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Fundamental Properties of the Molecular Au Clusters
Get PDFMonolayer-protected gold clusters (MPCs) are a very interesting and fascinating class of compounds, from the points of view of both the fundamental science and their possible applications. They are composed of a gold core, with a diameter smaller than a few nanometers, and are surrounded by a protecting organo-thiolate monolayer, bonded by covalent Au-S bonds. Due to their dimensions, these systems exhibit properties in between those of molecules and nanoparticles, therefore displaying unique physical and chemical behaviors. In this Thesis, some properties of the most stable and well-known molecular MPC are addressed and studied. The investigation focuses on fundamental features, including solid-state properties, optical behavior, reactivity and especially their magnetic properties. The investigation of the latter constitutes the major part of this work. Particular attention is dedicated to the effect of ligands on these phenomena. The main tool of our investigation was electron paramagnetic resonance (EPR) spectroscopy, which was used to study these MPCs both in solution and in the solid state. The topics addressed are to understand the magnetic interactions between gold core and the capping ligands in solutions phase and ferromagnetic and antiferromagnetic interactions between clusters in the solid state. Another magnetic resonance technique, nuclear magnetic resonance, was used for the study of the ligand exchange kinetics. The data obtained from a number of experimental techniques and computational calculations were used in conjunction with these two main tools
Probing firing-induced changes in non-carbonate clay through 55Mn EPR techniques
No full textNon-carbonate clay samples, fired in steps of 50 °C between 600 °C and 1000 °C, were examined through electron paramagnetic resonance (EPR) techniques (X- and Q-Band pulsed- and cw-EPR). X-band pulsed EPR and Q-Band pulsed- and cw-EPR techniques were able to spot 55Mn(II) signals out of the large iron background in the full range of firing temperatures, allowing one to follow the phase transformations through the evolution of the manganese signal. Q-Band pulsed EPR turned out to be particularly suitable to detect even small amounts of this ion in calcite, while the 55Mn hyperfine coupling constant, determined through Q-Band cw-EPR, could be used to detect transitions to the glassy phase
Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers
Get PDFSinglet oxygen, 1O2, can be generated by molecules that upon photoexcitation enable the 3O2 → 1O2 transition. We used a series of atomically precise Au24M(SR)18 clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, 1O2 was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the 1O2 lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate 1O2 by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer 1O2 lifetime; proper design of the cluster yields results analogous to those of a well-known reference photosensitizer, although more effectively. Comprehensive kinetic analysis provided important insights into the mechanism and driving-force dependence of the quenching of 1O2 by gold nanoclusters. Understanding on a molecular basis why th..
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
A magnetic look into the protecting layer of Au25 clusters
Get PDFThe field of molecular metal clusters protected by organothiolates is experiencing a very rapid growth. So
far, however, a clear understanding of the fine interactions between the cluster core and the capping
monolayer has remained elusive, despite the importance of the latter in interfacing the former to the
surrounding medium. Here, we describe a very sensitive methodology that enables comprehensive
assessment of these interactions. Pulse electron nuclear double resonance (ENDOR) was employed to
study the interaction of the unpaired electron with the protons of the alkanethiolate ligands in four
structurally related paramagnetic Au25(SR)0
18 clusters (R ¼ ethyl, propyl, butyl, 2-methylpropyl). Whereas
some of these structures were known, we present the first structural description of the highly symmetric
Au25(SPr)0
18 cluster. Through knowledge of the structural data, the ENDOR signals could be successfully
related to the types of ligand and the distance of the relevant protons from the central gold core. We
found that orbital distribution affects atoms that can be as far as 6 A from the icosahedral core. ˚
Simulations of the spectra provided the values of the hyperfine coupling constants. The resulting
information was compared with that provided by 1
H NMR spectroscopy, and molecular dynamics
calculations provided useful hints to understanding differences between the ENDOR and NMR results. It
is shown that the unpaired electron can be used as a very precise probe of the main structural features
of the interface between the metal core and the capping ligands.peerReviewe
Atomically precise Au144(SR)60 nanoclusters (R = Et, Pr) are capped by 12 distinct ligand types of 5-fold equivalence and display gigantic diastereotopic effects
Get PDFFor two decades, Au144(SR)60 has been one of the most studied and used thiolate (SR) protected gold
nanoclusters. In many ways, however, it proved to be a challenging and elusive case, also because of the
difficulties in solving its structure by single-crystal X-ray crystallography. We used very short thiols and
could prepare Au144(SC2H5)60 and Au144(SC3H7)60 in a very pure form, which was confirmed by UV-vis
absorption spectroscopy and very regular electrochemistry patterns. Inductively coupled plasma and
electrospray ionization mass spectrometries gave definite proof of the Au144(SR)60 stoichiometry. Highresolution
1D and 2D NMR spectroscopy in the solution phase provided the result of assessing the
presence of 12 ligand types in exactly the same amount (5-fold equivalence). Equally important, we
found that the two protons belonging to each methylene group along the thiolate chain are
diastereotopic. For the a-CH2 protons, the diastereotopic effect can be indeed gigantic, as it reaches
chemical-shift differences of 2.9 ppm. DFT calculations provided insights into the relationship between
structure and NMR results. In particular, the 12 ligand types and corresponding diastereotopic effects
may be explained by considering the presence of C–H/S hydrogen bonds. These results thus provide
fundamental insights into the structure of the thiolate layer capping this long-studied gold nanocluster
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