No description available.

Comportamento oscilatório manifesta-se em diversas escalas da experiência humana estando presente desde sistemas vivos, como no caso dos ritmos biológicos, até estruturas artificiais como a evolução de preços na bolsa de valores. Em eletroquímica e, em particular, na área de eletrocatálise, oscilações de potencial do eletrodo ou corrente que flui através deste são facilmente observáveis, requerendo apenas condições simples de controle experimental como a aplicação de uma corrente constante ou a utilização de uma resistência externa. Diversos trabalhos na literatura exploram as especificidades do comportamento oscilatório durante a oxidação de moléculas orgânicas importantes para a tecnologia de células a combustível, como metanol, ácido fórmico, etanol, entre outros, algumas vezes fazendo uso de simulações com modelos específicos. Por outro lado, é interessante o uso de modelos gerais para que se possa entender como propriedades comuns a todos os sistemas oscilatórios como frequência oscilatória e amplitude estão relacionadas, por exemplo, com o envenenamento do eletrodo e a recuperação autocatalítica de sítios livres. O presente trabalho foi desenvolvido tanto no âmbito experimental investigando as propriedades da oxidação oscilatória de metanol e etanol, quanto com o uso de simulações utilizando modelos generalistas e avaliando o resultado da introdução de efeitos específicos como o bloqueio da superfície por um veneno catalítico e respectiva liberação através da oxidação da espécie. Determinou-se, especificamente para o estudo de simulação, que o aumento das constantes de bloqueio e oxidação relativas ao veneno produziu duplicação no número de bifurcações encontradas em regime potenciostático, e levou ao aumento da frequência e diminuição da amplitude oscilatórias em regime galvanostático. Ferramentas numéricas foram propostas e validadas para avaliar a velocidade de envenenamento e o grau de harmonicidade das séries temporais. Os experimentos oscilatórios com metanol e etanol revelaram que a primeira molécula apresenta oscilações de maior frequência e menor amplitude. Esse comportamento foi explicado pela maior velocidade de envenenamento observada para a reação de metanol e os resultados demonstraram experimentalmente os achados de simulação numérica. Finalmente, o efeito do bloqueio da superfície por ânions foi estudado através de análise numérica. Determinou-se que a diminuição do tempo total oscilatório quando da adição da espécie é função da maior velocidade de variação do potencial médio e uma expressão relacionando concentração do ânion e tal velocidade foi encontrada. / Oscillatory behavior can be seen at several levels of the human experience stemming from living systems, in the case of biological rhythms, to artificial structures such as the evolution of prices at the market stock. In electrochemistry and, particularly, in the field of electrocatalysis, oscillations of the electrode potential or current can be easily observed and requires only simple experimental control conditions such as applying a fixed value of current or through the use of an external resistance. Several studies found in the literature explore the specificities of oscillatory behavior found during the oxidation of organic molecules that are important for the development of the technology of fuel cells, such as methanol, formic acid, ethanol, among several others, some of those studies even making use of numerical simulations for specific models. On the other hand, it is interesting to explore general models that can embrace oscillatory properties that are common to several systems such as how frequency and amplitude relate to the dynamics of surface poisoning and the autocatalytic recovery of free sites. The work presented in this thesis was developed on an experimental basis with the investigation of oscillatory properties for the reactions of methanol and ethanol, as well as theoretical one, with the proposal and use of general models and the evaluation of specific effects such as surface blockage by a catalytic poison and surface recovery through its oxidation. The simulation study determined specifically that increasing the rates of surface blockage and poison oxidation would duplicate the amount of oscillatory regions found in the bifurcation diagrams for potentiostatic conditions. For the galvanostatic one, it was found that the increase in the rate of those velocities would increase the frequency of oscillations and decrease their amplitudes. Numerical tools were proposed and validated to evaluate the velocity of poisoning and the degree of harmonicity of the time series. Oscillatory experiments employing methanol and ethanol revealed that the first molecule display oscillations with higher frequency and lower amplitude than for ethanol. This behavior was explained by the greater degree of self-poisoning observed for the methanol reaction and the results were comparable to those found in the numerical study. Finally, the effect of a surface blocking anion was studied with the use of numerical analysis. It was determined that the decrease in total oscillatory duration when the anion was added was the result of a greater rate of change of the average potential. An expression relating both quantities was devised.

