1,721,129 research outputs found
Metodologie quantistiche computazionali per lo studio di cristalli molecolari.
No full textI cristalli molecolari hanno applicazioni rilevanti sia nel contesto della ricerca accademica sia in quella industriale poiché vengono utilizzati, a seconda dei casi, in elettronica/optoelettronica (come per schermi o periferiche di archiviazione), sistemi host-guest, per esempio per applicazioni di rilascio dei farmaci, o per sistemi meccanici fotoattivati (attuatori).
Le proprietà menzionate, in particolare, sono largamente influenzate dal polimorfismo e dal comportamento strutturale ed elettronico che da esso derivano, per ogni composto che si trova in una forma cristallina definita. Questo è complicato dalla presenza delle forze vdW e/o dai legami a idrogeno, che governano la struttura molecolare all’interno del cristallo.
Con l’obiettivo di descrivere questo tipo di interazioni e predire proprietà elettroniche avanzate (come spettroscopia NMR di stato solido, assorbimento UV-Visibile), è stato adottato un approccio quantistico (QM) computazionale, a livello della Teoria del Funzionale della Densità (DFT) – sia nella sua formulazione indipendente dal tempo, sia in quella dipendente dal tempo. La DFT ha permesso di concludere con successo i diversi studi, bilanciando accuratezza e costo computazionale.
In particolare, il bisogno di tenere conto delle forze di vdW tramite la DFT corretta per le interazioni dispersive è stato messo in evidenza e differenti tipi di approccio (D2,D*,TS) sono stati adottati e comparati.
Questi approcci ci hanno permesso di ottenere un avanzamento notevole nella comprensione del polimorfismo e di altre proprietà dei sistemi studiati, in particolare di dimostrare che : i) una ragionevole combinazione tra DFT e correzioni semiempiriche per la dispersione a basso costo può portare ad una predizione della stabilità relativa tra polimorfi allo stesso livello di accuratezza di tecniche più rigorose e costose (per esempio, Diffusion Monte Carlo); ii) la stabilità relativa di composti complessi può essere calcolata con metodi DFT corretti per la dispersione: gli approcci disponibili offrono spesso risultati compatibili con quelli forniti dai metodi post-HF, ed il benchmarking con tali metodi può essere utile per l’individuazione di nuovi schemi per la dispersione iii) la predizione strutturale delle quattro forme cristalline di un sistema host-guest, per il quale abbiamo riportato e chiarito alcune lacune nell’interpretazione sperimentale della spettroscopia NMR, grazie al calcolo dei chemical shifts NMR; iv) l’ottimizzazione di un protocollo computazionale – basato sulla caratterizzazione QM di clusters di molecole estratte dal solido – per lo studio dei processi fotofisici e fotochimici di un cristallo molecolare termocromatico.Molecular crystals find relevant applications both in academic and industrial research since they can be utilized, depending on case, for electronics/optoelectronics (such as displays or storage devices), host-guest systems with, for example, applications for drugs release, or photo-activated mechanical systems (actuators).
The properties mentioned above are severely affected by polymorphism which influences the structural/electronic behaviour of each compound in a definite crystalline form. This is complicated by the dispersion and/or hydrogen bond interactions, which govern the molecular displacement within the solid.
In order to describe accurately these interactions, and to predict advanced electronic properties (such as solid-state NMR spectra, UV-Vis absorption), a quantum (QM) computational approach has been adopted, at the Density Functional Theory (DFT) level – both in its time-independent and in its time-dependent formulations. DFT furnishes a good balance between accuracy and computational cost.
Importantly, the need of accounting for vdW forces through dispersion-corrections has been highlighted, and different kinds of existing approaches (such as D2, D*,TS) were adopted and benchmarked.
