The purpose of the investigation described herein was to investigate the degree of delocalization of the unpaired electron in ion radicals formed in the oxidation process of compounds with aromatic rings connected by means of various groups and atoms not entering the ring; then, to establish the relationship of coupling constants in radical cations with substituent σ values. The parent cation radical, in which the co-planar ion was derived from N,N-dimethylaniline, was selected in order to maximize the substituent effects on coupling constants and to obtain couplings at several positions (specifically, CH3, N, and ring protons).

A series of dimolybdenum compounds having a Mo2 4+ core coordinated by various ligands, including formamidinate (e.g. DAniF = N, NN-di-p-ansisylformamidinate ), acetate and/or acetonitrile molecules, have been synthesized as building blocks for the construction of Mo2-containing supramolecular arrays. Compound Mo2(DAniF)3(O2CCH3) was specifically designed for the preparation of dimolybdenum pairs, whereas the others meet the needs of Mo2 4+ units for different geometry settings. Compounds described by a general formula [Mo2]L[Mo2], where [Mo2] = [Mo2(DAniF)3]+, have two dimetal units electronically coupled by the central unit L , which consequently engender significant impact on the redox property and electronic structure of the molecule. It is found that in the weakly coupled complex system, [Mo2]M(OCH3)4[Mo2] (M = Zn and Co), the mixed-valence complexes present asymmetric molecular structures with two distinct [Mo2] units corresponding to be a bond order 4.0 (F2B4*2) and 3.5 (F2B4*1), respectively. EPR and magnetic susceptibility measurements for the doubly oxidized species show that there is no significant antifferromagnetic spin coupling. Electron delocalization occurs in the complex system where a N, N'-dimethyloxamidate binds two [Mo2] units within two fused six-membered rings. In this case, the mixed-valence complex has a symmetric molecular structure, implying that the odd electron is fully delocalized over two [Mo2]units. Strong metal-metal interaction is also evidenced by intervalence charge transfer of the mixed-valence species and the diamanetism of the doubly oxidized complex. Remarkably, two isomers varying in linkage conformation, namely, alpha and beta, have been isolated as diaryloxamidate ligands are used as the linker. Studies on the neutral and the oxidized compounds of the two isomers by employing various techniques consistently show that in the alpha form intramolecular electron transfer is blocked , while in the beta form, the electrons are delocalized over the two [Mo2] units. Thus, the mixed-valence complexes of the two isomers are appropriately described by alpha-[Mo2]0(oxamidate)[Mo2]1+ and beta- [Mo2]0.5+(oxamidate)[Mo2]0.5+ respectively.

We study two interacting particles in a random potential chain by a transfer matrix method which allows a correct handling of the symmetry of the two- particle wave function, but introduces an artificial ¨bag¨ interaction. The dependence of the two-particle localization length lambta 2on disorder, interaction strength and range is investigated. Our results demonstrate that the recently proposed enhancement of lambta 2 as compared to the results for single particles is vanishingly small for a Hubbard interaction. For longer-range interactions, we observe a small enhancement but with a different disorder dependence than proposed previously.

No description available.

