Search results for "RF"
showing 10 items of 23683 documents
Formation and trapping of the thermodynamically unfavoured inverted-hemicucurbit[6]uril
2019
Amplification of a thermodynamically unfavoured macrocyclic product through the directed shift of the equilibrium between dynamic covalent chemistry library members is difficult to achieve. We show for the first time that during condensation of formaldehyde and cis-N,N'-cyclohexa-1,2-diylurea formation of inverted-cis-cyclohexanohemicucurbit[6]uril (i-cis-cycHC[6]) can be induced at the expense of thermodynamically favoured cis-cyclohexanohemicucurbit[6]uril (cis-cycHC[6]). The formation of i-cis-cycHC[6] is enhanced in low concentration of the templating chloride anion and suppressed in excess of this template. We found that reaction selectivity is governed by the solution-based template-a…
Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets
2013
The rhenium(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4(+) = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)](2-) anions and bulky NBu4(+) cations. Each [ReBr4(ox)](2-) anion is surrounded by six NBu4(+) cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re(IV) cent…
Viewpoint: Atomic-Scale Design Protocols toward Energy, Electronic, Catalysis, and Sensing Applications
2019
Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I "Atomic-scale design protocols towards energy, electronic, catalysis…
Unexpected Scholl Reaction of 6,7,13,14-Tetraarylbenzo[k]tetraphene: Selective Formation of Five-Membered Rings in Polycyclic Aromatic Hydrocarbons
2016
Cyclodehydrogenation is a versatile reaction that has enabled the syntheses of numerous polycyclic aromatic hydrocarbons (PAHs). We now describe a unique Scholl reaction of 6,7,13,14-tetraarylbenzo[k]tetraphene, which "unexpectedly" forms five-membered rings accompanying highly selective 1,2-shift of aryl groups. The geometric and optoelectronic nature of the resulting bistetracene analogue with five-membered rings is comprehensively investigated by single-crystal X-ray, NMR, UV-vis absorption, and cyclic voltammetry analyses. Furthermore, a possible mechanism is proposed to account for the selective five-membered-ring formation with the rearrangement of the aryl groups, which can be ration…
Determining Factors for the Unfolding Pathway of Peptides, Peptoids, and Peptidic Foldamers.
2016
We present a study of the mechanical unfolding pathway of five different oligomers (α-peptide, β-peptide, δ-aromatic-peptides, α/γ-peptides, and β-peptoids), adopting stable helix conformations. Using force-probe molecular dynamics, we identify the determining structural factors for the unfolding pathways and reveal the interplay between the hydrogen bond strength and the backbone rigidity in the stabilization of their helix conformations. On the basis of their behavior, we classify the oligomers in four groups and deduce a set of rules for the prediction of the unfolding pathways of small foldamers.
Helical supramolecular polymerization of C3-symmetric amides and retroamides: on the origin of cooperativity and handedness.
2016
The cooperative supramolecular polymerization of 1 and 2 yields P- or M-type helical aggregates depending on the absolute configuration (S or R) of the stereogenic centres attached to the side chains. The connectivity of the amide group does not affect the handedness of the helical aggregates, but determines a larger cooperativity for retroamides 1.
Surface sediment dynamics along the shore of Hammamet Gulf (Tunisia, southern Mediterranean)
2016
International audience; In the summer of 2015 the authors analysed grain size and surface sediment composition through high spatial resolution from samples taken at 53 stations along the Hammamet coast (southern Mediterranean Sea). The Acoustic Doppler Current Profiler deployed in this study showed that the surface current flows toward the north-east, parallel to the coast at a maximum speed along the main axis of about 5.9 cm s−1. Near the bottom the current flows toward the north-west at a maximum speed of 2.2 cm s−1. The tide plays a relatively small role in water circulation in Hammamet Gulf. Spatial distribution of particle size, along with speed and current direction analysis, furnish…
Annually resolved δ13Cshell chronologies of long-lived bivalve mollusks (Arctica islandica) reveal oceanic carbon dynamics in the temperate North Atl…
2011
Abstract The ability of the ocean to absorb carbon dioxide is likely to be adversely affected by recent climate change. However, relatively little is known about the spatiotemporal variability in the oceanic carbon cycle due to the lack of long-term, high-resolution dissolved inorganic carbon isotope ( δ 13 C DIC ) data, especially for the temperate North Atlantic, which is the major oceanic sink for anthropogenic CO 2 . Here, we report shell carbon isotope values ( δ 13 C shell ), a potential proxy for δ 13 C DIC , of old-grown specimens of the long-lived bivalve mollusk, Arctica islandica . This paper presents the first absolutely dated, annually resolved δ 13 C shell record from surface …
Modeling Rain Isotopic Composition under Orographic Control: A Landscape Approach for Hydrogeological Applications
2021
Oxygen isotopic composition is useful for individuating recharge areas of groundwater bodies by the comparison with those of local rainfalls. While on a global scale general relationships, such as the isotopic vertical gradient or continentality effects, efficiently describe spatial variations of the isotopic signature, hydrogeological applications need spatial models that are more focused on the effects of local topographic structures and/or subsoil geology. This work presents a case study in northeastern Sicily (Italy) characterized by complex geological and orographic structures, in which isotopic composition of rainfalls is governed by orographic effects and the varying initial composit…
Dimensionless Stage-Discharge Relationship for a Non-Linear Water Reservoir: Theory and Experiments
2020
In the field of hydrology, stage&ndash