As reações de cicloadição 1,3-dipolar são uma poderosa ferramenta para a síntese de uma variedade de anéis heterocíclicos de cinco membros. A cicloadição de óxidos de nitrila à olefinas, em particular, é de considerável interesse para a obtenção de isoxazolinas, que são intermediários versáteis na síntese de produtos naturais e de materiais com potencial aplicação como cristais líquidos. A Teoria do Funcional da Densidade foi utilizada para estudar o mecanismo da reação cicloadição [3+2] 1,3-dipolar que ocorre, inicialmente, entre o óxido de benzonitrila e o ácido vinilacético. Para tal, foram empregados os funcionais PBE1PBE, B3LYP e CAM-B3LYP, no nível 6-311+G(2d,p). O efeito do solvente foi avaliado através dos modelos PCM e CPCM, com os solventes THF, acetonitrila e formamida. A análise dos Orbitais Moleculares de Fronteira e do recente modelo da distorção e interação do estado de transição (TS), foram utilizadas para explicar a regioquímica dos produtos obtidos e a formação do bisaduto 2:1, originário de duas sucessivas cicloadições envolvendo o óxido de benzonitrila. Na primeira etapa da reação, os cálculos evidenciaram a formação do produto 3,5-dissubstiuído. Embora existam diferenças quantitativas nas barreiras de ativação e reação calculadas com os três diferentes funcionais, a previsão dos produtos majoritários e estados de transição mais favoráveis é a mesma, independentemente do funcional utilizado. Contudo, a conformação dos estados de transição e dos produtos intermediário e final da reação sofre uma pequena alteração com a inclusão do efeito do solvente. A energia de ativação nas duas cicloadições aumenta com o incremento da polaridade do solvente, porém, a possibilidade de formação de uma ligação de hidrogênio no estado de transição é responsável por uma diminuição na energia total de ativação. Esse resultado está diretamente vinculado à polaridade do TS. Quando comparados os resultados obtidos com os dois modelos de solvente, observou-se que ΔE≠ e ΔEreação são essencialmente as mesmas, embora as energias eletrônicas calculadas com CPCM sejam levemente inferiores às calculadas com PCM. / The 1,3-dipolar cycloaddition reactions are a powerful tool for synthesizing a wide range of 5-membered heterocyclic rings. Particularly, the cycloaddition of nitrile oxides to olefins is considerably interesting to obtain isoxazolines, which are versatile intermediaries in the synthesis of natural products and materials with potential application such as liquid crystals. The Density Functional Theory has been used to study the [3+2] 1,3-dipolar cycloaddition reaction mechanism that initially occurs between benzonitrile oxide and vinylacetic acid. To do that, PBE1PBE, B3LYP and CAM-B3LYP functionals have been used at level 6- 311+G(2d,p). The solvent effect was evaluated through the PCM and CPCM models, with the THF, acetonitrile and formamide solvents. The analysis of the Frontier Molecular Orbitals and of the recent distortion and interaction model of transition state (TS) have been used to explain the regiochemistry of the products obtained and the formation of the bisadduct 2:1, which is originated from two successive cycloadditions involving benzonitrile oxide. In the first reaction step, the calculations showed the formation of the 3,5-dissubstituted product. Although there are quantitative differences in the activation and reaction barriers calculated with the three different functionals, the forecasting of more favorable majoritary products and transition states is the same, no matter the functional used. However, the conformation of the transition states and of the final and intermediary products of the reaction is slightly changed by the inclusion of the solvent effect. The activation energy of both cycloadditions increases with the polarity increment of the solvent, but the possibility of formation of a hydrogen bond in the transition state is responsible for a reduction of the total activation energy. That result is directly linked to the TS polarity. When we compare the results obtained with the two solvent models, we observe that ΔE≠ and ΔEreaction are essentially the same, although the electronic energies calculated with CPCM are slightly smaller than the ones calculated with PCM.

