This thesis is concerned with dynamic electrochemical experiments with different concentrations of supporting electrolyte. Normally supporting electrolyte is added to a solution in order to avoid undesirable effects as migration and potential drop in solution. However, in the present thesis we focus on the study and understanding of such effects as the concentration of supporting electrolyte decreases. First a theoretical treatment is proposed, based on numerical simulation using the Nernst-Planck- Poisson system of equations. The theoretical treatment is compared with previous works as electroneutrality, the differences between both models are explained. The model is also compared with theoretical results to validate the theoretical treatment. Experimental results of chronoamperometry and cyclic voltammograms are compared with theoretical results obtaining remarkable agreement. Is noteworthy that to the best of the author’s knowledge this is the first time that experimental dynamic voltammetry under weakly supported conditions has been successfully modeled by a theoretical treatment. The electrochemical reaction of a non-charged electroactive species is presented for the system ferrocene/ferrocenium in acetonitrile in which the oxidized and reduced species are soluble in solution, the reaction is studied at different concentrations of supporting electrolyte. Comparison is presented between theoretical simulations and experimental results, for which potential drop in solution is studied. Then systems involving charged electroactive species are treated, in these cases the decrease of supporting electrolyte influence the mass transport of the electroactive species due to migration, comparison between different experimental systems as hexaammineruthenium (III)/(II), cobaltoceniun/cobaltocene and hexacyanoferrate (III)/(II) are presented in comparison with theoretical simulations. More complex mechanistic paths are also investigated, such as deposition and stripping, in which it is established that the level of support required to achieve ‘diffusion only’ voltammetry is on dependence of the concentration of amalgamated electroactive species prior to the stripping step. Comparison between theoretical simulation and experimental results of the deposition and stripping of thallium at a mercury hemisphere are presented, and found to be in good agreement for either chronoamperometry and cyclic voltammetry Simulations are also presented showing the necessary required amount of supporting electrolyte required to achieve ‘diffusion only’ cyclic voltammetry. This is obtained by comparison between diffusion only software and the simulation described in the present thesis. The required amount of supporting electrolyte is shown to depend on the concentration of the electroactive species and supporting electrolyte in the media, the electrode radius, the diffusion coefficient of species and the scan rate. Finally, the cyclic voltammetry in weakly supporting media is used to obtain mechanistic information, by using the migration of electroactive species to differentiate the mass transport of electroactive species to the electrode. The two single electron reductions of anthraquinone in acetonitrile is presented, and the comproportionation mechanistic path is observed in weakly supported media, diffusion only voltammetry is normally unable to present whether this mechanism path takes place, due to the similarity in diffusion coefficients of the electroactive species. In contrast in weakly support conditions the diffusion controlled comproportionation mechanistic path is observed experimentally and constraints for the rate constant are discussed.

The uniquely predictable and controllable binding mechanism of DNA strands has been exploited to construct a vast range of synthetic nanodevices, capable of autonomous motion and computation. This thesis proposes novel ideas for the control and observation of such devices. The first of these proposals hinges on introducing mismatched base pairs into toehold-mediated strand displacement – a fundamental primitive in most dynamic DNA devices and reaction networks. Previous findings that such mismatches can impede strand displacement are extended insofar as it is shown that this impediment is highly dependent on mismatch position. This discovery is examined in detail, both experimentally and through simulations created with a coarse-grained model of DNA. It is shown that this effect allows for kinetic control of strand displacement decoupled from reaction thermodynamics. The second proposal improves upon a previously presented strand displacement scheme, in which two strands perform displacement cooperatively. This scheme is extended to be cascadable, so that the output of one such reaction serves as input to the next. This scheme is implemented in reaction networks capable of performing fundamental calculations on directed graphs. The third proposal is exclusively concerned with a novel observation methodology. This method is based on single-molecule fluorescence microscopy, and uses quantum dots, a fluorescent type of semiconductor nanocrystal, as a label. These quantum dots display a set of characteristics particularly promising for single-molecule studies on the time- and length scales most commonly encountered in DNA nanotechnology. This method is shown to allow for highly precise measurements on static DNA devices. Preliminary data for the observation of a complex dynamic device is also presented.