The localized model of a chemical bond has had a long and prominent role in chemistry, but situations of extended charge delocalization and dipole effects remain topics in need of greater understanding. Both orbital delocalization in isolated molecules and induced molecular dipoles in condensed phases serve to move electron density and influence the chemical and physical properties of a system. This dissertation studies these aspects of electronic structure for selected organic, inorganic, and organometallic systems by means of electronic structure calculations and photoelectron spectroscopy, which is well-suited for studying both intramolecular and intermolecular effects by providing a direct probe of orbital energies and characters. Photoelectron spectra of P₄ and AsP₃ reveal differences in the molecular symmetry and cationic state effects between the two molecules in Chapter 3. Despite these differences, AsP₃ is found to have electron delocalization and vibrational structures that are comparable to P₄. A similar study of the delocalized -system of 2H-1,2,3-triazole in Chapter 4 relates the vibrational structure in photoelectron spectroscopy data to a series of Rydberg excitations in the vacuum UV photoabsorption spectrum. Chapters 5 and 6 examine extended electronic structures in organometallic complexes. The electron delocalization and charge transfer between two Ru centers along a bridging ethynediyl ligand is studied in [CpRu(CO)₂]₂(μC≡C). Details of the Ru-alkynyl interaction were explored by comparing the spectra of CpRu(CO)₂C≡CMe with CpRu(CO)₂Cl, including the -backbonding ability of alkynyl ligands. Chapter 6 moves from the realm of intramolecular effects to intermolecular interactions to understand how surrounding media affect electronic properties of molecules. The reversal of ionization energies between the gas and solid phases of M(CO)₄dmpe and M(CO)₄dppe, where M = Mo, W, is explored with photoelectron spectroscopy. The surrounding molecular environment stabilizes the cation, resulting in this reversal that extends to core ionization energies. The variety of systems presented illustrates the wide applicability of photoelectron spectroscopy and computations to different electronic structure studies, including how gas phase results can be related to condensed phase studies. This work continues the progress of photoelectron spectroscopy from small molecules to larger molecular systems and even further to bulk systems.

Interactions between electrons determine the structure and properties of matter from molecules to solids. Therefore, the understanding of the electronic structure of molecules will enable us to extract relevant chemical information. In the first part of this thesis, we focus our attention on the analysis of chemical bonding by means of the Electron Localization Function (ELF) and the Domain-Averaged Fermi Hole analysis (DAFH). In the second part, we assess the performance of some indicators of aromaticity by analyzing their advantages and drawbacks. We propose a series of tests based on well-known aromaticity trends that can be applied to evaluate the aromaticity of current and future indicators of aromaticity in both organic and inorganic species. Moreover, we investigate the nature of electron delocalization in both aromatic and antiaromatic systems in the light of Hückel’s (4n + 2) rule. Finally, we analyze the phenomenon of multiple aromaticity in all-metal clusters. / Les interaccions entre electrons determinen l’estructura i propietats de la matèria. Per tant, la comprensió de l’estructura electrònica de les molècules ens permetrà extreure informació química rellevant. En la primera part d’aquesta tesi, centrem la nostra atenció en l’anàlisi de l’enllaç químic per mitjà de la funció de localització electrònica (ELF) i l’anàlisi dels anomenats domain averaged Fermi holes (DAFH). En la segona part, s’avalua el comportament d’alguns indicadors d’aromaticitat analitzant els seus avantatges i inconvenients. Al llarg d’aquesta part, es proposen una sèrie de tests basats en tendències d’aromaticitat conegudes que es poden aplicar per avaluar el comportament dels indicadors actuals en espècies tan orgàniques com inorgàniques. A més a més, s’investiga la naturalesa de la deslocalització d’electrons en sistemes aromàtics i antiaromàtics que segueixen la regla 4n+2 que proposà Hückel. Finalment, analitzem el fenomen de l’aromaticitat múltiple en sistemes metàl•lics