Orientadores: Harry Westfahl Junior, Amir Ordacgi Caldeira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-10T23:02:41Z (GMT). No. of bitstreams: 1 Fernandes_RafaelMonteiro_D.pdf: 6829160 bytes, checksum: 4dfae2ffd4b06c5c3c1f89f399700913 (MD5) Previous issue date: 2008 / Resumo: Um dos objetivos centrais das Ciências Naturais é relacionar as estruturas dos mais diversos sistemas com as funções particulares que os caracterizam. Por exemplo, no que se refere a materiais, sejam eles sintéticos ou biológicos, a ciência está constantemente buscando a predição de diferentes propriedades macroscópicas a partir do conhecimento das suas estruturas mi- croscópicas. Nesta tese, investigamos as propriedades magnéticas e de transporte de sistemas que apresentam interações competitivas em diferentes escalas de comprimento. Como resultado desta competição, surge um estado termodinâmico caracterizado por um parâmetro de ordem modulado, dando origem a uma série de con.gurações espacialmente inomogêneas. A termo- dinâmica destes estados modulados pode ser descrita pelo chamado modelo de Brazovskii, que prevê uma transição de primeira ordem, induzida pelas .utuações do parâmetro de ordem, entre a fase homogênea e a fase modulada. Há uma vasta gama de sistemas encontrados na Natureza que parecem se encaixar nesta descrição de Brazovskii, compreendendo estruturas tão díspares quanto cristais líquidos e condensados de píons em estrelas de nêutrons. No presente trabalho, investigamos dois sistemas físicos particulares. Motivados pela rica variedade de domínios obser- vados experimentalmente em filmes finos magnéticos, estudamos as propriedades magnéticas de blocos ferromagnéticos dipolares com dimensões finitas e condições de contorno não-periódicas. Desenvolvendo uma modelagem baseada na solução da Hamiltoniana de Brazovskii, pudemos explicar, de maneira inédita e consistente, a estrutura de domínios magnéticos dos filmes finos de MnAs:GaAs, um promissor candidato a aplicações no campo da spintrônica. Além disso, estabelecemos uma relação clara entre o fenômeno de reorientação magnética e a mudança na forma das curvas de histerese observada nesses filmes. O segundo tipo de sistemas que in- vestigamos foram os isolantes de Mott, cujas propriedades de transporte foram determinadas a partir do modelo de redes de resistores correlacionados. Considerando que a transição de Mott térmica pertence à classe de universalidade de Ising, mostramos que a condutividade macroscópica depende não apenas da magnetização, mas também da densidade de energia, dando origem a um comportamento de crossover. Através destes resultados, lançamos luz sobre a aparente e misteriosa incoerência entre as previsões teóricas e as medidas experimentais recentes envolvendo isolantes de Mott não-dopados. Prosseguindo para as fases inomogêneas dos isolantes de Mott dopados, estudamos a condutividade macroscópica das mesofases eletrônicas com ordenamento de carga esmético e nemático, as quais são encontradas nos niquelatos e nos cupratos, respectivamente. Inspirados nos conceitos da Física dos cristais líquidos, expressamos de forma bastante intuitiva a relação entre as propriedades de transporte e a termodinâmica das mesofases eletrônicas anisotrópicas, descrita pelo modelo de Brazovskii / Abstract: Natural Sciences is to relate the structures of systems to example, in what concerns materials, either synthetic or for the prediction of different macroscopic properties from the knowledge of their microscopic structure. In this thesis, we investigate the magnetic and transport properties of systems with competing interactions in distinct length scales. As a result of such a competition, there is a thermodynamic state characterized by a modulated order parameter, originating a set of spatially inhomogeneous con½gurations. The thermodynamics of these modulated states can be described by the so-called Brazovskii model, which predicts a fluctuation induced first order transition from the homogeneous phase to the modulated phase. There is a large diversity of systems for which the Brazovskii description seems suitable, including utterly disparate structures such as liquid crystals and pion condensates in neutron stars. In the present work, we investigate two particular physical systems. Motivated by the rich variety of domains experimentally observed in magnetic thin films, we study the magnetic properties of ferromagnetic dipolar slabs with finite dimensions and non-periodic boundary conditions. After developing a model based on the solution of the Brazovskii Hamiltonian, we were able to explain, in a consistent and novel way, the magnetic domain structures of MnAs:GaAs thin films, which are promising candidates for spintronics devices. Moreover, we established a clear connection between the film's magnetic reorientation and the experimentally observed