Nas indústrias de celulose, o processo dominante é a polpação Kraft. A polpação ou o cozimento de madeira em digestores constitui a operação central da produção de celulose, onde se processam as reações de deslignificação dos cavacos de madeira. Em geral, aí os modelos são empíricos e baseados em experimentos em escala de laboratório, e o \"scale-up\" para nível de digestores industriais torna-se tedioso e às vezes impraticável, pelo número de experimentos necessários e pela dificuldade de reprodução de todas as condições operacionais. Propõe-se neste trabalho um modelo matemático de polpação Kraft de eucalipto. O modelo consiste num sistema de equações diferenciais parciais que descrevem os efeitos combinados da cinética de polpação e da difusão de reagentes, como um reator de mistura perfeita com fases heterogêneas. Apresentam-se as previsões das simulações, comparam-se seus valores com dados experimentais. Este modelo é heterogêneo e difere dos modelos homogêneos usualmente propostos para a polpação. O primeiros permite uma abordagem das influências da espessura, massa específica e porosidade de cavacos, e também da difusão de reagentes nos cavacos durante a polpação. O presente trabalho dá ainda alguns exemplos de aplicação e tece comentários quanto ao modelo. / Kraft pulping is the dominant process f the pulp industry. The pulping of wood in digesters is the central operation of the pulp production, which consists of the delignification of the wood chips. Usually, the models of the delignification are empiricals and derived from laboratory scale experiments . The scale-up for industrial applications is tedious and sometimes impracticable, requiring a large number of experiments and presenting difficulty in reproducing all the operational condictions. The present work suggests a mathematical model for the Kraft pulping of the eucalyptus. This model consists of a system of partial differential equations describing the combined effects of the pulping kinetics and the diffusion of reactants, in form of continuous stirred tank reactor with heterogeneous phases. The prediction of the values given by simulation and their comparison with experimental data are presented. Differing from the homogeneous model, the heterogeneous allows an approach to estimate the influence of the chip thickness, density, porosity and that of the diffusion of the reactants in the chips, during pulping. The present work gives further some examples of application and comments on the model.

A reação de hidrodescloração catalítica (HDC) de bifenilas policloradas (PCB) foi estudada em diversos sistemas reacionais: solventes orgânicos, dióxido de carbono supercrítico, presença e natureza de aditivos (bases e co-solventes) a temperaturas na faixa de 25 °C a 100 °C e pressões de 1 bar até 250 bar. Os reagentes utilizados foram um composto modelo (2,4- diclorobifenil), misturas comerciais de PCB (Aroclor, Ascarel) e outros compostos halogenados (clorobenzenos e bromoclorobenzeno). Os catalisadores avaliados, paládio suportado em carvão ativo (Pd/C), paládio suportado em sílica mesoporosa (Pd/SiO2), nanopartículas ferromagnéticas de paládio (FFSiNH2Pd), e nanopartículas ocas de paládio (PdNP) e de platina (PtNP), mostraram-se versáteis nas mais diversas condições, quer sejam condições supercríticas, presença ou não de água, presença e natureza da base e diferentes doadores de hidrogênio. Em dióxido de carbono, dois regimes reacionais foram avaliados: dióxido de carbono subcrítico, onde o gás atua como agente expansor da fase líquida ao se dissolver nela permitindo que a reação ocorra em uma fase líquida rica em gás hidrogênio; e dióxido de carbono supercrítico, sistema onde a temperatura influencia fortemente a reação e a pressão tende a ter efeito negativo sobre a velocidade de reação, devido a efeitos de diluição e de competição das moléculas do gás com as moléculas dos reagentes. A condição reacional ótima foi determinada para o regime supercrítico e dados cinéticos foram obtidos para os regimes sub- e supercrítico. / The catalytic hydrodechlorination reaction (HDC) of polychlorinated biphenyls (PCB) was studied in several reactional systems: organic solvents, supercritical carbon dioxide, presence and nature of additives (bases and co-solvents), in temperatures of 25 °C to 100 °C and pressures of 1 bar to 250 bar. The reagents used were: a model compound, 2,4-dichlorobiphenyl, commercial mixtures of PCB (Aroclor, Ascarel) and other halogenated compounds (chlorobenzenes and bromochlorobenzene). The catalysts evaluated, palladium supported in activated carbon (Pd/C), palladium supported in mesoporous silica (Pd/SiO2), palladium ferromagnetic nanoparticles (FFSiNH2Pd), and hollow palladium ((PdNP) and platinum (PtNP) nanoparticles, showed to be versatile in several conditions, either supercritical conditions, presence or not of water, nature of the base, and different hydrogen donors. In carbon dioxide, two reaction regimes were evaluated: subcritical carbon dioxide, where the gas acts as an expansion agent of the liquid phase when it dissolves in it, allowing that the reaction happens in a liquid phase rich in hydrogen; and supercritical carbon dioxide, a system where the temperature has a strong influence over the reaction and the pressure has a negative effect on the rate of the reaction, due to dilution and the competition of gas molecules with the reagents molecules for the catalyst. The optimal reaction condition was determined for the supercritical regime and kinetic data were obtained for both the sub and supercritical regimes.

Estudos do mecanismo de reações unimoleculares, das moléculas de acetato de metila e etila, induzidas por absorção multifotônica e de reações bimoleculares com os radicais OH e Cl, usando cálculos ab initio e de funcional de densidade foram realizados neste trabalho. A análise dos cálculos das barreiras de energia e das constantes de velocidade microcanônicas das reações unimoleculares com o formalismo da teoria RRKM, permitiu prever algumas reações cujos produtos não foram determinados experimentalmente. Além disso, semelhanças das geometrias e dos valores de energia de algumas estruturas entre diferentes superfícies de energia, analisadas com cálculos de IRC, permitiram a explicação da viabilidade de determinada via de reação em detrimento de outra. Em outro método, relatado por Forst(1), na determinação das constantes de velocidade variacionais, comparadas com a variação da energia de Gibbs, verificou-se constantes de velocidade microcanônicas menores e a localização dos estados de transição em comprimentos de ligação, também, menores. Nas reações com o radical OH e Cl, a via predominante de reação é o ataque aos hidrogênios do grupo ligado diretamente ao oxigênio da molécula, responsável pelo valor da constante final de mais de 93% nas reações com OH e 99% nas reações com Cl, a 298K, em concordância com os resultados experimentais, em detrimento do ataque ao grupo CH3 da porção carboxilato da molécula. Nas reações com OH, os valores de constante de velocidade calculados mais próximos dos valores experimentais foram obtidos a partir dos resultados com os funcionais mPW1B95-41, para o acetato de metila, e mPW1B95-44, para o acetato de etila, enquanto que nas reações com Cl foram obtidos com o os métodos CCSD(T)//B3LYP para o acetato de metila e CCSD(T)//MP2 para o acetato de etila. Os valores de constante de velocidade da reação com cloro são cerca de dez vezes maiores que aquelas para as reações com radicais OH. As reações com OH e Cl ocorrem em uma única etapa, sem estabilização do intermediário e sem efeito de tunelamento significante. Por sua vez, cálculos da constante de velocidade, pelo método da relação estrutura reatividade (SAR), sobreestimam a reatividade dos hidrogênios dos grupos CH3 na porção carboxilato, em ambos os acetatos, e na porção etóxido do acetato de etila. Este método falha ao descrever a participação de cada grupo na reação com Cl, pois prevê que a reação no grupo CH3 da porção alcóxido passa a ser predominante sobre a reação ao grupo CH2 nas reações do acetato de etila, oposto aos nossos cálculos onde a reação com o grupo CH2 é a predominante. Outra falha do método é na previsão de mesma reatividade para ambos os grupos CH3 no acetato de metila, pois por nossos cálculos a reação ocorre predominantemente no grupo CH3 na porção alcóxido da molécula. / Computational studies on methyl and ethyl acetates molecules using ab initio and density functional calculations exploring the unimolecular mechanism, induced by multiphoton absorption, and the bimolecular reactions with OH and Cl radicals, have been performed in this work. Analysis of the calculated energy barriers and rate constants of unimolecular reactions with the RRKM microcanonical theory, predicts the occurrence of some reactions whose products were not determined experimentally. Furthermore, similarities on geometrical and energetic of some structures between different energy surfaces, analyzed with IRC calculations, allowed the explanation of the viability of a particular reaction pathway over another. Forst\'s method, used to determine variational rate constants, when compared with the variation of Gibbs energy, generates microcanonical rate constants with