465,143 research outputs found
La Cosa Queer: saggio introduttivo
No full textIl tentativo di costruire un canone queer non può che essere proposto sotto il segno dell’ironia. Di fatto, nonostante alcuni degli autori e delle autrici che presentiamo abbiano assunto un ruolo quasi eroico nella rappresentazione della teoria queer, e nonostante i testi che presentiamo siano, ciascuno a suo modo, testi-chiave, sappiamo con certezza che questa è la nostra selezione e che ciò ha aggiunto un portato performativo al contenuto espressivo dei testi. Sappiamo che questo effetto è coestensivo a qualunque operazione di reiterazione, e dunque anche a quella forma particolare di reiterazione che va sotto il nome di traduzione culturale
Strategie per la rigenerazione dell'Albergheria
No full textIl saggio esplica l'importanza di azioni di 'partecipazione' attiva per la rigenerazione di un quartiere complesso del centro storico di Palermo, caratterizzato da una forte multiculturalità
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
Source function applied to experimental densities reveals subtle electron delocalization effects and appraises their transferability properties in crystals
No full textThe Source Function (SF) [1] enables the electron density (ED) to be seen at a point as determined by source contributions from the atoms of a system, and it is therefore well linked to the chemist’s awareness that any local property and chemical behaviour is to some degree influenced by all the remaining parts of a system [1-3]. The key feature of the SF is that its evaluation requires only knowledge of the ED of a system, enabling a comparison of ab initio and X-ray diffraction derived ED properties on a common, rigorous basis. We here apply the SF descriptor to X-ray derived EDs as a mean to reveal electron-delocalization effects (EDEs) in crystals. Use of the SF to detect them has been firmly assessed for isolated molecules and for theoretically-derived EDs [2,4-5], but extending to crystals and experimental EDs, although being reported at two conferences [6-7] and in two papers discussing heteroaromaticity in a benzothiazol-substituted phosphane [8] or antiaromaticity in cyclopentadienone derivatives [9] needs to be fully demonstrated. Still unanswered questions are whether the EDs from X-ray data may be accurate enough to reveal the subtle features caused by electron pairing and whether these are not only detectable, but also reproducible and transferable, whenever appropriate. To provide an answer we analyse the experimental SF patterns in benzene (BZ), naphthalene (NT) and (+/-)-8’-benzhydrylideneamino-1,1’-binaphtyl-2-ol (BAB) molecular crystals. We find that the SF tool recovers the characteristic SF% patterns caused by pi-electron conjugation in the first two paradigmatic aromatic molecules in almost perfect quantitative accord with those from ab initio periodic calculations [10]. Moreover, the effect of chemical substitution on the transferability of such patterns to the BZ- and NT-like moieties of BAB is neatly spotted by the observed systematic deviations, relative to BZ and NT, of only those SF contributions from the substituted C atoms [10]. The capability of the SF to reveal EDEs by using a promolecule ED (PED), rather than the “true” ED, is then tested; the PED seems unable to reproduce the SF trends anticipated by the increase of electron delocalization [10].
The SF has wider applications than those related to the nature of chemical bonds in more or less conventional situations [2-3]. Detection of EDEs is one such new direction, another being the extension of the SF machinery to retrieve the atomic sources of the spin ED [5,11].
Acknowledgements
Prof. Sine Larsen and Prof. Mark Spackman are both warmly thanked for providing us with, respectively, the 135 K X-ray diffraction dataset of naphthalene crystal and the 110 K X-ray diffraction data set of benzene. The Danish National Research Foundation is also thanked for partial funding of this work through the Center for Materials Crystallography (DNRF93).
References
[1] Bader, R. F. W. & Gatti, C. (1998). Chem. Phys. Lett. 287, 233-238.
[2] Gatti, C. (2012). Struct. Bond. 147, 193-286.
[3] Gatti, C. (2013). Phys. Scripta 87, 048102 (38pp).
[4] Monza, E., Gatti, C., Lo Presti, L. & Ortoleva, E. (2011). J. Phys. Chem. A 115, 12864–12878.
[5] Gatti, C., Orlando, A. M., Monza, E. & Lo Presti, L. (2016). Applications of Topological Methods in Molecular Chemistry, edited by R. Chauvin, C. Lepetit, B. Silvi & E. Alikhani, p. XXXX-YYYY, in Springer series Challenges and Advances in Computational Chemistry and Physics 22, Springer International Publishing, DOI 10.1007/978-3-319-29022-5_5
[6] Gatti, C., Saleh, G., Lo Presti, L. et al. (2012). In: Abstracts (page 42) of the Sagamore meeting XVII on Charge Spin and Momentum Densities, Daini Meisui Tei, Sapporo, Hokkaido, Japan, 15-20 July.
[7] Gatti, C. (2013). In: Abstracts of Natta’s Seeds Grow, From the crystallography and modelling of stereoregular polymers to the challenges of complex systems, International symposium on occasion of the 50th anniversary of the award of the Nobel Prize for Chemistry to Giulio Natta and Ziegler, Politecnico di Milano, Italy, 21-22 November.
[8] Hey, J., Leusser, D., Kratzert, D., Fliegl, H., Dieterich, J. M., Mata, R. A. & Stalke, D. (2013). Phys. Chem. Chem. Phys. 15, 20600-20610.
[9] Pal, R., Mukherjee, S., Chandrasekhar, S. & Guru Row, T. N. (2014). J. Phys. Chem. A, 118, 3479–3489.
[10] Gatti, C., Saleh, G. & Lo Presti, L. (2016). Acta Cryst. B72, doi:10.1107/S2052520616003450
[11] Gatti, C., Orlando, A. M. & Lo Presti, L. (2015). Chem. Sci. 6, 3845-3852
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
- …
