Band structure and optical properties of germanium sheet polymers

The band structure of H-terminated Ge sheet polymers is calculated using density-functional theory in the local density approximation and compared to the optical properties of epitaxial polygermyne layers as determined from reflection, photoluminescence, and photoluminescence excitation measurements. A direct band gap of 1.7 eV is predicted and a near resonant excitation of the photoluminescence is observed experimentally close to this energy

Structural Resolution Of The Stereochemistry Of A Spirooxirane Derived From An Alpha-Arylidene Heterocyclic Carbonyl - The Crystal And Molecular-Structure Of 2-(Para-Chlorophenyl)-5-Phenyl-7-Methyl-1-Oxa-5,6-Diazaspiro 2.4 Hept-6- En-4-One

The stereochemical assignment of molecular geometry for α‐arylidene carbonyl compounds and spirooxirane derived from them have continued to be a challenging problem for which the most satisfactory solution continues to be an x‐ray diffraction structure determination. In that regard, the title compound (a spirooxirane) has been found to crystallize in the monoclinic space group P2 1/c with cell dimensions of a = 5.989(1)Å, b = 27.625(4)Å, c = 9.374(2)Å, β = 99.06(1)°. The structure of the compound has been determined, with the refinement to R = 0.059. The previous, tentative assignment of structure has been confirmed substantiating our prediction that the oxidation of the enone system proceeds with rotation of the phenyl group on the β‐carbon away from the carbonyl group, minimizing adverse steric interactions and allowing orbitals of the carbonyl group to overlap with those of the carbanionic center during the closure of the oxirane ring. The agreement between predictions based on nmr data and the x‐ray diffraction determination will support a stronger reliance on the nmr data predictions in subsequent studies

Reappraisal of the origins of the polymodal molecular mass distributions in the formation of poly(methylphenylsilylene) by the Wurtz reductive-coupling reaction

Variations of yields and molecular weight parameters of poly(methylphenylsilylene) formed through sodium-mediated Wurtz reductive coupling of dichloromethylphenylsilane in toluene, xylene, diethyl ether, tetrahydrofuran, and diphenyl ether both in the presence and in the absence of 15-crown-5 and at different temperatures are described. The effects of various terminating reagents are also considered. The crown ether when it is present in the reaction mixture is shown to act as a phase transfer agent for the sodium and this is interpreted as having two effects. The first is to activate the alkali metal for reductive coupling, which at lower temperatures is a necessary condition for polymer formation. The other is to transport the alkali metal to points on the high molecular weight polymer chains where, as a prelude to backbiting, it induces a highly selective chain scission; this is explained by it occurring only at gauche defects in the otherwise rodlike all-trans sequences within the polymer in solution. The polymodal molecular weight distributions that are commonplace for polysilylenes are rationalized in terms of a competition between such activation and degradation processes

Reappraisal of the origins of the polymodal molecular mass distributions in the formation of poly(methylphenylsilylene) by the Wurtz reductive-coupling reaction

Variations of yields and molecular weight parameters of poly(methylphenylsilylene) formed through sodium-mediated Wurtz reductive coupling of dichloromethylphenylsilane in toluene, xylene, diethyl ether, tetrahydrofuran, and diphenyl ether both in the presence and in the absence of 15-crown-5 and at different temperatures are described. The effects of various terminating reagents are also considered. The crown ether when it is present in the reaction mixture is shown to act as a phase transfer agent for the sodium and this is interpreted as having two effects. The first is to activate the alkali metal for reductive coupling, which at lower temperatures is a necessary condition for polymer formation. The other is to transport the alkali metal to points on the high molecular weight polymer chains where, as a prelude to backbiting, it induces a highly selective chain scission; this is explained by it occurring only at gauche defects in the otherwise rodlike all-trans sequences within the polymer in solution. The polymodal molecular weight distributions that are commonplace for polysilylenes are rationalized in terms of a competition between such activation and degradation processes

Structural resolution of the stereochemistry of a spirooxirane derived from an α‐arylidene heterocyclic carbonyl. The crystal and molecular structure of 2‐( p ‐chlorophenyl)‐5‐phenyl‐7‐methyl‐1‐oxa‐5,6‐diazaspiro[2.4]hept‐6‐en‐4‐one

The stereochemical assignment of molecular geometry for α‐arylidene carbonyl compounds and spirooxirane derived from them have continued to be a challenging problem for which the most satisfactory solution continues to be an x‐ray diffraction structure determination. In that regard, the title compound (a spirooxirane) has been found to crystallize in the monoclinic space group P2 1/c with cell dimensions of a = 5.989(1)Å, b = 27.625(4)Å, c = 9.374(2)Å, β = 99.06(1)°. The structure of the compound has been determined, with the refinement to R = 0.059. The previous, tentative assignment of structure has been confirmed substantiating our prediction that the oxidation of the enone system proceeds with rotation of the phenyl group on the β‐carbon away from the carbonyl group, minimizing adverse steric interactions and allowing orbitals of the carbonyl group to overlap with those of the carbanionic center during the closure of the oxirane ring. The agreement between predictions based on nmr data and the x‐ray diffraction determination will support a stronger reliance on the nmr data predictions in subsequent studies.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96401/1/5570200314_ftp.pd