Search results for " inversion"
showing 10 items of 167 documents
Transmission of mechanical energy through polymeric liquid crystals and their blends
1988
Molecular composites, also called polymeric liquid crystals (PLC), are contrasted with the traditional composites; the name heterogeneous composites is proposed for the latter. Advantages of blending PLCs with ordinary engineering polymers are discussed. Results reported for blends of poly(ethylene terepthalate) (PET) with a PLC containing sequences of PET and p-hydroxybenzoic acid (PHB) include thermophysical properties, melt rheology, mechanical properties of solid blends, and scanning electron microscopy of fracture surfaces. A model called the island model was developed to explain the results: One assumes that the lines of force as well as propagating cracks tend to concentrate in the p…
Chiroptical inversion of a planar chiral redox-switchable rotaxane.
2019
Reversible redox-switching of a planar chiral [2]rotaxane with a tetrathiafulvalene-bearing crown ether macrocycle generates a complete sign reversal of the main band in the ECD spectrum, as shown by experiment and rationalised by DFT calculations.
Experimental and DFT study of the conversion of ephedrine derivatives into oxazolidinones. Double SN2 mechanism against SN1 mechanism
2010
Sulfonation of the N-Boc derivatives of 1,2-aminoalcohols, such as ephedrine, pseudoephedrine, norephedrine, norpseudoephedrine, thiomicamine, and chloramphenicol yields a mixture of the corresponding oxazolidinones with inversion (erythro derivatives) and/or retention of configuration (threo derivatives)at C5. We suggest a competition between two mechanisms: an intramolecular SN2 process initiated by attack of the carbonyl oxygen of the Boc group to the benzylic carbon and the other one through a double SN2 process. In the erythro derivatives the first mechanism is predominant, while in the threo derivatives both mechanisms have similar energy. This hypothesis is supported by theoretical c…
1988
The structure of 1-azabicyclo[4.2.0]octane (1) was evaluated by NMR spectroscopy. It was found that the six-membered ring has a chair conformation with the four membered ring in the N-axial and C-6-equatorial position, and as a consequence, the monomer allows neither a ring inversion nor a nitrogen inversion. It possesses two centers of chirality, one at the nitrogen atom and one at C-6. Homopolymerization was carried out in methanol at 60°C with N-alkyl ammonium salts of 1 affording the polymer in high yield within a few hours. Its NMR spectra reveal that the polymer chain is preferably arranged in the biequatorial position of the six-membered ring. As a polybase the polymer can be titrate…
Chiral Inversion of Thiolate-Protected Gold Nanoclusters via Core Reconstruction without Breaking an Au-S Bond
2019
Based on density functional theory computations of the well-known chiral Au38(SR)24 nanocluster and its Pd- and Ag-doped derivatives, we propose here a mechanism for chiral inversion that does not require breaking of a metal-sulfur bond at the metal-ligand interface, but features a collective rotation of the gold core. The calculated energy barriers for this mechanism for Au38 and Pddoped Au38 are in the range of 1 – 1.5 eV, significantly lower than barriers involving breakage of Au-S bonds (2.5 eV). For Ag-doped Au38, barriers for both mechanisms are similar (1.3 – 1.5 eV). Inversion barriers for a larger chiral Au144(SR)60 are much higher (2.8 eV). Our computed barriers are in a good agre…
Securitización e inversiones extranjeras: control del riesgo de ciertas inversiones para la seguridad nacional del Estado
2019
El aumento constante de las inversiones extranjeras durante las últimas décadas ha venido acompañado de un cambio en sus parámetros. El origen de las inversiones varía, como lo hace su naturaleza y destino. La posibilidad de que sectores claves de la economía del Estado receptor acaben en manos de inversores extranjeros, en ocasiones pertenecientes o controlados por un Estado foráneo, es vista con prevención en muchos países y acentúa la creciente oposición al libre comercio y a la inversión extranjera. La limitada respuesta -ex post- ofrecida por el marco normativo internacional, lleva a los Estados a diseñar respuetas ex ante, dirigidas a controlar ciertas inversiones que por su origen, c…
Revisiting the bromination of 3β-hydroxycholest-5-ene with CBr4/PPh3 and the subsequent azidolysis of the resulting bromide, disparity in stereochemi…
2023
Cholesterol reacts under Appel conditions (CBr4/PPh3) to give 3,5-cholestadiene (elimination) and 3β-bromocholest-5-ene (substitution with retention of configuration). Thus, the bromination of cholesterol deviates from the stereochemistry of the standard Appel mechanism due to participation of the Δ5 π-electrons. In contrast, the subsequent azidolysis (NaN3/DMF) of 3β-bromocholest-5-ene proceeds predominantly by Walden inversion (SN2) affording 3α-azidocholest-5-ene. The structures of all relevant products were revealed by X-ray single crystal structure analyses, and the NMR data are in agreement to the reported ones. In light of these findings, we herein correct the previous stereochemical…
Crystal structure and absolute configuration of (4S,5R,6S)-4,5,6-trihydroxy-3-methylcyclohex-2-enone (gabosine H)
2017
The absolute configuration of the title compound, determined as 4S,5R,6S on the basis of the synthetic pathway, was confirmed by single-crystal X-ray diffraction. The molecule is formed by a substituted six-membered cyclohexene ring adopting an envelope conformation and substituted by carbonyl, methyl and hydroxyl groups. The supramolecular structure is mainly built by a combination of O—H⋯O and weaker C—H⋯O hydrogen bonds.
Los beneficios del mar: negocios e inversiones económicas de la oficialidad naval en el siglo XVIII
2019
Throughout the eighteenth century, the Spanish navy was extensively reformed and enhanced as a consequence of the new model of Monarchy that the Bourbons introduced. Among the measures adopted, it is remarkable the erection of the naval officers corps according to the needs and obligations that maritime war had in this century. This group becomes the paradigm of the new professionalization and new training approach, and its members had great relevance due to its great mobility around the empire and the power of the naval institution. This means that a part of the naval officers were placed in privileged positions and at the intersection of diverse political, social and economic networks. Th…
Experimental introgression in Drosophila : Asymmetric postzygotic isolation associated with chromosomal inversions and an incompatibility locus on th…
2022
Interspecific gene flow (introgression) is an important source of new genetic variation, but selection against it can reinforce reproductive barriers between interbreeding species. We used an experimental approach to trace the role of chromosomal inversions and incompatibility genes in preventing introgression between two partly sympatric Drosophila virilis group species, D. flavomontana and D. montana. We backcrossed F1 hybrid females from a cross between D. flavomontana female and D. montana male with the males of the parental species for two generations and sequenced pools of parental strains and their reciprocal 2nd generation backcross (BC2mon and BC2fla) females. Contrasting the obser…