Comportamento oscilatório manifesta-se em diversas escalas da experiência humana estando presente desde sistemas vivos, como no caso dos ritmos biológicos, até estruturas artificiais como a evolução de preços na bolsa de valores. Em eletroquímica e, em particular, na área de eletrocatálise, oscilações de potencial do eletrodo ou corrente que flui através deste são facilmente observáveis, requerendo apenas condições simples de controle experimental como a aplicação de uma corrente constante ou a utilização de uma resistência externa. Diversos trabalhos na literatura exploram as especificidades do comportamento oscilatório durante a oxidação de moléculas orgânicas importantes para a tecnologia de células a combustível, como metanol, ácido fórmico, etanol, entre outros, algumas vezes fazendo uso de simulações com modelos específicos. Por outro lado, é interessante o uso de modelos gerais para que se possa entender como propriedades comuns a todos os sistemas oscilatórios como frequência oscilatória e amplitude estão relacionadas, por exemplo, com o envenenamento do eletrodo e a recuperação autocatalítica de sítios livres. O presente trabalho foi desenvolvido tanto no âmbito experimental investigando as propriedades da oxidação oscilatória de metanol e etanol, quanto com o uso de simulações utilizando modelos generalistas e avaliando o resultado da introdução de efeitos específicos como o bloqueio da superfície por um veneno catalítico e respectiva liberação através da oxidação da espécie. Determinou-se, especificamente para o estudo de simulação, que o aumento das constantes de bloqueio e oxidação relativas ao veneno produziu duplicação no número de bifurcações encontradas em regime potenciostático, e levou ao aumento da frequência e diminuição da amplitude oscilatórias em regime galvanostático. Ferramentas numéricas foram propostas e validadas para avaliar a velocidade de envenenamento e o grau de harmonicidade das séries temporais. Os experimentos oscilatórios com metanol e etanol revelaram que a primeira molécula apresenta oscilações de maior frequência e menor amplitude. Esse comportamento foi explicado pela maior velocidade de envenenamento observada para a reação de metanol e os resultados demonstraram experimentalmente os achados de simulação numérica. Finalmente, o efeito do bloqueio da superfície por ânions foi estudado através de análise numérica. Determinou-se que a diminuição do tempo total oscilatório quando da adição da espécie é função da maior velocidade de variação do potencial médio e uma expressão relacionando concentração do ânion e tal velocidade foi encontrada. / Oscillatory behavior can be seen at several levels of the human experience stemming from living systems, in the case of biological rhythms, to artificial structures such as the evolution of prices at the market stock. In electrochemistry and, particularly, in the field of electrocatalysis, oscillations of the electrode potential or current can be easily observed and requires only simple experimental control conditions such as applying a fixed value of current or through the use of an external resistance. Several studies found in the literature explore the specificities of oscillatory behavior found during the oxidation of organic molecules that are important for the development of the technology of fuel cells, such as methanol, formic acid, ethanol, among several others, some of those studies even making use of numerical simulations for specific models. On the other hand, it is interesting to explore general models that can embrace oscillatory properties that are common to several systems such as how frequency and amplitude relate to the dynamics of surface poisoning and the autocatalytic recovery of free sites. The work presented in this thesis was developed on an experimental basis with the investigation of oscillatory properties for the reactions of methanol and ethanol, as well as theoretical one, with the proposal and use of general models and the evaluation of specific effects such as surface blockage by a catalytic poison and surface recovery through its oxidation. The simulation study determined specifically that increasing the rates of surface blockage and poison oxidation would duplicate the amount of oscillatory regions found in the bifurcation diagrams for potentiostatic conditions. For the galvanostatic one, it was found that the increase in the rate of those velocities would increase the frequency of oscillations and decrease their amplitudes. Numerical tools were proposed and validated to evaluate the velocity of poisoning and the degree of harmonicity of the time series. Oscillatory experiments employing methanol and ethanol revealed that the first molecule display oscillations with higher frequency and lower amplitude than for ethanol. This behavior was explained by the greater degree of self-poisoning observed for the methanol reaction and the results were comparable to those found in the numerical study. Finally, the effect of a surface blocking anion was studied with the use of numerical analysis. It was determined that the decrease in total oscillatory duration when the anion was added was the result of a greater rate of change of the average potential. An expression relating both quantities was devised.