change in the hysterisis loops shape. The second class of systems we investigated were the Mott insulators, whose transport properties were determined from the correlated resistor network model. After considering that the finite temperature Mott transition belongs to the Ising universality class, we showed that the macroscopic conductivity depends not only on the magnetization, but also on the energy density, giving rise to crossover behaviour. Using these results, we shed light upon the apparent and mysterious inconsistency between the theoretical predictions and the experimental measurements regarding undoped Mott insulators. Proceeding to the inhomogeneous phases of doped Mott insulators, we studied the macroscopic conductivity of electronic mesophases with smectic and nematic charge ordering, which are found in the nickelates an in the cuprates, respectively. Inspired by the concepts from the Physics of liquid crystals, we expressed in an intuitive way the connection between the transport properties and the thermodynamics of anisotropic electronic mesophases described by the Brazovskii model / Doutorado / Física da Matéria Condensada / Doutor em Ciências

O campo magnético da Terra varia em diferentes escalas de tempo, de milissegundos a bilhões de anos. Os dados de observatórios magnéticos e satélites obtidos nos últimos 150 anos indicam que o momento do dipolo magnético terrestre está diminuindo continuamente. Essa queda está associada à presença de fontes não-dipolares do campo em uma extensa região que abrange todo o Atlântico Sul e uma porção da América do Sul, sendo que no Brasil a contribuição dessas fontes varia fortemente com a latitude. Em escala de tempo arqueomagnética (~1.000-10.000 anos) a evolução do campo magnético terrestre não é tão bem estabelecida, principalmente em função da escassez de dados no hemisfério Sul, que contribui com apenas 5% dos dados de intensidade obtidos para os últimos 4.000 anos. A América do Sul, com alguns poucos resultados no Peru, Equador e Bolívia, pode ser considerada a terra incógnita da arqueointensidade. Nesta tese são apresentados os primeiros resultados arqueomagnéticos para o território brasileiro. Foram escolhidas duas regiões de estudo, o Nordeste e o Sudeste do Brasil, situadas em diferentes faixas de latitude de modo a investigar diferentes contribuições de componentes não-dipolares do campo. No Nordeste, as amostras foram coletadas na cidade de Salvador (BA), a primeira capital do Brasil, fundada em 1549 AD. Na região Sudeste a amostragem foi efetuada nas cidades de Anchieta (ES), Rio de Janeiro (RJ), Niterói (RJ), Iperó (SP), Piracicaba (SP) e Botucatu (SP). Nas duas regiões, a paleointensidade do campo magnético terrestre foi obtida em materiais construtivos (tijolos e alguns fragmentos de telhas) datados entre 1550 AD e 1920 AD. As idades desses materiais foram estabelecidas com base em estudos arqueológicos e registros históricos das construções, fornecendo incertezas inferiores a 30 anos para a grande maioria das amostras. As paleointensidades foram estimadas utilizando-se dois métodos: (a) duplo aquecimento com medidas em temperatura ambiente, pelo protocolo de Thellier modificado por Coe; (b) duplo aquecimento com medidas contínuas em alta temperatura, pelo protocolo Triaxe. Após as medidas e correções magnéticas, todas as amostras foram analisadas com base em rigorosos critérios de seleção, que resultaram em 23 novas determinações de intensidade de alta qualidade (correspondendo a um total de 584 espécimes analisados, com uma taxa de sucesso de 57%). A partir desses resultados foram traçadas duas curvas de variação da intensidade do campo magnético para cada uma das regiões estudadas, abrangendo os últimos 500 anos. Essas curvas revelam uma oscilação do momento de dipolo nos últimos cinco séculos, que não foi prevista nos modelos de campo disponíveis atualmente, trazendo implicações importantes no entendimento da evolução dos campos dipolar e não-dipolar nessa escala de tempo. As variações rápidas descritas nessas curvas permitem aplicar o arqueomagnetismo como ferramenta de datação arqueológica, como exemplificado pela datação de uma casa do Pelourinho em Salvador. / The Earth\'s magnetic field varies in different timescales, from milliseconds to billions of years. Magnetic data from observatories and satellites indicate that the dipole moment has continuously been decreasing for the past 150 years. This decay is associated to the presence of non-dipole sources covering a wide region that encompasses the South Atlantic and part of South America; in Brazil, the contribution of the non-dipole fields varies strongly with latitude. In the archeomagnetic timescale (~1,000-10,000 years), the evolution of the Earth\'s magnetic field is not well established, mainly due to the scarcity of data from southern hemisphere, which contributes