De nombreuses substances chimiques sont susceptibles de mener à des phénomènes d'incompatibilité, lors de leur mise en contact avec d'autres produits ou matériaux. Pour maîtriser ces risques, une identification rapide et précise de ces incompatibilités est nécessaire. Elle est réalisée, jusqu’à présent, par des outils expérimentaux qui malgré leur incontestable importance, ne fournissent que des informations limitées. Pour les compléter et améliorer la compréhension des processus d'incompatibilité, ce travail de thèse, vise à étudier la réactivité du nitrate d'ammonium, produit très utilisé dans l'industrie, connu pour sa longue liste d'incompatibilités et impliqué dans des nombreux accidents majeurs (Toulouse 2001), à l'aide de la modélisation moléculaire. Une étude théorique approfondie, basée sur des calculs DFT, destinée à identifier les chemins réactionnels, les produits formés ainsi que la chaleur dégagée par les réactions a été menée.Après avoir caractérisé le mécanisme radicalaire de décomposition du nitrate d’ammonium pur en phase gaz, une étude détaillée de la réactivité du mélange du nitrate d’ammonium et du dichloroisocyanurate de sodium (DCCNa), a été réalisée. Un chemin réactionnel a été caractérisé, il s'agit de la réaction directe entre le nitrate d'ammonium et le DCCNa, en présence d'une molécule d'eau. Ensuite, ces méthodes théoriques ont été appliquées à d'autres systèmes d'incompatibilités chimiques avec le nitrate d'ammonium (NaNO2, substances chlorées) en focalisant sur les étapes limitantes des mécanismes afin de tester leur pertinence et potentiel en tant qu'outil de prédiction a priori du phénomène d'incompatibilité. Ce travail, pionnier dans l'étude microscopique des incompatibilités chimiques du nitrate d'ammonium, a permis de clarifier les mécanismes réactionnels lors de la décomposition du produit pur et en contact avec des contaminants. / Numerous chemical reactants tend to lead to undesired phenomena of incompatibility. In order to prevent the happening of those phenomena, a rapid and accurate identification of the incompatibilities is needed. Until now, experimental studies, which provide insightful, but limited information, were the only possible approach used for the study of incompatibilities. In this work a molecular modeling approach was used in order to complete and improve experimental results of incompatibility processes for the reactivity of ammonium nitrate. Ammonium nitrate is a widely used compound in the chemical industry, known for its long list of incompatibilities and often involved in major accidents (Toulouse, 2001).This theoretical work, based on Density Functional Theory (DFT) calculations, is intended to provide insights into the possible reaction pathways, enthalpies and products formed by incompatibility reaction between ammonium nitrate and other compounds. In this thesis the mechanism of the decomposition of pure ammonium nitrate in the gas phase was firstly characterized and then the reactivity of the mixture of ammonium nitrate – sodium dichloroisocyanurate (DCCNa) was considered. The results obtained suggest the existence of a reaction path, involving a direct reaction between the ammonium nitrate and DCCNa in the presence of a molecule of water.Furthermore, the theoretical approach was employed focusing on the study of the limiting steps of other chemical incompatibilities for ammonium nitrate systems (NaNO2, chlorinated substances). Those results were employed to understand the potentiality of the theoretical approach as a priori predicting tool for the incompatibility processes.In summary this work, pioneer in the microscopic study of chemical incompatibilities of ammonium nitrate, has clarified the reaction mechanisms involved in the decomposition of pure compounds, as well as in contact with other contaminants.

Nucleic acids, such as DNA, play an essential role in all known forms of life; however, despite their fundamental importance, there is still a significant lack of understanding surrounding their functional behaviour. This thesis explores the structure and dynamics of DNA by employing methods based on fluorescence and through the use of computational calculations. Time-resolved fluorescence experiments have been performed on dinucleotides containing 2-aminopurine (2AP) in various alcohol-water mixtures. 2AP, a fluorescent analogue of the nucleobase adenine, has been used extensively to investigate nucleic acids because of its ability to be incorporated into their structures with minimal perturbation and its high sensitivity to its local environment. Direct solvent effects on 2AP were established through measurements on the free fluorophore. Analysis of the complex fluorescence decays associated with the dinucleotides was challenging but has provided insight into their conformational dynamics. Solvent polarity was found to play a significant role in determining both photophysical and conformational properties in these systems. The complicated fluorescence decay of 2AP in nucleic acids highlights the need for accurate and unbiased analysis methods. Various time-resolved fluorescence analysis methods, including iterative reconvolution and the exponential series method, have been investigated with real and simulated data to obtain an overview of their benefits and limitations. The main outcome of the evaluation is that no single method is preferred in all situations and there is likely to be value in using a combination when there is ambiguity in the interpretation of the results. Regardless of the analysis technique used, the parameterised description of the observed fluorescence decay is meaningless if the underlying physical model is unrealistic. The advance of computational methods has provided a new means to rigorously test the viability of proposed models. Calculations have been performed at the M06-2X/6-31+G(d) level of theory to investigate the stability of 2AP-containing dinucleotides in conformations similar to those observed in the double-helical structure of DNA. The results help to explain the similarity of the time-resolved fluorescence behaviour of 2AP in dinucleotide and DNA systems but also bring to light subtle differences that could perhaps account for experimental discrepancies. The recent emergence of advanced optical microscopy techniques has offered the prospect of being able to directly visualise nucleic acid structure at the nanoscale but, unfortunately, limitations of existing labelling methods have hindered delivery of this potential. To address this issue, a novel strategy has been used to introduce reversible fluorescence photoswitching into DNA at high label density. Photophysical studies have implicated aggregation and energy-transfer as possible quenching mechanisms in this system, which could be detrimental to its future application. The reliability of fluorescence photoswitching was investigated at ensemble and single-molecule level and by performing optical lock-in detection imaging. These developments lay the foundations for improved and sequence-specific super-resolution microscopy of DNA, which could offer new insights into the 3D nanoscale structure of this remarkable biopolymer. In summary, the work presented in this thesis outlines important observations and developments that have been made in the study of the structure and dynamics of nucleic acids.