smaller values and location of transition states in smaller bond lengths. With OH and Cl, the predominant reaction route is the attack on the hydrogens of the group bonded directly to oxygen in the molecule, responsible for more than 93% of the final constant value in reactions with OH and 99% in reactions with Cl , at 298 K, in agreement with the experimental results, while the attack to the CH3 group in the carboxylate portion of the molecule is the less likely to occur. In reactions with OH, the values of the rate constant calculated closer to the experimental values were obtained from the results with functional mPW1B95-41, for methyl acetate, and mPW1B95-44 for ethyl acetate, whereas in reactions with Cl, were obtained with the CCSD(T)//B3LYP method for the methyl acetate and the CCSD(T)//MP2 method for ethyl acetate. The rate constant for the reaction with chlorine are about ten times larger than those for reactions with OH radicals. Reactions with OH and Cl occur in a single step, without stabilization of the intermediary and without significant tunneling effect. The rate constant obtained by the structure-reactivity relationship (SAR) overestimates the reactivity of the hydrogens of the CH3 groups at the carboxylate portion, in both acetates, and at the ethoxide portion of ethyl acetate. This method fails to describe the participation of each group in the reaction with Cl, predicting that the reaction on the CH3 portion of the alkoxide becomes predominant over the reaction on the CH2 group of ethyl acetate, relative to our calculations where the reaction with the CH2 group is predominant. Another flaw by providing the same reactivity for both the CH3 in methyl acetate, is in disagreement with our calculations which indicate that the reaction occurs predominantly in the CH3 group at the alkoxide portion of the molecule.

A emergência frequente de oscilações de corrente e potencial durante a eletro-oxidação de moléculas orgânicas pequenas tem implicações mecanísticas importantes, como por exemplo, na conversão reacional global e, portanto, no desempenho de dispositivos práticos de conversão de energia. Orientado nesse sentido, este trabalho desenvolveu-se por meio de duas frentes relacionadas: (a) utilizando-se medidas obtidas por meio do acoplamento de uma célula eletroquímica a um espectrômetro de massas, estudou-se a dinâmica da produção de espécies voláteis durante a eletro-oxidação oscilatória de ácido fórmico, metanol e etanol. Além da apresentação de resultados experimentais ainda não relatados, introduz-se o uso de regressão linear multivariada para se comparar a corrente faradaica total estimada, com a proveniente da produção de espécies voláteis detectáveis: dióxido de carbono para ácido fórmico, dióxido de carbono e metilformiato para metanol e, dióxido de carbono e acetaldeído para etanol. A análise fornece a melhor combinação das correntes iônicas detectadas para se representar a corrente global ou a máxima contribuição faradaica possível devido à produção de espécies voláteis. Os resultados foram discutidos em conexão com aspectos do mecanismo reacional de cada molécula. A incompatibilidade entre a corrente faradaica total estimada e a obtida pela melhor combinação das correntes parciais provenientes da produção de espécies voláteis foi pequena para ácido fórmico, quatro e cinco vezes maior para etanol e metanol, respectivamente, evidenciando, nestes dois últimos casos, o aumento do papel desempenhado por espécies solúveis parcialmente oxidadas; (b) investigou-se características gerais da eletro-oxidação de formaldeído, ácido fórmico e metanol sobre platina em meio ácido, com ênfase na comparação do desempenho eletrocatalítico global sob condições estacionária e oscilatória. A comparação procedeu-se por meio da interpretação de resultados tratados de diferentes formas e generalizada pela utilização das mesmas condições experimentais em todos os casos. Para todos os sistemas, o baixo potencial alcançado durante as oscilações evidenciou uma considerável diminuição do sobrepotencial associado à reação anódica, se comparado com o obtido na ausência de oscilações. Além do mais, o processo de reativação superficial do catalisador que ocorre durante as oscilações amplia o desempenho de todos os sistemas em termos de atividade eletrocatalítica. Por fim, também são discutidos alguns aspectos do mecanismo reacional das moléculas estudadas. / The frequent emergence of current/potential oscillations during the electrooxidation of small organic molecules has implications on mechanistic aspects such as, for example, on the overall reaction conversion, and thus on the