with only 5% of the intensity data for the past 4,000 years. South America is the terra incognita of archeointensity, counting only a handful of results from Peru, Ecuador and Bolivia. This thesis presents the first archeomagnetic results from Brazil. In order to investigate different contributions of non-dipolar sources, we concentrated our sampling in two regions located in different latitudes the Northeast and Southeast regions of Brazil. In the Northeast region, all samples were collected in the city of Salvador (BA), the first Brazilian capital settled in 1549 AD. In the Southeast region, sampling was conducted in the cities of Anchieta (ES), Rio de Janeiro (RJ), Niterói (RJ), Iperó (SP), Piracicaba (SP) and Botucatu (SP). All paleointensity data was obtained from architectural fragments (bricks and some tiles) dated between 1550 AD and 1920 AD. The age of bricks and tiles was established on the basis of archeological studies and the historical record of the buildings, providing age uncertainties of less than 30 years for most of the samples. Paleointensity estimates were obtained by two methods: (a) double-heating with measurements in room temperature, using the modified version of the Thellier protocol; (b) double-heating with measurements in high temperatures, using the Triaxe protocol. After measurements and magnetic corrections, all samples were screened using strict selection criteria resulting in 23 high-quality new site-mean intensity values (from 584 analyzed specimens, with a success rate of 57%). These results were integrated into two curves of geomagnetic intensity variation for each studied region over the past five centuries. These curves reveal an oscillating dipole moment for the past five centuries, a behavior not predicted in currently available geomagnetic field models, thus providing key information on the dipole and non-dipole field evolutions in this timescale. The rapid intensity changes described in these curves permit the application of archeointensity techniques as an archeological dating tool, as exemplified by the dating of a house from the Pelourinho area, in Salvador city.

Doctor of Philosophy / Department of Physics / Talat S. Rahman / In this work we present a study of the geometric, electronic, vibrational and magnetic properties of several nanostructured systems for which experimental data call for a theoretical understanding. In order to investigate the effect of magnetic dipolar interactions on the magnetization of nanomagnets arranged in finite lattices, we utilize a phenomenological classical approach, which is based on the Landau-Lifshitz equation. Dipolar interactions lead to hysteretic behavior of the magnetization curves and established that the external field sweep rate, sample temperature, and shape anisotropy play a role in determining the specifics. Our results (derived from a classical approach) for magnets arranged in a square lattice suggest that stepped hysteresis curves do not have necessarily a quantum origin (quantum tunneling of the magnetization). We also find that in the square lattice small changes in the dipolar strength introduce sudden transitions in the magnetic hysteresis. For the examination of geometric vibrational and electronic structure of systems of interest, we turn to density functional theory (DFT), which is the leading technique for modeling nanoscale systems from first principles. We have applied DFT to either address some old queries of surface science, such as the dynamics of the CO-chemisorbed Cu(001) surface, or to contribute to the forefront of hydrogen-based economy through the comprehension of the growth and diffusion of Pt islets on Ru(0001), or to predict the geometric and electronic properties of materials to-be-created, as in the case of core-shell bimetallic nanoclusters. In the case of CO on Cu(001), although the bond has been considered to be weak enough so as to treat the adsorbate and substrate separately, our calculations are able to reproduce measurements and provide evidence that the dynamics of the molecule is influenced by the substrate and vice versa, as well as by intermolecular interactions. Taking into account the adsorbate-substrate interplay, has furthermore clarified issues that were pending for the clean surface and led to the correct interpretation of some features in the phonon dispersion of the chemisorbed surface. DFT has also directed us to the conclusion that the catalytic properties of few-atom Pt islets on Ru nanoclusters are preserved by the low probability of these islets to diffuse through the edges of the Ru nanoclusters. Moreover, the analysis of the Ag_{27}Cu_7 nanoalloy from ab initio methods has opened a wide panorama in terms of the geometry, coordination, energetics, and electronic structure of alloyed phases, in general,that may aid in the assembling on new materials.

