1,721,007 research outputs found
Structure and bonding in coordination and nanocluster compounds
No full textStructure and bonding of chemical compounds has always been a central part of study in chemistry. Since the birth of the Lewis structure a century ago, chemists already have a systematic way to describe the features of chemical bondings for numerous chemical compounds. On the other hand, within the past few decades, a large multitude of atypical chemical compounds, many of those being coordination and nanocluster compounds, that do not follow the simple rules proposed by Lewis a century ago are discovered. In this dissertation, several representative classes of coordination and nanocluster compounds are selected for study, with governing rules ranging from simple two-center-two-electron bonds, through electron-deficient cases including the Wade-Mingos rule and the jellium model, to nanoclusters with fragments held together by more subtle orbital interactions. Within each of these classes, we have reviewed the applicability and limitations of each rule, and also considered possible extensions within each case that could find support from quantum chemical calculations as well as reported examples from literature. Over all these studies, we have explored the possibility of formulating localized models for understanding all these reported examples, and compared our localized approaches against the more delocalized molecular orbital descriptions available from quantum chemical calculations. We showed how a localized approach can complement usual quantum chemical calculations and give guidance on further research directions. We anticipate the proposed models and analysis could aid the understanding in structure and bonding of a much wider range of examples of coordination and nanocluster compounds.</p
DFT Studies on Copper-Catalyzed Dearomatization of Pyridine
No full textA systematic theoretical study has been performed with the aid of density functional theory (DFT) calculations on the mechanism of asymmetric 1,4-dearomatization of free pyridine with styrene and dimethoxy(methyl)silane (HSiMe(OMe)2) catalyzed by copper complexes. Two proposed mechanisms, dearomatization of pyridine catalyzed by cooperation of two copper(I) centers and by one copper(I), are calculated and analyzed. The results indicate that one copper(I) center is capable of catalyzing the dearomatization reaction. To achieve the dearomatization on one copper(I) center, the Cu-H addition "product"(derived from styrene insertion into Cu-H) undergoes 1,3- and 1,5-Cu migration on a benzylic ligand, facilitating the 1,2- and 1,4-dearomatization of pyridine. In the two metal-center mechanism, significant sterically repulsive interactions among the substituents on the ligands between the two metal fragments hindered the dearomatization process. The origin of selectivity between 1,2- and 1,4-dearomatization of pyridine has also been discussed.</p
Linking DNA-packing density distribution and TAD boundary locations
No full textDNA is heterogeneously packaged into chromatin, which is further organized into topologically associating domains (TADs) with sharp boundaries. These boundary locations are critical for genome regulation. Here, we explore how the distribution of DNA-packing density across chromatin affects the TAD boundary locations. We develop a polymer-physics-based model that utilizes DNA accessibility data to parameterize DNA-packing density along chromosomes, treating them as heteropolymers, and simulates the stochastic folding of these heteropolymers within a nucleus to yield a conformation ensemble. Such an ensemble reproduces a subset (over 60%) of TAD boundaries across the human genome, as confirmed by Hi-C data. Additionally, it reproduces the spatial distance matrices of 2-Mb genomic regions provided by FISH experiments. Furthermore, our model suggests that utilizing DNA accessibility data alone as input is sufficient to predict the emergence and disappearance of key TADs during early T cell differentiation. We show that stochastic folding of heteropolymers in a confined space can replicate both the prevalence of chromatin domain structures and the cell-to-cell variation in domain boundary positions observed in single-cell experiments. Furthermore, regions of lower DNA-packing density preferentially form domain boundaries, and this preference drives the emergence of TAD boundaries observed in ensemble-averaged Hi-C maps. The enrichment of TAD boundaries at CTCF binding sites can be attributed to the influence of CTCF binding on local DNA-packing density in our model. Collectively, our findings establish a strong link between TAD boundaries and regions of lower DNA-packing density, providing insights into the mechanisms underlying TAD formation.</p
Achieving Localization and Semi-localization via Orbital Alignment – Fragment Aligned Molecular Orbital Analysis
No full textPrincipal Interacting Orbital (PIO): A Chemically Intuitive Method for Deciphering Bonding Interactions
No full textUnderstanding of Co(I)-Catalyzed Hydrogenation of C=C and C=O Substrates
No full textThe first-row transition metal catalysis often serves as a complementary strategy to traditional precious metal catalysis. Other than their advantages in abundance and sustainability, their unique electronic properties often give rise to more complicated reactivities. Co-catalyzed hydrogenation is an example of these relatively underexplored first-row transition metal catalysis. In this work, we will focus on an example of Co-catalyzed hydrogenation, which was reported to be able to selectively hydrogenate C=C but not C=O. We have performed DFT calculations on the model reactions, accompanied by a series of structure and bonding analysis, to investigate the underlying reason for the observed selectivity. We have identified the predominant σ-donating nature of phosphine and the π-accepting nature of C=C/C=O fragments upon interaction with Co(I) center, and we have also analyzed why these differences would lead to different coordination geometry, stability, and reactivity. We anticipate that all these analyses would advance our understanding in Co(I) catalysis and are beneficial for future Co-based catalyst design.</p
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
- …