performance of practical devices of energy conversion. In this direction, this work is divided in two parts: (a) by means of on line Differential Electrochemical Mass Spectrometry (DEMS) it was studied the production of volatile species during the electrooxidation of formic acid, methanol and ethanol. Besides the presentation of previously unreported DEMS results on the oscillatory dynamics of such systems, it was introduced the use of multivariate linear regression to compare the estimated total faradaic current with the one comprising the production of volatile detectable species, namely: carbon dioxide for formic acid, carbon dioxide and methylformate for methanol and, carbon dioxide and acetaldehyde for ethanol. The introduced analysis provided the best combination of the DEMS ion currents to represent the total faradaic current or the maximum possible faradaic contribution of the volatile products for the global current. The results were discussed in connection with mechanistic aspects for each system. The mismatch between estimated total current and the one obtained by the best combination of partial currents of volatile products was found to be small for formic acid, 4 and 5 times bigger for ethanol and methanol, respectively, evidencing the increasing role played by partially oxidized soluble species in each case; (b) it was investigated general features of the electro-oxidation of formaldehyde, formic acid and methanol on platinum and in acid media, with emphasis on the comparison of the performance under stationary and oscillatory regimes. The comparison is carried out by different means and generalized by the use of identical experimental conditions in all cases. In all three systems studied, the occurrence of potential oscillations is associated with excursions of the electrode potentials to lower values, which considerable decreases the overpotential of the anodic reaction, when compared to that in the absence of oscillations. In addition, the reactivation of catalyst surface benefits the performance of all systems in terms of electrocatalytic activity. Finally, some mechanistic aspects of the studied reactions are also discussed.

Les 50 dernières années ont été caractérisées par le développement rapide de l’astrochimie. Plus de 150 réactions entre espèces neutres ont déjà été étudiées aux basses températures qui sont celles du le milieu interstellaire et des atmosphères planétaires. Néanmoins, les constantes de vitesse, et la nature des produits, restent inconnus pour de nombreuses réactions potentiellement importantes pour caractériser ces milieux. Nous avons effectué des études cinétiques pour des processus réactifs, et non réactifs, entre des atomes dans un état électronique excité [C(1D), O(1D) et N(2D)] et plusieurs molécules stables afin de quantifier leur importance dans la chimie des atmosphères planétaires. Nous avons aussi étudié la réaction entre les atomes de carbone dans leur état électronique fondamental (3P) et l’eau, confirmant l’importance, pour certaines réactions avec barrière, de l’effet tunnel pour la réactivité à basse température. Les constantes de vitesse et les rapports de branchement pour ces processus ont été déterminés dans la gamme de température entre 50 et 296 K en utilisant un appareil CRESU, les atomes étudiés ont été produits par photolyse à l’aide d’un laser pulsé (PLP) et détectés par fluorescence induite dans l’ultraviolet sous vide (VUV LIF). / The last 50 years have been characterized by the fast development of astrochemistry as a science. To date, more than 150 gas-phase neutral-neutral reactions have been investigated at low temperatures relevant to planetary atmospheres and in cold regions of the interstellar medium. However, the rate constants and nature of the products for many potentially important gas-phase processes remain unknown. We performed kinetic studies of reactive and non-reactive removal processes between electronically excited atoms [C(1D), O(1D) and N(2D)] with several molecules in order to quantify their importance in the chemistry of planetary atmospheres. Furthermore, we also investigated the reaction between carbon atoms in their ground electronic state (3P) with water, providing new evidence of a quantum mechanical tunnelling mechanism at low temperatures, which could play an important role in the chemistry of interstellar clouds. Rate constants and branching ratios for these processes were determined over the 50 - 296 K temperature range using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus coupled with pulsed laser photolysis (PLP) and vacuum ultraviolet laser induced fluorescence (VUV LIF).

