1,720,997 research outputs found
AIM Study and pumb-probe experiment for the keto-enol reaction in acetylacetone
No full textLa réaction de tautomérie est un équilibre chimique entre deux isomères de constitution. Très dépendantes de la température, du pH et de la nature du solvant, ces molécules sont souvent comparées à des caméléons pour leur aptitude à s’adapter à leur environnement. Dans les brins d’ADN, ces équilibres sont fondamentaux car ils constituent la base de la stabilité du réseau de liaisons hydrogène et par conséquent assurent sa structure tridimensionnelle, essentielle pour la bonne transcription des gènes. Dans le cadre de cette étude, nous avons étudié ce processus au sein d’une petite molécule organique “test” : l’acétylacétone (ACAC). Nous avons cherché à obtenir une première image « statique » de la réaction en étudiant son mécanisme réactionnel en phase gaz ainsi qu’en présence de molécules d’eau. Nos simulations DFT ont permis de vérifier, dans un premier temps, l’effet catalytique des solvants protiques impliqués dans la transformation. Dans un second temps, une analyse basée sur la topologie de la densité nous a aidés à dresser une ébauche rationnelle de l’origine catalytique des molécules de solvant en mettant en avant le rôle central joué par les liaisons hydrogène. Nous avons par la suite simulé une expérience pompe-sonde afin de proposer une seconde image plus « dynamique » permettant de suivre en temps réel la réaction de tautomérie. Pour cela, nous avons cherché à construire une première surface d’énergie potentielle réactive par Machine Learning. Ce type d’étude est essentielle afin d’obtenir une description précise des multiples voies réactionnelles. L’ensemble de notre étude jette les bases d’une compréhension globale et dynamique des réactions de tautomérie.The tautomerism reactions are very important processes in organic and biological chemistry. For example, in the DNA molecule these mechanisms are fundamental. They constitute the basements of the hydrogen bonds lattice and consequently they assure its three-dimensional structure. A small modification in the physical or chemical properties of the environment involves a change in the conformation and can lead to some failures in the transcription. Therefore it is really important to understand this reaction. In our study, we propose to investigate this phenomenon with a simple benchmark molecule : the acetylacetone (ACAC). The first aim was to build a “static” image of the reaction through a study of the mechanism in gas phase and in a micro-solvated environment. In a first hand, our DFT simulations have shown the catalytic effect occurring in solvents able to realise hydrogen bonds. In a second hand, an analysis based on the topology of the electronic density allowed us to pinpoint to central role played by these same hydrogen bonds. The second aim was to simulate the tautomerism reaction through a pump-probe experiment to follow this process in real time. To complete this project, we built our own potential energy surface of the reaction trough Machine Learning. This kind of investigation is essential to obtain a detailed description of the mechanism. Our study proposes a global understanding and a first dynamical approach of the tautomerism reaction
A theoretical study of Interatomic Coulombic Decay in helium clusters
No full textL'Interatomic Coulombic Decay (ICD) est une voie de désexcitation dans laquelle un ion, un atome ou une molécule excité(e) transfère son excès d'énergie à un atome voisin, ce qui l'ionise. La probabilité et la dynamique de l'ICD dépendent fortement de la structure du système chimique. Dans cette thèse, nous étudions l'ICD dans des agrégats d'hélium. Nous avons couplé une approche semi-classique de type "surface-hopping" avec une méthode "diatomics-in-molecule". Notre étude théorique sur des agrégats contenant entre 2 et 1000 atomes d'hélium montre comment les observables ICD dépendent du nombre d'atomes dans l'agrégat. Cela nous permet d'imaginer des expériences capables de mesurer la taille d'un agrégat d'hélium, la difficulté de cette mesure étant aujourd'hui une des limitations de l'utilisation des agrégats d'hélium pour diverses études de spectroscopie ultrafine.ICD is an ultrafast nonradiative decay process wherein an excited ion, atom or molecule transfers its excess energy to a neighbouring atom, ionizing it. The probability and dynamics of ICD strongly depend on the system's structure. This thesis focuses on ICD in helium clusters. We implemented a semi-classical model using Newtonian motion on quantum energy surfaces calculated within a Diatomics-in-Molecules approach. Helium clusters are an useful tool for high-resolution spectroscopies, however it is still an experimental challenge to measure their size. With our theoretical approach, we analyze the dependency between the ICD observables and the cluster size, for systems up to 1000 atoms. Whenever possible, we discuss how our results can be used to experimentally probe the number of atoms constituting a Helium cluster
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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