phase I to phase II structure phase transition at about 2.5 GPa.The compressibility of the two phases was compared.Based on the abnormal response of the C=O bond in phase II to pressure,the hydrogen bond between molecules was assumed to play an important role in phase II.At about 17 GPa,all Raman signals disappeared,indicating that nicotinic acid entered the amorphous state.Further decompression of nicotinic acid showed that the amorphous state could be preserved at normal pressure.Pyridine,a heterocyclic compound that substitutes only N atoms for a C-H group in a benzene ring,is also called azobenzene or azobenzene because its structure is similar to that of a benzene ring.Compared with nicotinic acid,because there are no substitutes in its heterocyclic structure,the weak interaction of hydrogen bonds between molecules is also lost.In this work,we have measured the high-pressure in-situ Raman spectra of pyridine.By analyzing the response of different Raman bands to pressure,we have found five possible phase transition points of pyridine,i.e.1.5,4.0,8.0,11.0and 15.0 GPa.We have determined the phase stability interval to obtain the effect of hydrogen bond on the structure stability of heterocyclic compounds under high pressure.The results show that most of the Raman vibration modes of pyridine disappear at pressures up to 21 GPa and all Raman vibration modes disappear when heated further.Therefore,we speculate that pyridine exhibits pressure-induced amorphization and eventually enters an amorphous form,which indicates that pyridine molecules without hydrogen bonds have better structural stability.After the sample was decompressed,only the unheated part could be restored to the atmospheric phase,while the heated part could not be restored,which indicates that the pyridine amorphization process caused by the combined action of high temperature and high pressure is irreversible.
Keywords/Search Tags:High-pressure, Raman spectroscopy, Pyridine, Niacin, Hydrogen bond, Phase transition
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