A arilação seletiva de compostos orgânicos confere-lhes propriedades com amplas aplicações: em compostos de atividade biológica, inibidora da protease do vírus HIV-1, e de interesse como agroquímicos ou na engenharia de materiais. Em razão disso, novos métodos e reagentes tem sido desenvolvidos com essa finalidade. Sabe-se que a N-arilação de várias aminas, catalisadas por paládio, com haletos de arila e triflatos é tida como ferramenta importante a disposição do químico sintético. Do mesmo modo, publicou-se que a N-arilação de certos -amino ácidos por haletos de arila, procede-se facilmente quando catalisada por CuI. Alternativamente, a literatura cita outros métodos eficientes usando ácidos borônicos, compostos arilbismuto e compostos organochumbo. Neste trabalho foi estudada a arilação seletiva de ânions ambidentados pelo sal de iodo polivalente cloreto de difenil iodônio, em diferentes condições de reação, visando a otimização da síntese: 1) reação térmica (agitação magnética), sem catalisador, e em diferentes solventes, ou mistura de solventes; 2) reação sonoquímica, sem catalisador e em diferentes soluções; 3) reação térmica e sonoquímica na presença de catalisador CuCl (10%). Sacarinato de sódio, acesulfame de potássio, e ftalimida potássica foram escolhidos como ânions ambidentados derivados de sulfoimidas e imidas a serem arilados. No caso de N-fenil sacarina a quemiosseletividade e o melhor rendimento são observados com acetonitrila/água (1:1, v/v), sob refluxo para formação do par iônico intermediário (ou o iodano correspondente), seguido por fusão térmica na ausência de solvente. O acesulfame potássico, por sua vez, forneceu seletivamente produtos de N- ou O-fenilação, de acordo com as seguintes condições estabelecidas: 1) produto de fenilação por via térmica ou sonoquímica em etanol; 2) produto de O-fenilação, por via térmica ou sonoquímica em acetonitrila. A ftalimida potássica, na ausência de catalisador, é arilada em baixos rendimentos. A melhor condição de síntese é encontrada com acetonitrila e CuCl (10%), sendo 92% o rendimento por via sonoquímica e 78% por via térmica. A seletividade verificada foi analisada em termos das interações dos ânions ambidentados e dos solventes em questão. / The arylation of organic compounds gives place to a wide number of applications: concerning to their biological properties as HIV-1, protease inhibitor, as well as synthetic intermediates in pharmaceuticals, agrochemical and polymer chemistry. New methodologies and reagents have been developed as consequence of this. It is already known that N-arylation of several amines, catalyzed by palladium, with aryl halides and trifflates is a valuable tool to various reported organic synthesis. In the same way, it has been published that the N-arylation of certain -amino acids by aryl halides, proceeds smoothly when catalized by CuI. Alternatively, the literature reports other efficient methods using boranic acids, arylbismuth and organolead compounds. In the present work, it was studied the selective arylation of ambident anions by hypervalent iodine salts (chloride of diphenyl iodonium), in a different set of conditions, seeking the synthesis optimizations: 1) thermal reactions (silent mode, magnetic stirring), without catalyst, and, in different solvents or mixture of then; 2) sonochemical reactions, without catalyst, and in a different solvent composition; 3) thermal and sonochemical reactions carried out in the presence of (10%) CuCl as the catalyst. Sodium saccharinate, acessulfame K and potassium phthalimide were chosen as the ambident anions (derived from sulfonimides and imide functional groups) to be studied. In the N-phenyl saccharin case the chemioselectivity was achieved along the best yields when water/acetonitrila (1:1, v/v) solvent was employed in the step to form the ion pair (or the 3-iodane, intermediate), followed by the thermal fusion in the absence of solvent. The acessulfame K, for its turn, supplied selectively products of N- and O-arylation under the following established conditions: 1) N-phenylation in etanol through thermal and sonochemical approach; 2) O-phenylation product, in acetonitrila, by thermal and sonochemical method. Finally, arylation of potassium phthalimide with diaryliodonium is sluggish and gives low yields without catalyst. The best protocol to this synthesis was found with the solvent acetonitrile and (10%) CuCl catalyst addition, being 92% the yield of sonochemical reaction and 78% of the thermal one. The selectivity achieving was analyzed in accord with the solution interaction between the ambident anion and the solvent molecules.