The thesis entitled “Exploring Diverse Facets of Small Molecules by NMR Spectroscopy” consists of six chapters. The main theme of the thesis is to exploit one and two dimensional NMR methodologies for understanding the diverse facets of small organic molecules, such as, weak intra- and inter- molecular interactions, chiral discrimination, quantification of enantiomeric excess and assignment of absolute configuration. Several new pulse sequences have also been designed to solve specific chemical problems, in addition to extensive utility of existing one and two dimensional NMR experiments. The results obtained on different problems, are discussed under six chapters in the thesis. The brief summary of each of these chapters is given below. Chapter 1 begins with the discussion on the importance of small molecules and their various facets, the analytical techniques available in the literature to study them. The role of NMR spectroscopy as powerful analytical technique to understand the diverse facets of organic molecules and their importance is set out in brief. A short introduction to the basic principles of NMR, the interaction parameters, the commonly employed one and two dimensional homo- and herero- nuclear NMR experiments are also given. The basic introduction to product operators essential for understanding the spin dynamics in the developed pulse sequences is given. The application of diffusion ordered spectroscopy (DOSY), the general problems encountered in the analysis of combinatorial mixtures and the matrix assisted method in circumventing such problems are discussed. Chapter 2 focuses on the chiral discrimination and the measurement of enatiomeric excess. The NMR approach to discriminate enantiomers using chiral auxiliaries such as, solvating agents, derivatizing agents, lanthanide shift reagents, the choice of such auxiliaries and the limitations are discussed in detail. The in-depth discussion on the new protocols developed using both the solvating and derivatizing agents for enantiomeric discrimination of chiral amines, hydroxy acids and diacids are discussed. The new three-component protocols that serve as chiral derivatizing agents for the discrimination of primary amines, diacids and hydroxy acids are discussed. Also the role of organic base such as DMAP in the chiral discrimination is explored for discrimination of acids using BINOL as a chiral solvating agent. Accordingly the discussion is classified into two sections. In the first section the protocol developed utilizing an enantiopure mandelic acid, a primary amine substrate and 2-formylphenylboronic acid that is ideally suited for testing the enantiopurity of chiral primary amines is discussed. The broad applicability of the protocols for testing enantiopurity has been demonstrated on number of chiral molecules using 1H and 19F NMR. The second section contains the results on the new concept developed for discrimination of hydroxy acids. The strategy involves the formation of three component protocol using chiral hydroxy acid, R-alphamethylbenzylamine and 2-formylphenylboronic acid for 1H-NMR discrimination of diacids. The section also includes the utility of ternary ion-pair complex for the discrimination of acids. The ternary ion-pair not only permitted the testing of enantiopurity of chiral acids, but is also found useful for the measurement of enantiomeric excess. Chapter 3 discusses the utilization of the developed three-component protocols for the assignment of absolute configurations of molecules of different functionality. The protocols for the assignments of absolute configuration of primary amines using 2-formylphenylboronic acid and mandelic acid yielded the substantial chemical shift differences between diastereomers. The consistent trend in the direction of change of chemical shifts of the discriminated proton(s) gave significant evidence for employing them as parameters for the assignment of spatial configuration of primary amines. Another protocol using 2-formylphenylboronic acid, hydroxy acids and enantiopure alphamethylbenzylamine permitted their configurational assignment. In the second section a novel solvating agent, obtained by the formation of an ion-pair complex among enantiopure BINOL, DMAP and chiral hydroxy acid for the assignment of the spatial configuration of hydroxy acids is discussed. Chapter 4 focuses on the development of novel NMR methodologies, and also the utility of existing two-dimensional experiments for addressing certain challenging problems. This chapter has been divided into three sections. In Section-I the utilization of well-known homonuclear 2D-J-resolved methodology for unravelling the overlapped NMR spectra of enantiomers, an application for chiral discrimination and the measurement of enantiomeric excess is discussed. The utilization of the chiral auxiliaries, such as, chiral derivatizing agents, chiral solvating agents and lanthanide shift reagents permits enantiodiscrimination and the measurement of excess of one form over the other. Nevertheless many a times one encounters severe problems due to small chemical shift difference, overlap of resonances, complex multiplicity pattern because of the presence of number of interacting