The more notable advancements in the understanding of polymorphism and other properties of the studied crystals are: i) the demonstration that a wise combination of DFT and costless semiempirical corrections for dispersion leads to predict accurately the relative stability between polymorphs, almost at the same level as higher and costly methods (e.g. Diffusion Monte Carlo); ii) the relative stability of challenging systems can be reliably computed with dispersion-corrected DFT: the available methods give often results compatible with those furnished by post-HF methods. The benchmarking against these last can be useful for the design of new dispersion-correction schemes for DFT iii) the structural prediction of four crystal forms of a solid host-guest compounds previously synthetized, for that we addressed and clarified some lacks on the experimental NMR interpretation, through calculated NMR chemical shifts; iv) the optimization of a computational protocol for the study of the photophysical-photochemical processes of a thermochromic molecular crystal – based on the QM characterization of clusters of molecules extracted from the bulk crystal
Il sistema di qualificazione delle imprese nei contratti pubblici tra diritto nazionale e comunitario
No full textLa ricerca si pone l’obbiettivo di esplorare la complessa problematica della qualificazione delle imprese che aspirano alla stipulazione di contratti pubblici cercando di analizzare il fenomeno da una prospettiva sia interna sia comunitaria; ciò al fine di cercare di cogliere i tratti evolutivi essenziali del sistema nonché possibili linee evolutive future. Affrontata la prima questione involvente l’ipotesi di una necessità della qualificazione diretto portato dell’assetto costituzionale, l’analisi quindi si dipana attraverso lo studio dell’evoluzione dei sistemi di qualificazione secondo la classica distinzione tra i cc.dd. “requisiti di ordine generale” e di “idoneità professionale”. A seguito di tale imprescindibile inquadramento d’insieme, l’ultima parte delle riflessioni sono rivolte ai particolari strumenti di favor legislativo alla partecipazione alle gare ed alle problematiche che questi mezzi, di matrice spiccatamente comunitaria, incontrano nel processo di integrazione con la disciplina normativa nazionale.The research wants to attain the goal of explore firms’ qualification issues that aspire to enter into public contracts by analyzing the phenomenon from an internal view and also the study of EEC’s jurisprudence, with the purpose to grasp the evolutionary essential features as well as a future possible evolution lines. Once cleared up the first point concerning the hypothesis, the research sorts itself out through the study of systems’ evolution according to the classic distinction between the so-called “requirement of general order” and from “professional suitability”. Following this unavoidable organization as a whole, the last part of the consideration’s bent to the particular instruments of legislative favor, to the participation in the tender for public contracts and to the troubles that these steps, with a clearly matrix EEC, meet in the integration process with the national jurisprudence
Unraveling the Polymorphism of [(p-cymene)Ru(κN-INA)Cl2] through Dispersion-Corrected DFT and NMR GIPAW Calculations
No full textThe structural and 13C/1H NMR parameters of the four crystal forms (1α, 1·H2O, 1β, and 1γ) of the solid wheel-and-axle (WAA) metal–organic compound [(p-cymene)Ru(κN-INA)Cl2] have been studied by means of periodic DFT calculations. The quality of the results obtained strongly depends on a correct description of long-range interactions; thus, in the geometry refinement protocol used, the pure DFT functionals need to be coupled with a dispersion-correction term (B3LYP-D2, B3LYP-D*). The solid-state 13C/1H NMR δiso parameters and 13C MAS NMR spectra, calculated by means of the PBE-GIPAW method, agree well with the experimental data for the four crystal forms (mean absolute deviations of the 13C and 1H δiso data values lie in the ranges 1.3–2.9 and 0.3–1.0 ppm, respectively). In this context, some revisions in the experimental assignment of the 13C/1H NMR δiso parameters of the 1·H2O, 1β, and 1γ crystal forms can be suggested. The mismatch in the assignment seems to be due to the rotation of the −COOH moiety, which occurs at the 1α–1·H2O transition and was not considered in the experiments. Finally, the results obtained suggest the presence of two COOH···Cl hydrogen bonds of comparable strength established by the two molecules in the asymmetric unit of the 1γ polymorph, in partial disagreement with previous findings
Manufacturing Terminal and Field Bitumen-Tyre Rubber Blends: The Importance of Processing Conditions
Full text linkNo-agitation Tyre Rubber Modified Bitumens (TR-MBs), also known as "terminal blends" or "field-blends", are quite new technologies leading to bituminous binders similar to polymer modified bitumens in terms of both rheology and needed efforts during paving operations. Their manufacturing, as well as other TR-MBs, is strongly dependent on the selected processing variables as well as on the chosen materials. This study presents a literature review to provide an overview of the TR-MB technologies which focuses on the influence of processing conditions on the modification process, storage stability and overall properties of the final bitumen - tyre rubber blends. (C) 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SIIV2012 Scientific Committe
On the ability of Periodic Dispersion-Corrected DFT Calculations to Predict Molecular Crystal Polymorphism in para-diiodobenzene
No full textPeriodic DFT calculations employing the PBE, PBE0 and B3LYP functionals coupled with different dispersion correction schemes have been applied to para-diiodobenzene molecular crystal in order to determine how they perform in reproducing the energetic and crystal geometry of its two well known polymorphs.Our results demonstrate that, provided that the dispersion correction scheme proposed by Tkatchenko and Scheffler [Phys. Rev. Lett. 102 (2009) 073005] is used, DFT can be successfully employed to predictthe geometric structure and energy ordering of the a and b forms of para-diiodobenzene molecular crystal, with an accuracy comparable to that yielded by diffusion Monte Carlo calculations
Developing a Life Cycle Management methodology for road pavements
No full textThe necessity of implementing sustainability is a main challenge in our society and it is taking up also in civil engineering works sector, although the infrastructure field is still understanding how to make the Sustainability Assessment (SA) a common practice within the National Road Authorities (NRAs).