1-Aza-2-azoniaallene salts are reactive intermediates that undergo [3+2] cycloaddition with many different types of multiple bonds. For the past several years, the Brewer group has studied the reactivity of these intermediates in intramolecular reactions, and have discovered that these cationic heteroallenes can react through a variety of other, mechanistically distinct, pathways to give different classes of nitrogen heterocycles. For example, prior work in the Brewer group revealed that 1-aza-2-azoniaallene salts could react in an intramolecular [4+2] cycloaddition reaction to give protonated azomethine imine salts containing a 1,2,3,4-tetrahydrocinnoline scaffold. Further study of the scope and limitations of this Diels-Alder-like reaction are described herein. These studies primarily focused on how varying the N-aryl ring and alkene substituents affected the reaction. We discovered that in several instances, the metal mediated reaction did not facilitate the cycloaddition very well, so we searched for alternative ways to facilitate the reaction. We discovered that a non-metallic Lewis acid (TMSOTf) provided very clean products with α-chloroazo compounds. I hypothesized that changing the leaving group adjacent to the azo might further improve the reaction. With this in mind, I developed a technique to prepare α-trifluoroacetoxyazo compounds by treating aryl hydrazones with trifluoroacetoxy dimethylsulfonium trifluoroacetate. This technique is compatible with all types of functional groups including nitro aryl compounds, which gave low yields of the corresponding chloroazo derivatives. Importantly, these α-trifluoroacetoxyazo compounds gave even better cycloaddition results when treated with TMSOTf, and this method is more practical, more environmentally friendly, and greener than the metal mediated technique. This process even returned sterically hindered products in high yield, and provide a dearomatized non-protonated azomethine imine salt, which further verified the proposed mechanism of the [4+2] cycloaddition. Azomethine imines are well known to undergo 1,3-dipolar cycloadditions with alkenes. We wondered if the protonated azomethine imine salts generated by the [4+2] cycloaddition could be used in a subsequent base-mediated [3+2] cycloaddition to generate structurally complex tetra- or pentacyclic products. We were pleased to find that the protonated azomethine imines indeed reacted smoothly with a variety of π-system in the presence of triethylamine to give the corresponding cycloadducts in high yields with moderate to high diastereoselectivities. In an attempt to understand the diastereoselectivity of these [3+2] cycloadditions better, I modeled them computationally.

In this thesis, different cycloaddition reactions, such as the enantioselective 1,3-dipolar-cycloaddition, which takes place between in situ generated stabilized azomethine ylides, and electrophilic alkenes, and the diastereoselective multicomponent reactions Amine-Aldehyde-Dienophile (AAD) or Phosphoramidate-Aldehyde-Dienophile (PAD) are described. In Chapter 1, an asymmetric 1,3-dipolar cycloaddition reaction involving an imino ester with tert-butyl acrylate was carried out using a silver(I) complex with double chirality, formed from a chiral phosphoramidite and chiral silver binolphosphate(I). The goal of this reaction is to synthesize key enantiomerically enriched structures to access the GSK-third generation of HCV inhibitors. In Chapter 2, the synthesis of polysubstituted cyclohex-2-enylamines using the multicomponent Amine-Aldehyde-Dienophile reaction involving benzyl or 4-methoxybenzylamine, is described. The study the diastereoselective version, employing commercially available chiral benzylic amines, or even a maleimide with the chiral information at the nitrogen atom, are also reported. In Chapter 3, the synthesis of polysubstituted cyclohex-2-enylamines derivatives using the multicomponent Phosphoramidate-Aldehyde-Dienophile (PAD), is described. Several series of N-substituted phosphoramidates reacted with α,β-unsaturated aldehydes, bearing hydrogen atoms at the γ-position, in good yields.

