Search results for "inversion"
showing 10 items of 312 documents
Inversion of the Triphenylamine Molecule
1964
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.
5,10-Dihydroindeno[2,1-a]indene
2019
The title compound, C16H12, crystallizes with four half molecules in the asymmetric unit, each of which is located on a crystallographic centre of inversion. The molecules are essentially planar. The crystal studied was a non-merohedral twin.
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…
Recognition of briefly presented familiar and unfamiliar faces
2009
Early processing stages in the perception of familiar and unfamiliar faces were studied in four experiments by varying the type of available facial information in a four alternative forced choice recognition task. Both reaction time and recognition accuracy served as dependent measures. The observed data revealed an asymmetry in processing familiar and unfamiliar faces. A markedly weak inversion effect and strong blurring effect suggest a limited usage of spatial relations within early processing stages of unfamiliar faces. Recognition performance for whole familiar faces did not deteriorate due to blurring or the presentation of isolated internal features, suggesting a low level of represe…
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…