Neste trabalho estudamos o espectro de absorção da clorofila d, incluindo os efeitos do solvente metanol, utilizando a Teoria do Funcional da Densidade Dependente do Tempo em combinação com o método s-QM/MM. Diferentes abordagens para a descrição do meio solvente, que vão desde o modelo contínuo polarizável até a inclusão de moléculas explícitas do solvente, são utilizadas. Observamos que a inclusão do solvente desloca o espectro, em relação ao calculado em vácuo, para o vermelho. A inclusão de 20 moléculas explícitas de metanol e 880 representadas como as cargas pontuais do seus átomos para a descrição do meio solvente foi a que melhor concordou com os valores experimentais. Mediante cálculos de mecânica quântica obtivemos para o complexo composto por a molécula de clorofila d e apenas uma de metanol explícita um deslocamento do átomo de magnésio da clorofila d em relação ao anel de 0,31 Å. Mediante a analise da função de distribuição radial de pares obtida da simulação clássica comprovamos que esse átomo de magnésio é penta-coordenado. Além disso, é usada uma estrutura simplificada da clorofila d baseado na localização dos orbitais moleculares participantes das transições eletrônicas com o objetivo de diminuir o custo computacional dos cálculos de mecânica quântica. Os espectros calculados para a clorofila d não apresentaram diferenças significativas com os calculados para a simplificação proposta. Como complementação o espectro Raman da clorofila d isolada é calculado mediante DFT e alguns dos modos normais são caracterizados. / In this work we study the absorption spectrum of chlorophyll d, including the effects of the solvent methanol, using the Time Dependent Density Functional Theory in combination with the method s-QM/MM. Different approaches for the description of the solvent medium, ranging from the polarizable continuum model to the inclusion of explicit solvent molecules, are used. We note that the inclusion of solvent shifts the spectrum, relative to that calculated in vacuum, to the red side of the spectrum. The inclusion of 20 explicit molecules of methanol and 880 represented as point charges of their atoms for the description of the solvent medium was the one that best agreed with the experimental values. By quantum mechanics calculations we obtained for the complex composed of the molecule of chlorophyll d and only one explicit methanol molecule a displacement of the magnesium atom of chlorophyll d in relation to the ring of 0,31 Å. By means of the analysis of the radial distribution function obtained from the classic simulation we can see that this atom of magnesium is penta-coordinated. In addition, a simplified structure of chlorophyll d is used based on the location of the molecular orbitals involved in the electronic transitions in order to reduce the computational cost of quantum mechanics calculations. The spectra calculated for chlorophyll d did not show significant differences with those calculated for the proposed simplification. As a complement the Raman spectrum of isolated chlorophyll d is calculated by DFT and some of the normal modes are characterized.

<p>One-step photoinitiated free radical carbonylation was employed for the rapid (5–7 min) labelling of aliphatic carboxylic acids, esters, and amides with a short-lived positron emitter ¹¹C (t_½ = 20.3 min) at the carbonyl position. The labelled compounds were synthesized from alkyl iodides (0.05–0.1 mmol), [¹¹C]carbon monoxide, and appropriate nucleophiles. Decay-corrected radiochemical yields were up to 74%; conversion of [¹¹C]carbon monoxide reached 85–90%; specific radioactivity was 158–192 GBq/mmol. The labelled compounds were identified and characterized using HPLC, LC-MS, and ¹H and ¹³C NMR. The effects of solvents, additives, photoirradiation, temperature, and reaction time were studied and discussed.</p><p>[<i>carbonyl-</i>¹¹C]Amides were synthesized using amines in 1–2 equiv. to iodides, exploiting solvent effects to control reactivity. [<i>carboxyl-</i>¹¹C]Acids were synthesized using water as a nucleophile, in binary and ternary aqueous solvent mixtures; the addition of TBAOH or KOH was necessary to obtain high radiochemical yields. [<i>carbonyl-</i>¹¹C]Esters were synthesized using primary and secondary alcohols, <i>tert-</i>butanol, and phenol. Bases were KOH, BuLi, LiHDMS.</p><p>The effects of photosensitizers were studied and exploited to accelerate the labelling of carboxylic acids and esters resulting in 75–85% decay-corrected radiochemical yields under mild conditions without the use of bases.</p><p>A mild procedure for the ¹¹C-carboxylation of alkyl iodides using DMSO as an oxygen nucleophile was developed. This method is expected to be suitable in the macroscale synthesis of carboxylic acids using isotopically unmodified carbon monoxide.</p><p>Radical carbonylation was applied to improve the synthesis of an extensively used PET tracer, [<i>carbonyl</i>-¹¹C]WAY-100635. The tracer was synthesized in one step, whereas a common approach via Grignard reagents requires three steps.</p><p>In addition, several (¹³C)compounds were synthesised using the described methods.</p><p>Free radical carbonylation may be used for the ¹¹C-carbonylation of alkyl iodides, whereas transition-metal carbonylation – of aryl halides and triflates. Thus, the two carbonylation methods are complementary with respect to the scope of synthetic targets.</p>