No description available.

<p>This thesis evaluates several solid-state NMR spectroscopy approaches to studying lithium ion dynamics in solid-state electrolytes. With the goal of reducing the risks associated with current liquid electrolytes, solid-state electrolytes provide non-flammable materials that are also stable against attack by cathode and anode materials. Solid-state NMR spectroscopy offers a versatile method to determine structural details and can also provide information about ion mobility in solid-state electrolytes. Challenges involved in the study of solid-state electrolytes include the difficulty in distinguishing between ^6,7Li resonances due to the small chemical shift range of diamagnetic lithium species. The NMR methods selected in this thesis aim to circumvent some of these issues in order to determine structural and dynamic properties in solid-state electrolytes. Several different electrolytes have been examined including LaLi_0.5Fe_0.2O_2.09 and related materials, which exhibit intricate structural properties. ¹³⁹La NMR spectroscopy, in combination with ⁷Li MAS NMR spectroscopy, was used to determine the nature of this disorder. In addition, studies of the quadrupolar framework ⁸⁷Rb nucleus, which take advantage of its large electric field gradient, have been used to indirectly probe the activation energy for Ag⁺ ion hopping in the solid-state silver ion electrolyte RbAg₄I₅. Alternatively, dipolar coupling between ⁶Li and ⁷Li has been used to compare lithium ion hopping rates in Li₆BaLa₂M₂O₁₂ (M = Ta, Nb) using ⁶Li{⁷Li}-REDOR NMR studies. Finally, T₂ relaxation studies have been used to probe ion dynamics in Li₃V₂(PO₄)₃ and LiVO₃ in order to determine if this is a viable method to study dynamics in these materials.</p> / Doctor of Philosophy (PhD)

This thesis presents analyses of hydrogen-bonded materials using solid-state nuclear magnetic resonance (NMR) spectroscopy. Proton dynamics were investigated in two classes of phosphate-based proton conductors: phosphate solid acids and tin pyrophosphates. These materials have the potential to be used as solid state proton conductors in fuel cells. Proton dynamics in phosphate solid acids were probed based on the attenuation of homonuclear dipolar coupling with increasing temperature. These studies showed that homonuclear dipolar recoupling NMR techniques can be employed in complex multi-spin systems. Additionally, two pathways for proton hopping in monoclinic RbH2PO4, a sample with two proton environments, were identified and quantified for the first time using a combination of dipolar recoupling and proton exchange NMR methods. Tin pyrophosphates, another class of solid-state proton conductor with analogous phosphate tetrahedral structure, were studied. Proton dynamics had to be analyzed via exchange-based NMR techniques as a result of low proton concentration in these materials. Proton mobility in tin pyrophosphate was found to increase with increased protonation. Furthermore, hydrogen bonding was investigated as a coordination mode in silicone boronic acid (SiBA) elastomers, potential materials for contact lens manufacture. As in the phosphate-based proton conductors, hydrogen bonding played an important role in the structure of the SiBA elastomers as one of the mechanisms through which these materials crosslink. In addition to hydrogen bonding, covalent bonding between boronic acids was found to occur at three- and four-coordinate boron centers. The purpose of this study was to determine the influence of boronic acid loading and packing density on crosslinking in SiBA elastomers. Boron coordination environments were investigated by 11B quadrupolar lineshape analysis. The incidence of four-coordinate dative bonding, a predictor of the stress-strain response in these materials, increased with boronic acid loading but was most heavily influenced by boronic acid packing density. / Thesis / Doctor of Philosophy (PhD) / Hydrogen bonds are intermolecular interactions that are significant in many structural (low crystal density in ice) and dynamic (enzymatic processes occurring under biological conditions) processes that are necessary to maintain life. In this thesis, solid-state nuclear magnetic resonance (NMR) spectroscopy is used to explore proton dynamics of hydrogen-bonded networks in various materials. Advanced NMR experiments that probe homo- and heteronuclear dipolar coupling interactions revealed possible pathways for proton transport in phosphate-based proton conducting materials. This study provided a better understanding of ion conducting mechanisms that can be used in intermediate-temperature fuel cell applications. Additionally, solid-state NMR was used in the identification of hydrogen bonding and other coordination modes in silicone boronate acids (SiBA), a class of elastomers with potential applications as contact lens. Boron coordination in SiBA elastomers was dependent on both boronic acid loading and boronic acid packing density.