<p>The computational success of density functional theory relies on the construction of suitable approximations to the exchange-correlation energy functional. Use of functional approximations depending explicitly upon the density alone appear unable to address all aspects of many-body interactions, such as the fundamental constraint that the ground state energy is a piecewise linear function of the total number of electrons, and the ability to model nonlocal effects. Functionals depending explicitly upon occupied and unoccupied Kohn–Sham orbitals are considered necessary to address these and other issues. This dissertation considers certain issues relevant to the successful implementation of explicitly orbital-dependent functionals through the optimized effective potential (OEP) approach, as well as extending the potential functional formalism that provides the formal basis for the OEP approach to systems in the presence of noncollinear magnetic fields.</p><p>The self-consistent implementation of orbital-dependent energy functionals is correctly done through the optimized effective potential approach—minimization of the ground state energy with respect to the Kohn–Sham potential that generates the set of orbitals employed in the energy evaluation. The focus on the potential can be problematic in finite basis set approaches as determining the exchange-correlation potential in this manner is an inverse problem which, depending upon the combination of orbital and potential basis sets employed, is often ill-posed. The ill-posed nature manifests itself as nonphysical exchange-correlation potentials and total energies. We address the problem of determining meaningful exchange-correlation potentials for arbitrary combinations of orbital and potential basis sets through an L-curve regularization approach based on biasing towards smooth potentials in the energy minimization. This approach generates physically reasonable potentials for any combination of basis sets as shown by comparisons with grid-based OEP calculations on atoms, and through direct comparison with DFT calculations employing functionals not depending on orbitals for which OEP can also be performed. This work ensures that the OEP methodology can be considered a viable many-body computational methodology.</p><p>A separate issue of our OEP implementation is that it can suffer from a lack of size-extensivity—the total energy of a system of infinitely separated monomers may not scale linearly with the total number of monomers depending upon how we construct the Kohn–Sham potential. Typically, a fixed reference potential is employed to aid in the convergence of a finite basis set expansion of the Kohn–Sham potential. This reference potential can be utilized to ensure other desirable properties of the resulting potential. In particular, it can enforce the correct asymptotic behavior. The Fermi–Amaldi potential is often used for this purpose but suffers from size-nonextensivity owing to the explicit dependence of the potential on the total number of electrons. This error is examined and shown to be rather small and rapidly approaches a limiting linear behavior. A size-extensive reference potential with the correct asymptotic behavior is suggested and examined.</p><p>We also consider a formal aspect of the potential-based approach that provides the underlying justification of the OEP methodology. The potential functional formalism of Yang, Ayers, and Wu is extended to include systems in the presence of noncollinear magnetic fields. In doing so, a solution to the nonuniqueness issue associated with mapping between potentials and wave functions in such systems is provided, and a computational implementation of the OEP in noncollinear systems is suggested.</p><p>Finally, as an example of an issue for which orbital-dependent functionals seem necessary to obtain a correct description, we consider the ground state structures of C_{4<italic>N</italic> + 2} rings which are believed to exhibit a geometric transition from angle-alternation (<italic>N</italic> ≤ 2) to bond-alternation (<italic>N</italic> > 2). So far, no published DFT approach has been able to reproduce this behavior owing to the tendency of common density functional approximations to bias towards delocalized electron densities. Calculations are presented with the rCAM-B3LYP exchange-correlation functional that correctly predict the structural evolution of this system. This is rationalized in terms of the recently proposed delocalization error for which rCAM-B3LYP explicitly attempts to address.</p> / Dissertation

To better understand the optical and electronic properties of thiophene- and pyrrole-based organic compounds on a molecular level, several aromatic compounds and their corresponding monocations were analyzed by a variety of solution-based spectroscopic techniques. The derivatives were initially synthesized using palladium-catalyzed amination reactions, condensation reactions, Horner-Emmons reactions, and Stille coupling reactions. Once isolated, the neutral compounds were analyzed by UV-visible-NIR absorption spectroscopy, fluorescence spectroscopy, cyclic voltammetry, and / or differential pulse voltammetry. Monocations were generated by chemical oxidation and were analyzed by visible-NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy. By quantifying the extent of the electron-donor abilities of some chromophores and the electron delocalization of positive charge in the monocations, a more thorough understanding of the optical and electronic properties of the compounds was obtained.