spins, and enormous line broadening due to paramagnetic nature of the metal complex. This section is focused on combating such problems utilizing 2D-J-1JNH resolved spectroscopy where a 450 tilting of the spectrum in the F2 dimension, yielded the pure shift NMR spectrum. The method circumvents several problems involved in chiral discrimination and allows the accurate measurement of enantiomeric excess. In Section-II, the development of novel NMR experimental methodology cited in the literature as C-HetSERF and its application for the study of symmetric molecules, such as, double bonded cis- and trans- isomers, and extraction of magnitudes and signs of long range homo- and hetero- nuclear scalar couplings among chemically equivalent protons in polycylic aromatic hydrocarbons is discussed. The extensive utility of the new pulse sequence has been demonstrated on number of symmetric molecules, where the conventional one dimensional experiment fails to yield spectral parameters. In section III, yet another novel pulse sequence called RES-TOCSY developed for unravelling of the overlapped NMR spectrum of enantiomers and the measurement of enantiomeric contents, has been utilized for the accurate measurement of magnitudes and signs of 1H-19F couplings in fluorine containing molecules. The method has distinct advantages as the strengths of the couplings and their relative signs could be extracted on diverse situations, such as, couplings smaller than line widths, the spectrum where the coupling fine structures are absent. Chapter 5 covers the study of nature of intra- and inter- molecular hydrogen bond in amide and its derivative. The chapter is accordingly divided into two sections. In the first section the study of acid and amide hydrogen bonding is discussed and the hydrogen bonded interactions are probed by extensive utility of 1H, 13C and 15N-NMR. The temperature perturbation experiments, measurements of the variation in the couplings, monitoring of diffusion coefficients and the association constants, detection of through space correlation have given unambiguous evidence for the hydrogen bond formation. The results were also supported by DFT calculations. Similar interaction in the solid state has also been derived by obtaining the crystal structure of complex phenylacetic acid with benzamide. In the second section of the chapter the hydrogen bond interaction of organic fluorine in trifluoromethyl derivatives of benzanilides has been explored and the involvement of CF3 group in the hydrogen bonding has been detected. The evidence for the participation of CF3 group in hydrogen bond has been confirmed by number of experiments, such as, the detection of through space couplings, viz., 1hJFH, 1hJFN, and 2hJFF , where the spin polarization between the interacting spins is transmitted through hydrogen bond, the temperature and solvent dependent studies, variation in the 1JNH and two dimensional heteronuclear correlation experiments. In an interesting example of a molecule containing two CF3 groups situated on two phenyl rings of benzanilide, the simultaneous participation of fluorines of two CF3 groups in hydrogen bond has been detected. The confirmatory evidence for such an interaction, where hydrogen bond mediated couplings are not reflected in the NMR spectrum, has been derived by 19F−19F NOESY. Significant deviations in the strengths of 1JNH, in addition to variable temperature, and the solvent induced perturbation studies yielded additional evidence. The NMR results are corroborated by both DFT calculations and MD simulations, where the quantitative information on different ways of involvement of fluorine in two and three centered hydrogen bonds, their percentage of occurrences, and geometries have been obtained. The hydrogen bond interaction energies have also been calculated. The study revealed the rare observation and the first example of the C-F…H-N hydrogen bond in solution state in the molecules containing CF3 groups. Chapter 6 focuses on the mixture analysis using the diffusion ordered spectroscopy (DOSY). High Resolution-DOSY works when the NMR spectrum is well resolved and the diffusion coefficients of the combinatorial mixtures are substantially different from each other. DOSY technique fails when the mixture contains the molecules of nearly identical weights and similar hydrodynamic radii. Thus, the positional isomers, enantiomers consequent to their nearly identical rates of diffusion, are not differentiated. Some of these problems can be overcome by Matrix-Assisted Diffusion Order Spectroscopy (MAD-spectroscopy), where an external reagent acts as a matrix and aids in their diffusion edited separation, provided the molecules embedded in it possess differential binding abilities with the matrix. Such different binding properties of the matrix are the basis for resolution of many isomeric species. In the present study three different novel auxiliaries, micelles-reverse micelles, crown ether and cyclodextrin are introduced for the resolution of positional isomers, double bonded isomers, viz., fumaric acid and maleic acid and also enantiomers. Accordingly, the results of each of these studies are discussed in three different sections.