Showing papers by "Robert Berger published in 2009"
TL;DR: Two alternative routes are discussed for the treatment of conformational equilibrium in a moderately flexible compound (see the superposition of the two conformers of the butyrolactone studied) when residual dipolar couplings are used.
Abstract: Mission possible! The motional averaging of NMR spectroscopic data complicates the determination of conformation and relative configuration in flexible organic molecules. Two alternative routes are discussed for the treatment of conformational equilibrium in a moderately flexible compound (see the superposition of the two conformers of the butyrolactone studied) when residual dipolar couplings are used.
68 citations
TL;DR: A NMR route toward molecular PV appears promising once suitable compounds have been identified and the typical sin(2alpha)-like dependence of the PV NMR frequency splittings on the dihedral angle alpha is observed for the entire series.
Abstract: In this paper, a quasirelativistic two-component zeroth order regular approximation (ZORA) density functional theory (DFT) approach to the calculation of parity violating (PV) resonance frequency differences between the nuclear magnetic resonance (NMR) spectra of enantiomers is presented and the systematics of PV NMR shielding constants in C2-symmetric dihydrogen dichalcogenides (H2X2 with X=O17, S33, S77e, T125e, P209o) are investigated. The typical sin(2α)-like dependence of the PV NMR frequency splittings on the dihedral angle α is observed for the entire series. As for the scaling behavior of the effect with the nuclear charge Z of X, the previously reported Z2.5±0.5 scaling in the nonrelativistic limit is reproduced and a scaling of approximately Z3 for the paramagnetic and Z5 for the spin-orbit coupling contribution to the frequency splitting is observed in the relativistic framework. The paramagnetic and spin-orbit coupling contributions are typically of opposite sign for the molecular structures s...
37 citations
TL;DR: A series of six 2,3-diphenyl-4-neopentyl-1-silacyclobut-2-enes with different 1,1-substituents has been prepared and characterized by single-crystal X-ray crystallography, demonstrating these compounds' potential as chromophores for condensed-phase luminescence applications.
Abstract: A series of six 2,3-diphenyl-4-neopentyl-1-silacyclobut-2-enes with different 1,1-substituents has been prepared and characterized by single-crystal X-ray crystallography These compounds possess a cis-stilbene-like chromophore involving also the four-membered ring, and exhibit a photophysical behavior similar to that of previously reported 1,2-diphenyl-cycloalkenes This chromophore system is confirmed by a theoretical investigation of the electronic structure and excitation spectra The absorption and photoluminescence of selected derivatives were studied in solution, as solid powder samples, and in doped-polymer thin films In well-dissolved solution, the silacyclobutenes show only very weak fluorescence emission (quantum yield approximately 01%), due to competition with photochemical and non-radiative photophysical relaxation When the solubility is degraded in a poor (aqueous) solvent, the formation of nanoscale aggregates leads to a significant enhancement factor in the emission intensity, due to the suppression of the photoreactivity in the more rigid molecular environment, although the quantum yield still remains below a few percent In the solid-state, however, photoreactivity is completely suppressed leading to fluorescence quantum efficiencies of 8-23% depending on the 1,1-substituents, which demonstrates these compounds' potential as chromophores for condensed-phase luminescence applications Two dominant competing photochemical reactions have been identified in solution (for excitation in the lowest-energy absorption band, >260 nm), which are analogous to related (sila-)cyclobutenes and stilbenoids The first involves ring-opening due to cleavage of the 1,4-Si-C bond to form metastable silabutadienes, which was confirmed by isolating the stereospecifically formed allylsilane which results from a secondary reaction with trapping agents such as methanol or water The second photochemical reaction involves ring closure of the 2,3-diphenyl substructure to form a dihydrophenanthrene analogue, which was confirmed by isolating the phenanthrene derivative that results following subsequent hydrogen abstraction in the presence of oxygen Measurements of the silacyclobutenes in doped-polymer thin films reveal a spectroscopic behavior ranging from that in solution to the nano-aggregate case as the silacyclobutene dopant concentration is increased
8 citations
TL;DR: In 2008, Nambu won the Physiknobelpreis 2008 for the theoretische Beschreibung von spontanen Symmetriebrechungen with Methoden der Quantenfeldtheorie.
Abstract: Fur die theoretische Beschreibung von spontanen Symmetriebrechungen mit Methoden der Quantenfeldtheorie erhielt Yoichiro Nambu den Physiknobelpreis 2008. Er teilt sich den Preis mit Makato Kobayashi und Toshihide Maskawa, welche die Brechung einer fundamentalen Symmetrie im Standardmodell der Teilchenphysik verankerten. Diese Symmetrieverletzung hat wiederum Konsequenzen fur molekulare Systeme.