La mesure de couplage dipolaire permet d’accéder à la structure tridimensionnelle d’un composé solide. Cependant, en présence d’une forte densité de spins couplés, le phénomène de troncature dipolaire rend difficile l’obtention de ces informations par RMN du solide. Ce problème peut être affranchi par l’étude de spins rares en abondance naturelle. En effet, avec une abondance naturelle de 1.1 %, la probabilité que trois 13C soient couplés, et avec elle la troncature dipolaire, devient négligeable. Une méthodologie basée sur la séquence de recouplage dipolaire POST-C7 permet d’accéder à des informations structurales d’échantillons en abondance naturelle sensibles à la fois à la conformation moléculaire et à l’empilement cristallin par mesure de couplages dipolaires 13C-13C. La sensibilité de détection des signaux RMN 13C est augmentée à l’aide la polarisation dynamique nucléaire ce qui permet de réduire considérablement les temps d’expériences. De plus, la séquence de recouplage R20_9_2 aidée de supercycles s’est montrée être plus robustes que POST-C7 face à de fortes anisotropies de déplacement chimique ou de forts couplages hétéronucléaires 1H-13C. La seconde problématique abordée concerne l’attribution de signaux 13C. En effet, il existe seulement quelques exemples de détermination de connectivités 13C -13C en abondance naturelle. Nous montrons ici que des spectres de corrélations dipolaires 13C-13C peuvent être obtenus en quelques jours à l’aide de la séquence de recouplage R20_9_2. Contrairement aux méthodologies basées sur le couplage J, notre séquence requiert un temps d’excitation DQ plus court ce qui la rend adaptée à l’étude de solides désordonnés. / Measurment of dipolar coupling provides 3D structural information of powder samples. However, in practice, the high density of spins in organic compounds prevents the measurements of long-range dipolar couplings in solid-state NMR by the so-called dipolar truncation effect. The study of rare spins on natural abundance allows to overcome this problem. In fact, with a natural abundance of 1.1 %, the probability for three 13C to be coupled is negligible. We developed a methodology based either on the dipolar recoupling NMR pulse sequence POST-C7 or on the dramatic increase in sensitivity provided by dynamic nuclear polarization. We demonstrated that its methodology provides a measure of 13C-13C dipolar couplings in natural abundance powder samples and that the so-obtained distance information is sensitive to both molecular conformation and crystal packing of powder samples. Moreover, we show that the recoupling pulse sequence R20_9_2 is more robust to strong chemical shift anisotropy and also to strong 1H-13C heteronuclear dipolar couplings than POST-C7. The second challenge involves 13C signal assignment for natural abundance. In fact, there are only a few examples of 13C-13C correlation spectra obtained for natural abundance samples. Here, we show that 13C-13C correlation spectra sequence based on the reintroduction of 13C−13C dipolar couplings can be obtained with standard MAS probe and within few days using R20_9_2 pulse sequence. Contrary to pulse sequences based on 13C-13C J coupling, our pulse sequence requires shorter DQ excitation time and hence, is more suitable for samples having short T2 relaxation times such as amorphous solids.