This research aims at providing a specific methodology for the Life Cycle Management (LCM) of road pavements materials and activities, starting from the available ISO and EN standards, by using Life-Cycle based techniques and Multi-Criteria Decision Making (MCDM), with the aim of assessing performance under the three pillars of sustainability
Electronic and optical properties of the Au22[1,8-bis(diphenylphosphino) octane]6 nanoclusters disclosed by DFT and TD-DFT calculations
No full textTime-dependent density functional theory calculations have been used to investigate the electronic and optical properties of a nanocluster composed of two directly bonded Au11 subunits, held together by six bidentate diphosphine ligands: 1,8-bis(diphenylphosphino) octane. Three exchange–correlation functionals have been adopted, a general hybrid (PBE0) and two range-separated hybrids (ωB97X and CAM-B3LYP). The results obtained show that the aforementioned properties are significantly different from those of a previously studied Au11-based nanocluster formed by just one single subunit. In particular, charge transfer excitations from the inner metal core to the outer ligands affect most of the UV–visible spectrum and occur for both alkyl and aromatic ligands. This is particularly evident when thiazole molecules are bonded to the gold core: In this case Au → ligand transitions affect also the first HOMO → LUMO excitation. Moreover, the gold core of this Au22 nanocluster has eight under-coordinated Au surface atoms not engaged in bonds with the ligands. No other known organic-protected gold nanocluster has a similar feature. These gold atoms can be considered as potential in situ active sites for catalysis, their catalytic efficiency and selectivity being modulated by charge distribution
Advanced Mixing Rheometry for Dynamic Shear Rheometer Testing of Highly Heterogeneous Bituminous Binders
No full textIn the era of asphalt mixtures designed with various admixtures and secondary materials, bitumen
technologists often encounter challenges in testing new materials and interpreting unconventional
results. One such challenge arises with highly heterogeneous bituminous
binders (HHBBs), including crumb rubber-modified bitumen, which is widely used despite technical
drawbacks such as poor stability during storage at high temperatures in asphalt plants.
Similar issues may arise during laboratory-scale high-temperature rotational viscosity testing,
in which conventional methods designed for homogeneous binders may not be suitable for
HHBBs. This study aims to address these challenges by introducing enhanced mixing rheometry
for cup-and-bob systems as an optimal solution tailored for dynamic shear rheometers’
rotational testing of HHBBs. The investigation evaluates two types of HHBBs (an 18 % crumb
rubber modified binder and a 6 % polymer-modified binder) by testing them with four different
impellers to record possible differences in viscosity measurement trends. Laboratory experiments
are augmented by numerical analysis to visually assess the stability of HHBBs during
rotational testing. Results from this combined empirical and numerical approach indicate that
current rotational measurement standards are inadequate for HHBBs because of poor mixing
efficiency. However, the use of the dual helical ribbon (ST-26.5-LPG) stirrer, which exhibits
improved mixing performance, appears to resolve this issue. Therefore, the authors recommend
adopting this specific testing geometry to obtain more accurate high-temperature viscosity
measurements of HHBBs, thereby enhancing the realism of the result
New simplified approach for obtaining a reliable plateau value in fatigue analysis of bituminous materials
Full text linkThe Plateau Value (PV) based on the Ratio of Dissipated Energy Change (RDEC) approach has been proven to provide a unique relationship with the fatigue life, Nf, independent of loading mode, temperature and frequency. In this paper, a new simplified approach is proposed to compute the unique energy parameter PV. The results of this study show that applying the proposed method allows a unique fatigue power law to be produced and eliminates the ambiguity and variability in calculating PV. However, the study also shows that the PV-Nf fatigue curves for bituminous materials are dependent on material type which is contradictory to other studies dealing with PV. The study also highlights the importance of appropriately identifying the fatigue failure point prior to commencing any fatigue analysis
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