1,720,963 research outputs found
Study of chirality with computational methods
No full textChirality is influencing our life every second. Chiral molecules, also called enantiomers, can be found everywhere in the world, even within our bodies. A pair of enantiomers will react differently only in a chiral environment thus they initiate, for example different body responses. Accordingly, there exists a clear demand to obtain products in a single enantiomeric form, which can be achieved by either separating the racemic mixture of enantiomers or by producing single enantiomeric product. In this thesis both options are discussed. The first part of the thesis deals with the separation of racemic products. Stochastic Molecular Dynamics simulations were performed to study the chiral recognition process that takes place on a chiral stationary phase (CSP). In-depth analysis unveiled that for cinchona-motif chiral stationary phases, electrostatic interactions are the major contributor for enantiodiscriminations. In addition, it was shown that the fragment most responsible for the stereodiscrimination process is the carbamate group of the chiral selector. A CoMFA study was also performed to assess how the carbamate group could be modified to improve the enantioselectivity of newly designed CSPs. It was found that the most important field for enantiodiscrimination is the steric field. With that in mind, novel and potentially improved CSPs were suggested. The second part of the thesis deals with the production of single enantiomeric products. One way to synthesize single enantiomers involves using asymmetric synthesis, in which an asymmetric catalyst is used for the stereodifferentiating step. A way to quantify chirality (instead of considering chirality as a either/or property of a molecule) was used to establish a relationship between the chirality content of chiral catalysts for a series of reactions and their ability to carry out a stereodifferentiating task (e.g. to induce asymmetry during a chemical reaction). A relationship could be established for biaryl-Ti-complexes and also for bisoxazoline-Cu2+-complexes. During these investigations it was discovered that ligands coordinated to the metal have more chirality than they have in the unbound state. Therefore, several ligand distortion modes were studied to explore the dependence of these distortions and the chirality content of chiral catalyst, and hence the enantiomeric excess of the reaction. It was found that for bisoxazoline-Cu2+-complexes the twisting motion has the largest impact on the chirality content. For the Jacobsen-Katsuki system it was found that twisting and step-induced kinks have the greatest influence of chirality content
Atomistic modeling in tribology
No full textThe purpose of this study is to understand friction at the atomic level through computer simulations. The focus of this thesis is on sliding materials whose surfaces have been modified by various functional groups. Accordingly, a suitable force field was needed and we selected MM3 because it had been tested for a vast range of organic and inorganic molecules. Thus by relying on the MM3 potential functions, we constructed a computer program (SLICK99) to investigate atomistic friction. This software allowed us to simulate frictional effects of organic and other systems through molecular dynamics with the MM3 force field. The results from these studies are discussed
The role of computations in catalysis
No full textThis chapter examines the successes and the challenges of computational design of catalysts. It explores and learns from a crude example of experimental screening for catalysts for an exothermic reaction. There are several rules that make such rapid computational screening possible: the Sabatier principle, linear‐scaling, and the Brønsted‐Evans‐Polanyi (BEP) relation. Scaling relations can be developed for larger molecules, which make two bonds with the solid surface, through two different atoms. Oxide catalysts have numerous applications. The chapter discusses few rules discovered through computations. It illustrates many of the problems faced by most large‐scale catalytic processes. A useful catalyst must be cheap to make and it should not contain expensive or rare ingredients. Another important property of a good catalyst is its resistance to poisoning. Density functional theory (DFT) is the only practical option. DFT is approximate, especially when calculating activation energies
Machine Learning, Quantum Mechanics, and Chemical Compound Space
No full textA number of machine learning (ML) studies have appeared with the commonality that quantum mechanical properties are being predicted based on regression models defined in chemical compound space (CCS). The quantum mechanical framework is crucial for the unbiased exploration of CCS since it enables, at least in principle, the free variation of nuclear charges, atomic weights, atomic configurations, and electron number. This chapter first gives a brief tutorial summary of the employed ML model in Kernel Ridge Regression. A discussion on the various representations (descriptors) used to encode molecular species, in particular the molecular Coulomb‐matrix (CM), sorted or its eigenvalues follows. The chapter also reviews quantum chemistry data of 134k molecules. The local, linearly scaling ML models for atomic properties such as forces on atoms, nuclear magnetic resonance (NMR) shifts, core‐electron ionization energies, as well as atomic charges, dipole‐moments, and quadrupole‐moments for force‐field predictions are finally discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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