Orientador: Peter Alexander Bleinroth Schulz / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T13:08:11Z (GMT). No. of bitstreams: 1 Caetano_RodrigoAndre_D.pdf: 1870062 bytes, checksum: 8848ab4c022cee347bb886bd1b5d15c5 (MD5) Previous issue date: 2007 / Resumo: Nesse trabalho estudamos as propriedades eletrônicas de alguns sistemas de dimensão nanométricas. Primeiramente, vemos os efeitos do emparelhamento de base em cadeias duplas desordenadas. Dois aspectos são considerados: Para um conjunto de parâmetros que simulam o DNA, o emparelhamento de base aumenta significativamente o comprimento de localização da função de onda. Para um outro determinado conjunto de parâmetros, há uma forte sugestão de uma transição de estados localizados-delocalizados. Investigamos tamb'em os efeitos da correlação no grau de localização em sistemas 2D. Propomos um modelo minimamente realista para descrever a correlação na formação de pontos quânticos auto formados. O modelo que propomos é a proibição de dois pontos quânticos serem primeiros vizinhos. Revelamos tanto os efeitos estruturais, como a formação de regiões de redes binárias ordenadas, quanto os efeitos na estrutura eletrônica. Verificamos que a Razão de Participação é cerca de três vezes maior quando a correlação é considerada. Finalmente, consideramos os efeitos do acoplamento spin-órbita em nanotubos de carbono. Levamos em conta dois tipos de acoplamento: O intrínseco, devido ao arranjo dos átomos de carbono no nanotubo e o acoplamento spin-órbita Rashba, devido a um campo elétrico externo. Mostramos que o acoplamento intrínseco abre um gap, se o nanotubo for metálico na ausência de spin-órbita, ou aumenta o gap se, na ausência de spin-órbita, o nanotubo não for metálico. Mostramos também que para nanotubos metálicos (na ausência do acoplamento spin-órbita) a dependência da amplitude do gap depende de forma universal com a amplitude do campo elétrico externo, no entando, para nanotubos que já apresentam gap mesmo sem levar o acoplamento spin-órbita em conta, a amplitude do gap depende fortemente da quiralidade do nanotubo. Por fim, mostramos que a degenerescência de spin é quebrada pelo acoplamento spin-órbita Rashba / Abstract: In this work, we study the electronic properties of some nanometric systems. Firstly, we look the effects of the base pairing em disordered double chain. Two aspects are considered: For a set of parameters which mimics the DNA, the base pairing enlarge significantly the spread of the wave function. For another determined set of parameters, there is a strong suggestion of a truly localization-delocalization transition. We also investigate the effects of correlation in the localization degree in 2D systems. We propose a minimally realist model in order to describe the correlation in the growth of self-assembled quantum dots. The model that we propose is: There are no quantum dots first neighbor. We reveal as the structural effects, like the formation of "quantum crystals", as the effects in the electronic structure. We report that the Participation Ratio is around three times bigger whenever the correlation is imposed. Finally, we consider the effects of the spin-orbit coupling in carbon nanotubes. We take into account two kinds of coupling: The intrinsic, due to the hexagonal arrangement of the carbon atoms in carbon nanotubes and the Rashba spin-orbit coupling, due to the external electric field. We show that the intrinsic opens a gap, if the nanotube is metallic in the absence of spin-orbit coupling, or increase the gap, if the nanotube is not metallic in the absence of spin-orbit coupling. We also show that for metallic nanotubes (in the absence of spin-orbit coupling) the gap amplitude depends in an universal way with the external electric field amplitude. However, for non metallic nanotubes (in the absence of spin-orbit coupling), the gap amplitude strongly depends on the carbon nanotube chirality. By the end, we show that the Rashba spin-orbit coupling lifts the spin degenerescency / Doutorado / Física da Matéria Condensada / Doutor em Ciências

We study two interacting particles in a random potential chain by a transfer matrix method which allows a correct handling of the symmetry of the two- particle wave function, but introduces an artificial ¨bag¨ interaction. The dependence of the two-particle localization length lambta 2on disorder, interaction strength and range is investigated. Our results demonstrate that the recently proposed enhancement of lambta 2 as compared to the results for single particles is vanishingly small for a Hubbard interaction. For longer-range interactions, we observe a small enhancement but with a different disorder dependence than proposed previously.