1,723,806 research outputs found
Using X-ray derived charge densities to detect electron delocalization effects and non-covalent interactions
No full textBeing based on a quantum observable and measurable quantity, the Electron Density (ED) based descriptors retain the advantage of enabling a direct comparison of theoretical predictions with experimental results. We review here our most recent work aimed at evaluating whether two of such descriptors, the Source Function, SF, [1] and the Reduced Density Gradient (RDG), are able to unveil electron delocalization effects (EDEs) and non covalent interactions (NCI), respectively.
Making use of ab-initio EDs, we recently proved that the SF clearly detects EDEs in a series of supposedly electron-conjugated compounds [2]. That study is here extended to molecular crystals (benzene, a substituted binaphtyl-2-ol, citrinin), whose ED is derived from X-ray diffraction data. Regardless of the ED origin, the SF appears a useful tool to study fine details of EDEs, and independently from any symmetry constraint (e.g. / separation of the ED).
A novel NCI descriptor, based on the RDG and enabling an easy-to-catch image of either the supposedly attractive (dispersive, hydrogen bonding) or allegedly repulsive (steric) interactions, was recently proposed [3]. We have applied [4] this same tool to experimentally-derived ED’s of molecular solids (austdiol, benzene, famotidine), and discussed its performance in synergy with Bader’s analysis. We have also explored [5] the amount and type of information that is lost when the IAM replaces the “true” ED in evaluating the RDG.
References
[1] R.F.W. Bader, C. Gatti, Chem. Phys. Lett. 287 (1998) 233-238.
[2] E. Monza, C. Gatti, L. Lo Presti, E. Ortoleva, J. Phys. Chem. A 115 (2011) 12864-12878.
[3] E.R. Johnson, S. Keinan, P. Mori-Sanchez, J. Contreras-Garcia JACS 132 (2010), 6498.
[4] G. Saleh, C. Gatti, L. Lo Presti, J. Contreras-Garcia, submitted (2012)
[5] G. Saleh, C. Gatti, L. Lo Presti, submitted (2012
New descriptors for an “unbiased” and chemically insightful comparison of ab-initio and X-ray derived charge densities
No full textThe results of modelling always need to be compared and validated against the experiment. To be meaningful, the comparison should be as much as possible unbiased, and, hopefully, should use tools also able to provide chemical insight. Being based on a quantum observable and measurable quantity, the Electron Density (ED) based descriptors enable a direct comparison of ab-initio and X-ray derived EDs. They also provide a description of chemical paradigms which is, in principle, freed at the outset from any model preconception or arbitrariness. Furthermore, as for their nature rooted in physics, they are ideally suited to validate or reject the interpretive models of chemistry based on useful, but arbitrary objects. In this lecture, we will discuss the capability of two of such descriptors, the Source Function (SF) [1] and the Reduced Density Gradient (RDG), to unveil electron delocalization effects and to detect non covalent interactions, respectively. The SF enables one to view chemical bonding and other chemical paradigms under a new perspective [1,2]. We recently addressed the question of whether the SF is also capable to reveal electron delocalization effects (EDEs) in a series of supposedly electron-conjugated compounds, investigated through ab-initio methods [2]. The study is here extended to various molecular crystals (benzene, naphtalene and a substituted binaphtyl-2-ol), whose ED and SF results were obtained from X-ray diffraction data. Regardless of the derivation of the ED, the answer to the question above is convincingly positive. The capability of the SF to reveal EDEs is independent from a / separation and EDEs may be recovered even when such separation is unfeasible or when symmetry reasons would preclude it. Use of the SF to test the concept of hypervalency in the K2SO4 crystal [4] will also be discussed. A novel non covalent interaction (NCI) descriptor, based on the RDG and enabling an easy-to-catch pictorial visualization of either the supposedly attractive (dispersive, hydrogen bonding) or allegedly repulsive (steric) intermolecular interactions, was recently developed [5]. We apply for the first time [6] this same tool to experimentally-derived ED’s of molecular solids (austdiol, benzene, famotidine) discussing its performance in synergy with Bader’s analysis and using our software code, NCI-Milano [7], purposedly developed for such an extension.
[1] R.F.W. Bader, C. Gatti Chem Phys. Lett. 1998, 287, 233-238.
[2] C. Gatti Struct. Bond. 2012, 147, 193-286.
[3] E. Monza, C. Gatti, L. Lo Presti, E. Ortoleva J. Phys. Chem. A 2011, 115, 12864-12878.
[4] M.S. Schmøkel, S. Cenedese, J. Overgaard, M.R.V. Jørgensen, Y-S Chen, C. Gatti, D. Stalke, B.B. Iversen Inorg. Chem. 2012, 51, 8607-8616.
[5] E.R. Johnson, S. Keinan, P. Mori-Sanchez, J. Contreras-Garcia, A.J. Cohen, W. Yang J. Am. Chem. Soc 2010, 132, 6498-6506.
[6] G. Saleh, C. Gatti, L. Lo Presti, J. Contreras-Garcia Chem. Eur. J. 2012,18,15523-15536.
[7] G. Saleh, L. Lo Presti, C. Gatti, D. Ceresoli J. Appl. Cryst. 2013, 46, 1513-1517
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
Progettazione di nuovi materiali per l'abbattimento di inquinanti : Come proteggere i nostri monumenti
No full textSi illustra una parte dei risultati del progetto LISA 2013 di cui M. Ceotto è il PI. Co-PI L. Lo Presti e D. Tamascelli
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
Simulation of aggregation phenomena in solution
Get PDFHow does crystallization begin? An answer to this apparently simple question is out of reach for current experimental techniques, which are blind to transient supramolecular aggregates that may occur in the no man’s land between a homogeneous liquid state and a suspension of crystal nuclei. Despite an increasing understanding of the thermodynamics of crystallization in the framework of Classical and Multi-Step nucleation theories, a predictive theory of nucleation has never been developed so far.
Classical Molecular Dynamics can shed light on the events that initiate aggregation phenomena, as in principle it can simulate the very elementary acts that lead to nucleation. The recently developed MiCMoS platform , provides several flexible and cheap tools to investigate small organic molecules in condensed phase. MiCMoS relies on accurately calibrated intermolecular force fields and was successfully applied to investigate several systems, including liquids, crystals, nanodroplets and nanoparticles . In this contribution, the implementation of a biased Molecular Dynamics algorithm to speed up the self-recognition process is reported. The procedure is employed to probe pre-nucleation aggregation phenomena in solutions of benzoic acid, selected as a suitable test case. The algorithm is already available in the most recent release MiCMoS v2.0, which can be downloaded for free at https://sites.unimi.it/xtal_chem_group/index.php/research/5-micmos
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
Segnali dall'invisibile : una breve storia della diffrazione
No full textLa diffrazione di raggi X è oggi uno strumento preziosissimo per studiare l'intima struttura della materia. Nel seminario 'Segnali dall'invisibile: una breve storia della diffrazione' ripercorreremo insieme la storia, affascinante e a tratti avventurosa, che ha portato questa tecnologia dalle prime, pioneristiche scoperte del '600 fino all'avvento dei moderni sincrotroni. Come vedremo, si è trattato di un percorso non sempre lineare, ma che ha dato un importante contributo nel risolvere problemi antichissimi, quali la natura della luce e la struttura atomica della materia
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