1,720,961 research outputs found
Molecular entanglement can strongly increase basicity
Full text linkBr & oslash;nsted basicity is a fundamental chemical property featured by several kinds of inorganic and organic compounds. In this Review, we treat a particularly high basicity resulting from the mechanical entanglement involving two or more molecular subunits in catenanes and rotaxanes. Such entanglement allows a number of basic sites to be in close proximity with each other, highly increasing the proton affinity in comparison with the corresponding, non-entangled counterparts up to obtain superbases, properly defined as mechanically interlocked superbases. In the following pages, the development of this kind of superbases will be described with a historical perusal, starting from the initial, serendipitous findings up to the most recent reports where the strong basic property of entangled molecular units is the object of a rational design.Br & oslash;nsted basicity can be greatly enhanced by the mechanical entanglement of two or more interlocked molecular subunits within catenanes and rotaxanes. Here, the authors discuss the development of such mechanically interlocked superbases, and outline challenges and opportunities for future directions of research
Signal Transduction Allows Temporal Control of the Potential of a Concentration Cell Driven by the Decarboxylation of an Activated Carboxylic Acid
Full text linkThe use of Activated Carboxylic Acids (ACAs) allows the time-controlled operation of dissipative chemical systems based on the acid-base reaction by providing both the stimulus that temporarily drives a physicochemical change and, subsequently, the counter-stimulus with a single reagent addition. However, their application is inherently limited to acid-sensitive systems. To overcome this limitation, we herein develop a straightforward device for the transduction of the acid-base stimuli delivered by an ACA into a voltage signal that, in turn, is used to control voltage-sensitive circuits that are not pH-responsive by themselves. The signal transductor can be easily assembled from common laboratory equipment and employs aqueous solutions of readily available chemicals. Furthermore, the operator can simply and intuitively tune the amplitude of the voltage signal, as well as its duration and offset by varying the concentration of the chemical species involved in the transduction process.Time-controlled pH variations enabled by the decarboxylation of nitroacetic acid are transduced into time-controlled voltage variations. Such variations are exploited to govern the operation of simple electric devices.+ imag
A Doubly Dissipative System Driven by Chemical and Radiative Stimuli
No full textThe operation of a dissipative network composed of two or three different crown-ether receptors and an alkali metal cation can be temporally driven by the use (combined or not) of two orthogonal stimuli of a different nature. More specifically, irradiation with light at a proper wavelength and/or addition of an activated carboxylic acid, are used to modulate the binding capability of the above crown-ethers towards the metal ion, allowing to control over time the occupancy of the metal cation in the crown-ether moiety of a given ligand. Thus, application of either or both of the stimuli to an initially equilibrated system, where the metal cation is distributed among the crown-ether receptors depending on the different affinities, causes a programmable change in the receptor occupancies. Consequently, the system is induced to evolve to one or more out-of-equilibrium states with different distributions of the metal cation among the different receptors. When the fuel is exhausted or/and the irradiation interrupted, the system reversibly and autonomously goes back to the initial equilibrium state. Such results may contribute to the achievement of new dissipative systems that, taking advantage of multiple and orthogonal stimuli, are featured with more sophisticated operating mechanisms and time programmability
Cyclization reactions in confined space
Full text linkCyclization of chain substrates into ring products is an important reaction in organic synthesis. The ease of this process is measured by the Effective Molarity, which indicates that some rings (5, 6 and 7-membered ones) are especially facile to close, while others (>8 membered) are far more challenging. Confinement of the cyclization inside the nanometric cavity of capsules, coordination cages or porous materials can greatly facilitate the reaction via preorganization of the reagents and stabilization of the transition states, also offering the chance to alter the innate selectivity of the reaction. Moreover, the use of chiral cavities enables control over cyclization stereoselectivity. Eventually, pre-organization of the substrate chain inside the cavity can facilitate the closure of medium and large rings, a longstanding goal in organic synthesis. This review analyzes examples of the use of nanoconfinement to improve cyclization reactions, control their selectivity and close challenging rings. © 2023 Elsevier Lt
Combining X-ray Absorption and NMR spectroscopies to investigate a chemical reaction in solution
No full textknowledge on the structural and electronic properties of the relevant reaction intermediates requires the use of innovative and complementary spectroscopic probes. Herein, we combine X-ray Absorption Spectroscopy (XAS) and 1H-NMR to investigate the exchange reaction between two prototypical Cu coordination complexes in solution. By means of multivariate and theoretical analyses, we identify the number, nature and concentration time evolution of the relevant reaction intermediates and quantitatively determine their structures. The XAS and NMR techniques are proven to be complementary experimental tools to track the fate of both the metal and organic portions of reactive systems involving transition metals. The employed combined experimental and theoretical method may be useful in the study of reactive systems involving intermediates that may be labile or silent to conventional detection
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
Coupled X-ray Absorption/UV-vis Monitoring of a Prototypical Oscillating Reaction
No full textOscillating reactions are among the most intriguing phenomena in chemistry, but many questions on their mechanisms still remain unanswered, due to their intrinsic complexity and to the low sensitivity of the most common spectroscopic techniques toward the reaction brominated species. In this work, we investigate the cerium ion-catalyzed Belousov-Zhabotinsky (BZ) oscillating reaction by means of time-resolved X-ray absorption spectroscopy (XAS), in combination with UV-vis spectroscopy and unsupervised machine learning, multivariate curve resolution, and kinetic analyses. Altogether, we provide new insights into the collective oscillatory behavior of the key brominated species involved in the classical BZ reaction and measure previously unreported oscillations in their concentrations through Br K-edge XAS, while simultaneously tracking the oscillatory Ce4+-to-Ce3+ transformation by coupling XAS with UV-vis spectroscopy. Our work evidences the potential of the XAS technique to investigate the mechanisms of oscillatory chemical systems whose species are often not detectable with conventional experimental methods
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