Cette thèse porte sur le problème fondamental du mottage des poudres suite aux mécanismes de transition de phase. Le projet vise à étudier l'impact des facteurs intrinsèques (structure moléculaire des matériaux, propriétés physiques et/ou physicochimiques, etc.) ou des facteurs environnementaux (conditions de stockage ou paramètres de procédé) sur la stabilité de la structure des poudres. Plus précisément, notre étude a mis en évidence le rôle prépondérant du phénomène de cristallisation et des transitions entre les différents polymorphes du lactose. L'accent a été mis sur le rôle des phénomènes de cristallisation et de la transition de phase dans l'apparition du mottage des poudres de lactose. Deux cas ont particulièrement retenu notre attention: (1) des poudres de lactose monohydrate contenant une fraction de particules amorphes et (2) des échantillons de poudre anhydre composés des anomères α et β du lactose. Dans les deux cas, le mottage a été induite par l'exposition des échantillons à l'air humide, soit dans un dispositif de sorption dynamique de vapeur (SPS), soit par des tests accélérés utilisant deux appareils conçus et réalisés dans notre laboratoire (CLAIR & OLAF). Nos résultats ont montré que, dans les deux cas, la principale cause de prise en masse était la formation de lactose monohydrate, qui est la forme la plus stable parmi tous les polymorphes de lactose. Cependant, les mécanismes élémentaires, les étapes limites et la cinétique du processus de transformation étaient différents dans chaque cas. Les paramètres les plus déterminants étaient l’humidité relative et la température alors que la pression n’a pas eu d’effet significatif. La résistance mécanique des échantillons mottés était étroitement liée au taux et à la cinétique de cristallisation. Enfin, des simulations numériques basées sur la méthode des éléments discrets (DEM) de la résistance mécanique des échantillons mottés ont été réalisées. Le modèle permet de décrire le comportement des échantillons mottés soumis à des contraintes mécaniques de compression ou de traction. / This PhD study focuses on the fundamental problem of powder caking due to phase transition mechanisms. The project aims to study the impact of intrinsic factors (molecular structure of materials, physical and/or physicochemical properties, etc.) or environmental factors (storage conditions or process parameters) on the stability of the structure of powders. More precisely, our study has highlighted the preponderant role of the crystallization phenomenon and the transitions taking place between the different polymorphs of lactose. Emphasis was placed on the role of crystallization phenomena and phase transition on the advent of lactose powder caking. Two cases attracted particular attention: (1) lactose monohydrate powders containing a fraction of amorphous particles and (2) anhydrous powder samples composed of ð and anomers of lactose. In both cases, the caking was induced by exposure of the samples to moist air, either in a Dynamic Vapor Sorption device (SPS) or in accelerated caking tests using two home-made equipment (CLAIR & OLAF). Our results showed that in both cases, the main cause of caking was the formation of lactose monohydrate, which is the most stable form among all lactose polymorphs. However, the elementary mechanisms, the limiting steps and the kinetics of the transformation process were different in each case. The more influencing parameters were the relative humidity and the temperature whereas the pressure has no significant effect. The yield stress of caked samples was closely linked with crystallization extent and kinetics. Finally, numerical simulations based on Discrete Element Method (DEM) of mechanical resistance of caked samples were performed using the "beam model". The model allows describing the behavior of the caked samples subjected to compressive or tractive mechanical stresses.

