1,720,966 research outputs found
Visualizing dispersion interactions through the use of local orbital spaces
No full textThe interpretation of chemical properties/phenomena can often be aided through the use of imagery. The mapping of molecular electrostatic potentials is a prime example, serving as a guideline in the design of novel compounds or understanding transition state stabilization effects. It is today a common tool for theoreticians and experimentalists alike. With the emergence of concepts such as dispersion energy donors, and the overall importance of dispersion in chemical systems, representations targeting such a class of interactions are warranted. In this work, we make use of local orbital analysis to extract dispersion interactions and represent them in a scalar quantity, the Dispersion Interaction Density (DID). A particular advantage of the method is the possibility to represent at the same footing intermolecular and intramolecular interactions in a straightforward fashion from wave function calculations. We present examples for the benzene dimer, several substituted benzenes and a coupled diamondoid molecule. (C) 2016 Wiley Periodicals, Inc.Deutsche Forschungsgemeinschaft [SPP 1807, MA 5063/3-1
Dispersion forces in chirality recognition – a density functional and wave function theory study of diols
No full textWe analyse how dispersion interactions impact chirality recognition, both in the structure and energy of diol clusters.In the discussion of chirality recognition, steric considerations and strongly directed interactions such as hydrogen bonds are primarily discussed. However, given the sheer size of biomolecules, it is expected that dispersion forces could also play a determining role for aggregate formation and associated chirality recognition. With the example of diol molecules, we explore different factors in the formation of homo- and hetero-dimers as well as their relative stability. By comparing density functional results with the analysis of local correlation methods, we infer the impact of dispersion not only on the energies but also on the structures of such chiral aggregates. A local orbital based scheme is used to calculate wave function dispersion-free gradients and compare to uncorrected density functional structures
All That Binds Is Not Gold—The Relative Weight of Aurophilic Interactions in Complex Formation
No full textOften have you heard that complexes containing close Au(I) contacts are strongly influenced by what has come to be known as aurophilic interactions. In this work, local orbital analysis is carried out to separate competing metallophilic and other weak interactions at the correlated level in three selected molecular crystals. We carefully separate and discuss the different contributions to the total interaction energy of dimers and trimers according to their spatial location, and identify the relative weight in binding. Changes according to the orientation of the monomers have also been computed. Our results show that although metallophilic contacts contribute to the overall stability and the structure of the crystals, they are not at all dominant, and ligand-ligand interactions can easily outweigh the latter
Internal dynamics and guest binding of a sterically overcrowded host
No full textSubstituent control in self-assembled host systems allows for a fine-tuning of structure, dynamics and guest preference. Flat banana-shaped ligands L1 assemble with Pd(II) cations into the interpenetrated coordination cage dimer [3BF4@Pd4L1 8], capable of sequential guest uptake. In contrast, the introduction of bulky adamantyl groups in ligand L2 prevents dimerization and results in the clean formation of monomeric cage species [Pd2L2 4]. Owing to steric crowding, the adamantyl substituent is considerably bent sideways with respect to the ligand backbone, and is rapidly flipping between both faces of the free ligand giving rise to two energetically degenerate conformers. Surprisingly, the flipping is preserved in the cage, albeit at a lower rate due to entropic reasons. Despite the very dense packing within the self-assembled structure, the cage is able to encapsulate a series of bis-anionic guests in an induced-fit fashion. Electronic structure calculations revealed a substantial contribution from dispersion interactions between the guest and the surrounding adamantyl groups that stabilize the host–guest complex. Guest exchange kinetics were quantified and the influence that encapsulated guests imparted on the ligand flipping dynamics was examined by a series of 2D NMR experiments. Four synchrotron X-ray structures of the cage and its host–guest complexes are presented, allowing for unprecedented insight into the host– guest interactions of a sterically overcrowded host and its guest-induced distortion.DFG [CL 489/2-1, MA 5063/3-1, SPP 1807]; Fonds der Chemischen Industri
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
Temperature-Dependent Dynamics of Push-Pull Rotor Systems Based on Acridinylidene Cyanoacetic Esters
No full textA series of asymmetrically substituted acridinylidene cyanoacetic esters was synthesized and analyzed for dynamic behavior and spectroscopic features. Although formally connected by a C=C double bond, the cyanoacetic ester groups behave as molecular rotors due to the electronic push–pull situation in the system. The donor–acceptor character lowers the rotational barrier and allows controlling the rotational rate by adjusting the electronic contribution of the ester substituent and changing the solvent polarity. Besides rotation, a much faster flipping motion can be assigned to the tricyclic acridinylidene motif, both of which lead to a rapid interconversion between axially chiral conformers of the system. Treatment with acid induces an intense fluorescence of the system, due to protonation of the C=C carbon on the rotor site, concomitant with the formation of a fluorescent acridinium ion. The dynamic, fluorogenic, and electrochemical behavior of three systems is compared with the aid of NMR, UV/Vis and fluorescence measurements, X‐ray structural analysis, cyclic voltammetry, and molecular modelling
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
Manganese(I)‐Catalyzed Dispersion‐Enabled C−H/C−C Activation
No full textC-H/C-C Functionalizations were achieved with the aid of a versatile manganese(I) catalyst. Thus, an organometallic manganese-catalyzed C-H activation set the stage for silver-free C-H/C-C transformations with ample substrate scope and excellent levels of chemo-, site-, and diastereo-selectivities. The robust nature of the manganese(I) catalysis regime was reflected by the first C-H/C-C functionalization on amino acids under racemization-free reaction conditions. Detailed experimental and computational mechanistic studies provided strong evidence for a facile C-H activation and a rate-determining C-C cleavage, with considerable contribution from London dispersion interactions
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
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