One-step photoinitiated free radical carbonylation was employed for the rapid (5–7 min) labelling of aliphatic carboxylic acids, esters, and amides with a short-lived positron emitter 11C (t½ = 20.3 min) at the carbonyl position. The labelled compounds were synthesized from alkyl iodides (0.05–0.1 mmol), [11C]carbon monoxide, and appropriate nucleophiles. Decay-corrected radiochemical yields were up to 74%; conversion of [11C]carbon monoxide reached 85–90%; specific radioactivity was 158–192 GBq/mmol. The labelled compounds were identified and characterized using HPLC, LC-MS, and 1H and 13C NMR. The effects of solvents, additives, photoirradiation, temperature, and reaction time were studied and discussed. [carbonyl-11C]Amides were synthesized using amines in 1–2 equiv. to iodides, exploiting solvent effects to control reactivity. [carboxyl-11C]Acids were synthesized using water as a nucleophile, in binary and ternary aqueous solvent mixtures; the addition of TBAOH or KOH was necessary to obtain high radiochemical yields. [carbonyl-11C]Esters were synthesized using primary and secondary alcohols, tert-butanol, and phenol. Bases were KOH, BuLi, LiHDMS. The effects of photosensitizers were studied and exploited to accelerate the labelling of carboxylic acids and esters resulting in 75–85% decay-corrected radiochemical yields under mild conditions without the use of bases. A mild procedure for the 11C-carboxylation of alkyl iodides using DMSO as an oxygen nucleophile was developed. This method is expected to be suitable in the macroscale synthesis of carboxylic acids using isotopically unmodified carbon monoxide. Radical carbonylation was applied to improve the synthesis of an extensively used PET tracer, [carbonyl-11C]WAY-100635. The tracer was synthesized in one step, whereas a common approach via Grignard reagents requires three steps. In addition, several (13C)compounds were synthesised using the described methods. Free radical carbonylation may be used for the 11C-carbonylation of alkyl iodides, whereas transition-metal carbonylation – of aryl halides and triflates. Thus, the two carbonylation methods are complementary with respect to the scope of synthetic targets.

The structural, electronic, and optical properties of a series of organic pi-conjugated polymer, oligomer, or molecular materials of interest for applications in organic electronics are described. For this purpose, quantum-chemical techniques ranging from Density Functional Theory to Hartree-Fock ab initio and semiempirical methods are used to evaluate the charge-transport, charge-transfer, and electroluminescence properties of pi-conjugated organic materials. First, the effect of electronic polarization on the charge-transport parameters of organic semiconductors is discussed. A generalized methodology based on a basis set orthogonalization procedure is developed to determine reliable charge-transport characteristics. The charge-transport parameters of a number of organic semiconductors such as oligoacenes and derivatives are studied with this methodology. Then, triplet emitters, in particular iridium complexes, that achieve high efficiency electroluminescence in organic light-emitting diodes are discussed. The effects of ligand substitution and orientation on the luminescent properties of iridium compounds are investigated in order to develop structure-properties relationships. The emission properties of these complexes are found to be governed by an interplay between metal-to-ligand charge transfer excitations and ligand-centered and/or interligand excitations. The extent of mixing of these various excitations turns out to be highly dependent on the nature of the substituents. Design strategies to shift the emission color towards deep blue are proposed. Finally, several classes of materials acting as hosts for phosphorescent emitters are studied. It is shown that restricting the conjugation length leads to high energy gap host materials suitable for blue phosphorescent emitters.

