Studying On Aromaticity Using Information-Theoretic Approach In Density Functional Reactivity Theory

As a fundamental concept in modern chemistry,which is closely related to various other chemical properties,aromaticity is a widely used yet still very controversial chemical concept from both experimental and theoretical chemistry perspectives.Simlar to other chemical concepts such as bond order,molecular similarity,atomic charge,and oxidation state,aromaticity lacks a rigorous definition.Becasuse of this reason,it can neither be directly measured by experiments,nor be uniquely computed quantum mechanically.The narration of IUPAC?International Union of Pure and Applied Chemistry?on aromaticity is the following.Aromaticity is“a concept of spatial and electronic structure of cyclic molecular systems displaying the effects of cyclic electron delocalization which provide for their enhanced thermodynamic stability?relative to acyclic structural analogues?and tendency to retain the structural type in the course of chemical transformations”.In short,aromaticity demontrates five distinctive characteristics from the viewpoint of chemical behavior,energetics,geometry,electronic and magnetic properties:1)Chemical behavior:Aromatic electronic structures tend to be less reactive in chemical transformations,and ectrophilic aromatic substitutions are less favorable than additions;2)Geometric criterion:Bond length equalization is seen on aromatic rings;3)Energetic criterion:Enhanced stability with smaller heat of combustion;4)Electronic criterion:Electron cyclic delocalization with induced cyclic current;5)Magnetic criterion:Ring current effects cause anormalous chemical shifts,large magnetic anisotropies,and diamagnetic susceptibility exaltation.Based on these properties,numerous aromaticity indexes have been proposed in the literature to quantify aromaticity,four of which,the Aromatic Fluctuation Index?FLU?,Harmonic Oscillator Model of Aromaticity?HOMA?,Aromatic Stabilization Energies?ASE?,and Nucleus-Independent Chemical Shifts?NICS?,will be employed and discussed multiply in this dissertation.On the other hand,in this dissertation,we will make sure of the recent developments in the Information-Theoretic Approach in Density Functional Reactivity Theory?ITA-DFRT?,and apply them to quantify and appreciate aromaticity from the aspect of electron density.The ITA quantities we will apply in this dissertation include Shannon Entropy?S_S?,Fisher Information?I_F?,Ghosh-Berkowitz-Parr Entropy(S_GBP),Information Gain?I_G?,Onicescu Information Energy of order n?E_n?,and Relative Rényi Entropy of order n?R_n^r?.In this dissertation,fulvenes,benzene fused fulvenes,triplet fulvenes,acenes,singlet porphyrinoids,and triplet porphyrinoids have been selected as our model structures to investigate the evolution of aromaticity in these different compounds.Closely related to what was recently reported in the literature,a three-dimensional coordination system has been proposed to appreciate molecular aromaticity with regard to the number of conjugated?-electron,geometry,and spin-state.A brief summary for each of these systems is the following.?1?Fulvenes.Using seven series of substituted fulvene derivatives as the model,we analyze the correlation between aromaticity indexes and ITA quantities.Our results indicate that ITA quantities are often strongly correlated with aromaticity indexes,and the sign of strong linear correlations is opposite between[4n]?and[4n+2]?structures,independent of the ring size.?2?Benzene fused fulvenes.Several fulvene derivatives fused with one,two,and three benzene rings have been employed as the model system to explore the evolution of aromaticity.Our results indicate that aromaticity of the entire molecule is usually dictated by the fulvene moiety.This study also demonstrated the validity of ITA quantities in dealing with the systems satisfying both Clar and Hückel rules.?3?Triplet fulvenes.Seven triplet fulvenes have been selected as the model structures in this Section.Aromaticity governed by Baird's rule for triplet states and Hückel rule for singlet states is investiagted using ITA quantities.Our results indicate that triplet fulvenes illustrate opposite aromaticity tendencies,and both singlet and triplet aromaticity can be adequately described by ITA quantities.?4?Acenes.Ten acenes from 2-acene to 11-acene have been selected as the models to study their global and local aromaticity.Based on the correlation analysis between aromaticity indexes and ITA quantities,we unveil that local aromaticity in acenes can be“laminated”by one benzene width.In addition,the averaged aromaticity of acenes can be used to explain their nonlinear optical properties.?5?Singlet porphyrinoids.With two porphyrinoids of benzi[28]hexaphyrin?1.1.1.1.1.1?and benzi[30]hexaphyrin?1.1.1.1.1.1?as the model,we achieve the macrocyclic conformation switching from Hückel aromaticity and M?bius aromaticity by rotating one phenylene motif.Analyses from energetic decomposition,aromaticity indexes,and ITA quantities are applied to understand the evolution of aromaticity during the conformation switching process.?6?Triplet porphyrinoids.Similar to the singlet porphyrinoids,here two porphyrinoids of triplet benzi[30]hexaphyrin?1.1.1.1.1.1?and its two-electron reduced structures with the same skeleton are employed as the models.Rotating two phenylenes leads to the macrocyclic conformation with Hückel,M?bius,and Twisted topologies.Analyses in energetic components,molecular orbitals,aromaticity indexes,and ITA quantities are conducted to appreciate aromaticity and antiaromaticity properties,enabling us to undertant singlet and triplet aromaticity for macrocyclic compounds using the same theoretical framework.