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Abstract Detailed derivations of the Legendre-Hadamard necessary conditions for energy-minimizing states of fiber-reinforced three-dimensional solids and two-dimensional shells are presented. The underlying conceptual framework is Cosserat elasticity theory in which the Cosserat rotation field controls the orientation of the embedded fibers. This is partially coupled to the continuum deformation gradient by the requirement that the fibers convect as material curves with respect to the matrix material in which they are embedded. The conditions obtained combine the effects of deformation and rotation and subsume previously obtained decoupled inequalities involving these effects separately.
Significant melting point depression and large hysteresis during the melting–recrystallization cycle are observed for semiconductor Ge nanowires encapsulated within carbon nanotubes. The capability of cutting, linking (see cover), and welding (see Figure) nanowires at relatively modest temperatures may provide a new approach to integrating these 1D nanostructures into functional devices and circuitry.
Enge Bandlücken und hohe Elektronenaffinitäten zeichnen konjugierte Oligomere aus, die Thiophen-1-oxid- und Thiophen-1,1-dioxid-Einheiten enthalten und durch Zirconocen-vermittelte Kupplungsreaktionen synthetisiert wurden. Eine Reihe von Oligomeren mit drei bis elf Ringen (X=2 H, S=O, S(=O)2; siehe Bild) ermöglichte es, elektronische Eigenschaften konjugierter Ketten zu untersuchen. Besonders interessant ist, dass die weniger Elektronen ziehende Sulfoxidgruppe die kleinste Bandlücke zur Folge hat.
The impact of social media on adolescent mental health is an area of intense interest and controversy, especially since the COVID-19 pandemic has increased youth’s reliance on online communications. Current evidence, mostly from cross-sectional studies in the Global North, is mixed. Thus, the aim of this study was to investigate the relationship between social media and mental health in adolescents in Perú across 15 months of COVID-19 lockdowns. In this observational study, we first examined associations between social media intensity (SMI), feelings of loneliness, and anxiety at the beginning of the pandemic (May 2020) in 1603 early adolescents (10 – 14 years). Hypotheses of a positive association between all outcomes, particularly for girls, were derived from exploring one half of the sample (n =807), preregistered, and then confirmed in the second half of the sample (n = 806). In May 2020, SMI was associated with more frequent loneliness for girls, and SMI was associated with more frequent anxiety for both sexes. In a longitudinal follow up (n = 455) we then investigated prospective and directional associations across three waves (May 2020, November 2020, and July 2021). Longitudinal analyses revealed a more complex pattern. Across 15 months of COVID-19 in Perú, feelings of loneliness in girls were associated with an increase in SMI. In contrast, higher SMI among girls was associated with an increase in feelings of anxiety. We did not find the reverse relationships. Our findings with early adolescents in low-and middle-income urban settings in Latin America, an underrepresented population, underscore the importance of longitudinal research and contribute to understanding these important issues globally.
Palladium complexes of trio-o-tolyl phosphine and bis(diphenylphospino)ferrocene catalyze the reaction between aryl halides and either tin amides or amines in the presence of base to form aryl amines by halide substitution. This account describes our mechanistic and synthetic studies related to the amination reactions. These studies include kinetic behavior of the catalytic systems as well as direct observation of the primary stoichiometric reactions comprising the catalysis - including the rare C-N bond-forming reductive eliminations - and the mechanisms of these individual reactions. This paper also describes the development of tin-free amide sources and second generation amination catalysts that have resulted from our mechanistic understanding of the amination chemistry.
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