Les complexes à transition de spin sont des molécules qui peuvent être commutées entre deux états, l’un diamagnétique et l’autre paramagnétique. Cette commutation peut s’effectuer, entre autre, à l’aide d’une excitation lumineuse, ouvrant la voie vers un possible stockage de l’information au niveau d’une molécule unique. Toutefois, l’information photo-inscrite n’est stable qu’au-dessous d’une certaine température appelée T(LIESST). L’objectif principal de cette thèse a constitué en des modifications chimiques (modification du ligand organique, désolvatation, modifications d’anion,…) d’un complexe macrocyclique de Fe(II) qui présente à l’heure actuelle l’un des T(LIESST) les plus élevés (134 K), et ce afin d’augmenter encore cette valeur. Parmi les divers résultats expérimentaux, un T(LIESST) de 160 K a été obtenu. / Spin transition complexes are molecules that can be commuted between two different states, one being diamagnetic and the other one paramagnetic. This commutation can be triggered by different ways, including light irradiation, opening a way to the storage of information at molecular level. However, the photo-inscribed information is stable only below a given temperature called T(LIESST). The main goal of this thesis has been to modify chemically (modification of organic ligand, desolvation, modification of anions, …) a Fe(II) macrocyclic ligand presenting one of the highest T(LIESST) known today (134 K) with the aim to further increase this temperature. Among the different results obtained, a T(LIESST) of 160 K has been reached.

Cette thèse porte sur la modélisation multi-échelle des réactions en catalyse hétérogène de polyols sur une surface de platine 111, en phase aqueuse. Ce travail s’inscrit dans l’utilisation de la biomasse lignocellulosique. Les réseaux réactionnels issus du traitement de cette matière première sont très grands en taille, et en complexité. D’autre part, la nature même des molécules la composant impose une chimie en phase aqueuse. Il n’est donc pas possible de cartographier l’ensemble des ces chemins réactionnels avec des méthodes de calcul ab initio, tant les conditions aqueuses et le grand nombre de simulations que cela impliquerai demanderaient d’effort computationnel. Le changement d’échelle de simulation peut répondre à ce problème. Nous avons ainsi développé un modèle d’additivité par groupes permettant la prédiction des propriétés thermodynamiques de réactions de polyols sur platine 111 en phase aqueuse, à partir de la seule structure topologique des réactifs. Ce modèle a été construit sur un jeu de données issus de calculs en théorie de la fonctionnelle de la densité. Nous avons également étudié la cinétique de réactions de mêmes types, afin d’en prédire l’énergie d’activation à partir de leur thermodynamique. En parallèle, nous avons décrit l’influence de la micro-solvatation par rapport à un modèle de solvant implicite sur la réactivité d’alcools sur platine 111, ce qui couplé avec le modèle d’additivité par groupe permettra des simulations micro cinétiques complètes. Enfin, la réactivité des isomères du butanediol a été traitée afin de comprendre l’influence de l’espacement des fonctions alcools sur la réactivité d’un polyol. / This thesis focuses on multi scale simulations heterogeneous catalysis reactions of polyols on platinum (111) in aqueous phase. This work is about lignocellulosic biomass valorisation. The reaction networks raising from these materials are extremely large and complex. Also, the properties of the molecules forming this biomass make aqueous conditions mandatory. Ab initio methods as we know them forbids us to treat entire networks at the finest calculation level, the computational cost would be way beyond feasibility. Changing the simulation scale can tackle this problem. Thus, we developed a group additivity model assessing for thermochemical properties of polyols at a platinum (111) surface under aqueous conditions, with a topology as the only input. This model was built upon a density functional theory set. Kinetics of reactions was also covered, and the difference in terms of impact was investigated between implicit and explicit micro solvation models, in order to predict reaction barriers. The two models might be used together in order to feed a micro kinetic simulation, allowing a drastic decrease of reaction networks complexity. Finally, the influence on reactivity and selectivity, of hydroxyl groups on butanediol isomers was investigated.