Nas indústrias de celulose, o processo dominante é a polpação Kraft. A polpação ou o cozimento de madeira em digestores constitui a operação central da produção de celulose, onde se processam as reações de deslignificação dos cavacos de madeira. Em geral, aí os modelos são empíricos e baseados em experimentos em escala de laboratório, e o \"scale-up\" para nível de digestores industriais torna-se tedioso e às vezes impraticável, pelo número de experimentos necessários e pela dificuldade de reprodução de todas as condições operacionais. Propõe-se neste trabalho um modelo matemático de polpação Kraft de eucalipto. O modelo consiste num sistema de equações diferenciais parciais que descrevem os efeitos combinados da cinética de polpação e da difusão de reagentes, como um reator de mistura perfeita com fases heterogêneas. Apresentam-se as previsões das simulações, comparam-se seus valores com dados experimentais. Este modelo é heterogêneo e difere dos modelos homogêneos usualmente propostos para a polpação. O primeiros permite uma abordagem das influências da espessura, massa específica e porosidade de cavacos, e também da difusão de reagentes nos cavacos durante a polpação. O presente trabalho dá ainda alguns exemplos de aplicação e tece comentários quanto ao modelo. / Kraft pulping is the dominant process f the pulp industry. The pulping of wood in digesters is the central operation of the pulp production, which consists of the delignification of the wood chips. Usually, the models of the delignification are empiricals and derived from laboratory scale experiments . The scale-up for industrial applications is tedious and sometimes impracticable, requiring a large number of experiments and presenting difficulty in reproducing all the operational condictions. The present work suggests a mathematical model for the Kraft pulping of the eucalyptus. This model consists of a system of partial differential equations describing the combined effects of the pulping kinetics and the diffusion of reactants, in form of continuous stirred tank reactor with heterogeneous phases. The prediction of the values given by simulation and their comparison with experimental data are presented. Differing from the homogeneous model, the heterogeneous allows an approach to estimate the influence of the chip thickness, density, porosity and that of the diffusion of the reactants in the chips, during pulping. The present work gives further some examples of application and comments on the model.

Os cianopoliinos (HCnN, n = 1, 3, 5, ...) e seus isômeros, os isocianopoliinos (HCn-1NC), constituem duas famílias de moléculas já identificadas no meio interestelar. No intuito de auxiliar a detecção e investigar a formação destas moléculas neste ambiente, foram obtidas as geometrias, as constantes rotacionais, os momentos de dipolo, as frequências vibracionais e as intensidades fundamentais de infravermelho em níveis MP2/cc-pVTZ (n = 1 a 17), CCSD/cc-pVDZ (n = 1 a 13) e CCSD/cc-pVTZ (n = 1 a 7). Além disso, foi empregado o modelo de partição em carga - fluxo de carga - fluxo de dipolo, CFCFD, em termos dos multipolos atômicos advindos da Teoria Quântica de Átomos em Moléculas, QTAIM, para compreender os efeitos do tamanho da cadeia sobre as propriedades elétricas destas moléculas, como suas intensidades de infravermelho. Os resultados indicam que o nível CCSD/cc-pVTZ é o que melhor descreve as propriedades mencionadas para os menores cianopoliinos e isocianopoliinos. Contudo, devido à demanda computacional, é preciso optar entre os níveis MP2/cc-pVTZ e CCSD/cc-pVDZ para abordar espécies maiores. Assim, os momentos de dipolo destes maiores cianopoliinos são mais bem descritos pelo nível MP2/cc-pVTZ, enquanto CCSD/cc-pVDZ é mais indicado para tal propriedade em respectivos isocianopoliinos. Por sua vez, as intensidades de infravermelho destas famílias apresentam melhor concordância com dados experimentais quando determinadas em nível CCSD/cc-pVDZ. Além disso, tanto para cianopoliinos quanto para isocianopoliinos, o aumento do tamanho das cadeias resulta num incremento das intensidades do estiramento CH, o que é explicado por variações no fluxo de carga eletrônica. O estiramento das ligações triplas CC