A Study On The Aromaticity Of Annulenes

Aromaticity is one of the most important concepts in organic chemistry.Since the19th century,this concept has been continuously expanded from its shallow beginnings to what is a rich and deep field today.With the development of scientific research,the methods of determining aromaticity have also been continuously improved and updated.The concept of aromaticity has been employed in the fields of organic fluorescent materials,electrochemistry,and drug carriage.In general,the aromaticity of organic compounds can be determined both by experimental and theoretical criteria,including energetic,magnetic,chemical,and geometric criteria.Annulenes,a famous group of monocyclic compounds contains an alternating ring double bond that makes them completely conjugated system.The chemistry of annulenes has long been a subject of great interest to both synthetic and theoretical chemists.The[18]annulene has been regarded as a fundamental structure in annulene chemistry and has been extensively used as a model system for the study of aromaticity.In this paper,energetic criteria,such as TRE,%TRE,and REPE,are used to study the global aromaticity of annulene compounds.BRE and CRE methods are used to analyze the local aromaticity of[18]annulenes,[16]annulenes,and[14]annulenes.The magnetic properties of[18]annulene and[14]annulene,as well as of related compounds are predicted using the RC method and compared with the theoretical and experimental results reported elsewhere.The Hückel and M(?)bius aromaticity of annulene compounds is studied using the TRE and%TRE methods.Our results are compared with the NICS results reported in the literature,and the differences in the results obtained using different methods is discussed.This paper is divided into the six chapters as overviewed below.In Chapter 1,the concept,developmental history,and some common criteria of aromaticity are systematically introduced.The common criteria of aromaticity are analyzed and discussed.The methods which we use in this paper,such as TRE,BRE,CRE,and RC are described in detail.Finally,the significance and purpose of this paper are introduced.In Chapter 2,the global aromaticity of bridged[18]annulene compounds in which either two,four,or six internal hydrogen atoms are replaced by heteroatoms O,NH,and S are studied using the TRE and%TRE methods.At the same time,the aromaticity of these compounds is explained using TCS rules.In addition,the TRE method is used both to study the global aromaticity of the ring-opened structure of these compounds and to determine the main source of aromaticity.Following that,the BRE and CRE methods are used to study the local aromaticity and the main conjugation pathways of bridged[18]annulenes.Finally,the magnetic properties are predicted using the RC method.For the synthesized compounds,the correlation between the RCs and the~1H-NMR chemical shifts is discussed,and good consistency was found between the calculated RC and the experimental ~1H-NMR data.In Chapter 3,the global aromaticity,local aromaticity,and magnetropicity of different types of benzo[18]annulene compounds are studied.The effects of the introduction of benzene rings on the aromaticity of[18]annulene is studied.The local aromaticity is investigated using the BRE and CRE methods.Finally,the correlation between the experimental ~1H NMR chemical shift and the RC strength is discussed,with good correlation having been obtained.In Chapter 4,the global aromaticity of bridged[16]annulene compounds formed by the substitution of either two or four internal hydrogen atoms with heteroatoms O,NH,or S are investigated using the TRE,%TRE,and REPE methods.The BRE and CRE methods are used to analyze local aromaticity.The influence of the introduction of five-membered heterocycles on the antiaromatic[16]annulene is discussed,and the main factors affecting aromaticity are explored.In Chapter 5,the aromaticity of Hückel and M(?)bius structures of[n]annulene as well as its anions and cations is studied using the TRE method.The aromaticity results of the two structures is compared.The trend in aromaticity obtained by the two methods is explained using a broken line diagram.Our results are compared with the results obtained using the NICS(0)and ISE methods,and the correlation between the results from the two energetic criteria are discussed.Finally,the strength of the magnetically induced RC is calculated using the Hückel-London model.In Chapter 6,we study the aromaticity of different benzo[14]annulenes.The global aromaticity of the benzo[14]annulenes is studied using the TRE,REPE,and%TRE methods.The local aromaticity is studied by the use of the BRE and CRE methods.The magnetic properties are predicted using the RC method.Finally,the correlation between the observed ~1H NMR chemical shift and the diatropic RC strength is discussed,and good correlation was obtained.