No description available.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-05-20T12:56:17Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Ayyaz-Mahmood.pdf: 9023674 bytes, checksum: beecc131e286e19ce6d03dec91f319d8 (MD5) / Made available in DSpace on 2016-05-20T12:56:17Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Ayyaz-Mahmood.pdf: 9023674 bytes, checksum: beecc131e286e19ce6d03dec91f319d8 (MD5) Previous issue date: 2015-11-27 / Realizamos estudos do mecanismo, da seletividade e regiosseletividade da metilação de nitronatos [R1R2CNO2]− com CH3I, em que R1 = R2 = H (1), R1 = CH3 e R2 = H (2), R1 = R2 = CH3 (3); e R1 + R2 = c-(CH2)2 (4), usando método quântico ab initio MP2/CBS método e as teorias ADO e RRKM e simulações numéricas. As estruturas de todos os pontos estacionário no perfil da reação, incluindo os estados de transição, foram obtidas com sucesso e forneceram as energias de ativação (Δ‡) e de reacção (Δr) para todos os caminhos reacionais. Estas reações iniciam com a formação de complexo de reagente do tipo íon-molécula (RC@C ou RC@O) que se converte no respectivo complexo de produto (PC@C e PC@O), através do estado de transição (TS@C e TS@O) correspondente. As constantes de velocidade para a conversão do RC ao PC foram calculadas com a teoria RRKM. Ao contrário da proposta da literatura para o mecanismo de reação, a C-metilação é a via termodinâmica e cinética preferencial, com as seguintes seletividades: PC@C:PC@O = 89:11 (1), 97:03 (2-E), 97:03 (2-Z), 100:0 (3) e 100:0 (4). Isto é corroborado pela concordância qualitativa e quantitativa entre a ordem da reatividade calculada e experimental: 4 >> 3 > 2 > 1. A regiosseletividade para a C-alquilação é explicada pela maior exotermicidade desse canal de reação em comparação com a O-alquilação. O aumento da reatividade com o aumento do impedimento estérico ao redor do carbono central é explicado pelas diferenças de carácter sp3 deste átomo e pela diminuição da frequência vibracional associada à piramidalização ao redor deste átomo de carbono. Uma avaliação de funcionais híbridos- e duplo-híbrido DFT e do método MP2 para estas reações foi realizada a fim de verificar a confiabilidade e precisão destes métodos e sua utilização em modelagens similares. Os métodos mais precisos e robustos, que apresentam menor demanda computacional, foram M06- 2X and MP2 com as funções de base aug-cc-pVTZ-PP. A fim de revelar os efeitos solvente e do agente alquilante, a metilação do nitronato [MeCHNO2]− (2) com Me3O+ (oxônio) foi estudada em fase gás e em solução com o método B2PLYP/6-311+G(d,p). Os resultados mostraram regiosseletividade para a O-metilação em fase gás e principalmente em solução (diclorometano), o que concorda com os dados experimentais. Além disso, eles mostram que a regiosseletividade é fortemente dependente do agente alquilante (MeI vs. Me3O+). Este mesmo método foi utilizado para as reações [XC6H4CHNO2]− + MeCl (X = H, p-MeO, p-NO2) em fase gás e em solução (DMF). Os resultados em solução mostram uma pequena seletividade favorável à O-metilação, contudo, considerando que o agente metilante usado experimentalmente era o metil-p-bromobenzenossulfonato (MeOBS), o qual é muito mais reativo que o MeCl, é esperado que a utilização do MeOBS nas modelagens moleculares deva aumentar esta regiosseletividade para a O-metilação. / We performed a mechanistic, selectivity and regioselectivity study of methylation of nitronates [R1R2CNO2]− by CH3I in the gas-phase, where R1 = R2 = H (1), R1 = CH3 and R2 = H (2), R1 = R2 = CH3 (3); and R1 + R2 = c-(CH2)2 (4), using ab initio MP2/CBS method, ADO and RRKM theories and kinetics simulations. The structures of all stationary points, including transition states, were successfully obtained to provide energies of activation (Δ‡) and of reaction (Δr) for all reaction pathways. These reactions proceed with the formation of ion-neutral reactant complex (RC@C or RC@O) that is converted into the respective product complex (PC@C and PC@O) through the corresponding transition state (TS@C and TS@O). The rate constants for the conversion of RCs into PCs were calculated with the RRKM theory. Contrary to a previous proposal for the reaction mechanism, the C-methylation is the thermodynamics and kinetics preferred pathway with the following PC@C:PC@O selectivites 89:11 (1), 97:03 (2-E), 