centrais é o modo mais intenso em grandes cianopoliinos, o que também se deve ao fluxo de carga observado. Ademais, o espectro dos isocianopoliinos apresenta um número maior de bandas relevantes na região de estiramento de ligações triplas CC. Um estudo também foi conduzido, onde foram estimados os dados termodinâmicos, as geometrias do estado de transição e as constantes de velocidade da reação HCnN → HCn-1NC (n = 1 a 9) para temperaturas entre 298,15 e 3000 K. Os níveis B3LYP/aug-cc-pVsZ, MPW1K/aug-cc-pVsZ (s = Q) e CCSD(T)/aug-cc-pVmZ (m = T e Q) foram adotados em um tratamento composto para as reações com n = 1, 3 e 5, enquanto as reações com n = 7 e 9 foram investigadas de forma semelhante, porém com s = T e m = D e T. O método B3LYP foi o que apresentou melhor desempenho comparado ao MPW1K na determinação de frequências e geometrias. Os resultados indicam que esta reação é exotérmica no sentido em que os cianopoliinos são formados e, portanto, é mais fácil de ocorrer em ambientes mais frios do meio interestelar. Por outro lado, suas constantes de velocidade no sentido direto e inverso tendem a ser de mesma magnitude em temperaturas elevadas, indicando que estes ambientes mais quentes são mais propícios para detecção de isocianopoliinos. Por fim, equações parametrizadas foram ajustadas para reproduzir nossos dados de constantes de velocidade das reações com n = 1 até 9 dentro do intervalo de temperaturas considerado. / Cyanopolyynes (HCnN, n = 1, 3, 5, ...) and their isomers, isocyanopolyynes (HCn-1NC), constitute two families of molecules already identified in the interstellar medium. In order to provide data for their detection and to investigate the formation of these molecules in this environment, geometries, rotational constants, dipole moments, vibrational frequencies and infrared fundamental intensities were obtained at MP2/cc-pVTZ (n = 1 to 17), CCSD/cc-pVDZ (n = 1 to 13) and CCSD/cc-pVTZ (n = 1 to 7) levels. Furthermore, the partition model in charge - charge flux - dipole flux, CFCFD, from atomic multipoles given by the Quantum Theory of Atoms in Molecules, QTAIM, was applied to understand the effects of chain size on electrical properties of these molecules, such as intensities. Results indicate that the best description of the properties mentioned is achieved at the CCSD/cc-pVTZ level for smaller cyanopolyynes and isocyanopolyynes. However, due to computational demand, one needs to choose between MP2/cc-pVTZ and CCSD/cc-pVDZ levels to deal with larger species. In this sense, the dipole moments of large cyanopolyynes are better described by the MP2/cc-pVTZ level, while CCSD/cc-pVDZ is indicated for such property in respective isocyanopolyynes. In addition, the infrared intensities of both families from CCSD/cc-pVDZ calculations are in better accordance with experimental data. Moreover, chain size increases of cyanopolyynes and isocyanopolyynes result in intensity increments of CH stretching, which is explained by electronic charge flux changes. The stretching of central CC triple bonds is the strongest mode for large cyanopolyynes and this is also due to charge flux. A study was also conducted, in which thermodynamic data, transition state geometries and rate constants of the reaction HCnN → HCn-1NC (n = 1 to 9) were estimated for temperatures between 298.15 and 3000 K. B3LYP/aug-cc-pVsZ, MPW1K/aug-cc-pVsZ (s = Q) and CCSD(T)/aug-cc-pVmZ (m = T and Q) were adopted in a combined treatment for reactions with n = 1, 3 and 5, while reactions with n = 7 and 9 were investigated in a similar way, but with s = T and m = D and T. Frequencies and geometry data from the B3LYP method exhibited a better performance than MPW1K. The results indicated that this reaction is exothermic in the direction that leads to cyanopolyynes, and therefore it is easier to occur in colder environments of the interstellar medium. On the other hand, rate constants of forward and reverse reactions tend to show the same magnitude at higher temperatures, indicating these warmer environments are more amenable to detection of isocyanopolyynes. Finally, parameterized equations were fitted to reproduce our rate constant data for reactions with n = 1 to 9 in the temperature range considered.