Topic
Steric effects
About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.
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TL;DR: The fluorescent properties of tetrakis(4-N-methylpyridy1)porphyrin (H2TMPyP(4)) in aqueous solution have been the subject of considerable discussion as discussed by the authors.
Abstract: The fluorescent properties of the cationic free base tetrakis(4-N-methylpyridy1)porphyrin (H2TMPyP(4)) in aqueous solution have been the subject of considerable discussion. Conclusions by various authors on the presence of homo-aggregation of H2TMPyP in these solutions are contradictory. The present work reports spectroscopic data for three isomers of H2TMPyP(n) (n = 2,3, or 4) at varying concentrations, solvent polarity, and temperature. 'H NMR spectra of H2TMPyP(4) show no ground-state monomers below M in water. Fluorescence spectra of the three isomers in aqueous solutions indicate the absence of aggregates both in the ground and excited state. Fluorescence lifetimes of the three isomers both in solution as well as adsorbed on solid surfaces can be explained by taking into account their dependence on the steric hindrance for rotation of the pyridinium groups of the three isomers with respect to the porphyrin macrocycle. Molecular mechanics confirms a higher degree of steric hindrance of H2TMPyP(2) as compared to the two other isomers. From the experimental results it is concluded that the first excited singlet state SI of the porphyrin mixes with a nearby CT state slightly above this SI state. In this CT state an electron is transferred from the porphyrin macrocycle to the pyridinium group. The amount of SI-CT mixing, responsible for the spectroscopic differences, is determined by the degree of coplanarity and resonance interaction of the porphyrin and the pyridinium n-systems, and by the solvent polarity which determines the energy difference between the two states.
167 citations
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TL;DR: In this article, the existence of these species is presumably due to interactions between the free electron pairs on the nitrogen atoms and the carbene carbon atom or the silicon atom, and the properties of these molecules are compared with the results of quantum chemical calculations.
Abstract: The isolation and characterization of thermally stable diaminocarbenes and diaminosilylenes has reawakened interest in the low-coordinated compounds of group-14 elements. The existence of these species is presumably due to interactions between the free electron pairs on the nitrogen atoms and the carbene carbon atom or the silicon atom. The present review is mainly concerned with the heavier analogues R2Ge:, R2Sn:, and R2Pb:, systems without intramolecular donor stabilization that owe their existence principally to steric shielding by the voluminous alkyl or aryl groups R. Dimerizations of these electron-sextet molecules give rise to the double-bond systems of the digermenes, distannenes, and diplumbenes; the latter species have for a long time been considered as being incapable of existence. The properties of these molecules are compared with the results of quantum chemical calculations. A separate section is devoted to the isomers of Si4R6, which include novel tetrasilacyclobutenes and the diene analogues, the tetrasilabuta-1,3-dienes.
166 citations
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166 citations
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TL;DR: In this article, complexes of the form (Cp†)TiCl2(NPR3) and the analogous dimethyl derivatives (cp*)TiMe2( NPR3) have been prepared.
165 citations
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TL;DR: Catalytic oxidative coupling between acrylamides and alkynes was achieved using 0.5 mol % loading of [RhCp*Cl(2)](2) with Cu(OAc)(2) as an oxidant.
165 citations