97:03 (2- Z), 100:0 (3), and 100:0 (4). This is corroborated by the agreement between the calculated and experimental reactivity trend 4 >> 3 > 2 > 1. The regioselectivity towards the C-alkylation is explained by the much larger exothermicity of this reaction channel compared to the Oalkylation. The increase of reactivity upon the increase of crowdedness of the central carbon atom is explained by differences of sp3 character at this atom and the decrease of the vibrational frequency associated with pyramidalization around this carbon atom. An assessment of hybrid- DFT functionals and MP2 method for these reactions was performed in order to ascertain the reliability of these methods for similar molecular modeling. The most accurate and robust methods, with the least computational demand, were M06-2X and MP2 with aug-cc-pVTZ-PP basis sets. In order to reveal the solvent and alkylating agent effects, the methylation reaction [MeCHNO2]− (2) + Me3O+ (oxonium) was studied in the gas-phase and in solution with B2PLYP/6-311+G(d,p) calculations. The results show regioselectivity towards O-methylation in the gas-phase and more significantly in solution (dichloromethane), which is in agreement with the experimental observation. In addition, it shows that the regioselectivity is highly dependent upon the reactivity of the alkylating agent (MeI vs. Me3O+). This same method was employed for the [XC6H4CHNO2]− + MeCl (X = H, p-MeO, p-NO2) reactions in the gas-phase and in solution (DMF). The results in solution show a slight regioselectivity towards O-methylation; however, considering that the methylating reagent used in the experiments was methyl pbromobenzenesulfonate (MeOBS), which is much more reactive than MeCl, it should thus be expected an increase of the calculated selectivity.

Made available in DSpace on 2014-07-29T15:07:07Z (GMT). No. of bitstreams: 1 Orlei Luiz dos Santos.pdf: 953529 bytes, checksum: 9ae33a7c834d1eff1f7b0fdf621d088c (MD5) Previous issue date: 2010-07-27 / In this work we study the electric properties of mesoionic compounds in the gaseous phase in different solvents, with a special attention to the first hyperpolarizability (&#946; ). The Hartree-Fock (HF) method and the second-order Møller Plesset perturbation theory (MP2) were used to describe the electronic structure. The solvent effects were modelled using the selfconsistent reaction field approach with the Polarizable Continuum Model (PCM) and the electric properties were calculated numerically with the finite field method and analytically with the coupled perturbed Hartree-Fock (CPHF) procedure. The results obtained show the importance of inclusion of the solvent effects with significant alteration in the values of the electric properties as compared with the results in the gaseous phase. Nevertheless, such properties are almost not affected by changes in the solvent polarity. Comparisons between the HF and MP2 results indicate that the first hyperpolarizability is particularly sensitive to the inclusion of the electron correlation effects. In addition, a substantial increase in the &#946; values is observed with the incorporation of a strong electron acceptor group. / Neste trabalho estudamos as propriedades elétricas de compostos mesoiônicos em fase gasosa e em diferentes solventes, com uma atenção especial para a primeira hiperpolarizabilidade (&#946; ). Utilizamos o Método de Hartree-Fock (HF) e a Teoria de Perturbação de Møller-Plesset de segunda ordem (MP2) para descrição da estrutura eletrônica. Os efeitos de solvente foram tratados por meio do Modelo Contínuo Polarizável e os cálculos das propriedades elétricas foram realizados usando os métodos de Campo Finito e de Perturbação Acoplada de Hartree-Fock. Os resultados obtidos mostram a importância da inclusão dos efeitos de solvente com significativas alterações nos valores das propriedades elétricas quando se compara com os resultados em fase gasosa. Entretanto, tais propriedades quase não são afetadas por mudanças na polaridade do solvente. Comparações entre os resultados MP2 e HF indicam que a primeira hiperpolarizabilidade é particularmente sensível à inclusão dos efeitos de correlação eletrônica. Adicionalmente, um aumento substancial nos valores de &#946; é observado com a incorporação de um grupo com um forte caráter aceitador de elétrons.