Search results for "Periodic table"
showing 10 items of 262 documents
Tetra-p-tolyl-verbindungen p-Tol4Si und p-Tol4Ge: Ein beitrag zur konfiguration der tetraaryl-methan-analoga Ar4M (M = C, Si, Ge, Sn, Pb)
1992
Abstract p-Tol4Si and p-Tol4Ge have been obtained from the reaction of p-TolLi with SiCl4 and of p-TolMgBr with GeCl4, respectively. The crystal structures have been determined and compared with those of the homologous compounds p-Tol4Sn and p-Tol4Pb. The latter two molecules (space group I 4 ) exhibit a S4 axis running between the aromatic groups and are shortened along this axis: all four p-Tol groups are closer together than in an undistorted tetrahedron. In the cases of the silane and germane (space group Pc), the tetrahedron is lengthened along one of the bond axes CM: only three of the four p-Tol groups are closer together, and the set up of a special molecular symmetry is blocked. A…
13C-NMR Based Evaluation of the Electronic and Steric Interactions in Aromatic Amines
2005
Abstract: Chemical shifts of the para carbon atoms, δ(13C-4), in a series of aromatic amines were used to calculate the Ãp, ÃR and ÃOR substituent constants for different amino groups. 1-Pyrrolidino, N,N-di-n-butylamino and N,N-diethylamino groups were found to be the most strong electron-donors. ortho-Substitution decreases the donor properties of the amino group. The amino groups in 2,6-di-i-propylaniline and N,N-2,6-tetramethylaniline have very weak electron-donor properties. The nitrogen atom in benzoquinuclidine and N,N-dimethyl-2,6-di-i-propylaniline have an electron-acceptor character. The calculated substituent constants of the amino groups studied are consistent with the s…
Synthesis, microwave spectrum, quantum chemical calculations, and conformational composition of a novel primary phosphine, cyclopropylethynylphosphin…
2014
International audience; The microwave spectrum of cyclopropylethynylphosphine, C3H5C≡CPH2, has been investigated in the 26-120 GHz spectral region. The spectrum is dominated by very rich and complex a-type R-branch pile-ups. There must be insignificant steric interaction between the phosphino group and the cyclopropyl ring due to the long distance between these two groups. However, the phosphino group does not undergo free or nearly free internal rotation. Instead, the spectra of two distinct conformers were assigned. Both these two forms have CS symmetry. The symmetry plane bisects the cyclopropyl ring and the phosphino group in both conformers, and the lone electron pair of the phosphino …
Linked Amido—Cyclopentadienyl Complexes of Group 3 and 4 Metals: The First “Post-Metallocenes”
1999
Linked amido—cyclopentadienyl ligands were introduced by Bercaw and Shapiro in the late eighties with the catalysts of the type Sc(η5:η1-C5Me4SiMe2N t Bu)X (X = H, alkyl) [1]. These electronically more unsaturated and sterically more accessible analogs of ansa-scandocene complexes were found to be capable of catalyzing the living oligomerization of the α-olefins propylene, 1-butene, and 1-pentene. In these complexes the steric constraint [2] of Brintzinger-type ansa-metallocenes is alleviated by the replacement of one cyclopentadienyl ligand by an amido ligand NR’ (Scheme 1). In order to explore sterically demanding derivatives of this dianionic ligand (C5R4ZNR’)2-, iron and titanium comple…
Structural diversity in coordination chemistry of tridentate and tetradentate polyphosphines of Group 6 to 10 transition metal complexes
2003
Abstract Tridentate and tetradentate polyphosphines offer a huge variety of coordination modes to transition metals which lead, depending on the metal, to very different structural features in the resulting complexes. Steric effects being crucial in metal–phosphine complexes reactivity, a good knowledge of the molecular structures of the species is required both in the solid state and in solution. This article reviews from a structural point of view the monometallic and symmetrical homobimetallic complexes of the transition elements of Group 6 to 10 with tridentate and tetradentate phosphines. Concerning the classical triphosphines and tetraphosphines, emphasis was put on advances reported …
N-Methyl-N-(2-nitrophenyl)nitramine andN-methyl-N-(3-nitrophenyl)nitramine
2005
The structures of the two title isomeric compounds (systematic names: N-methyl-N,2-dinitroaniline and N-methyl-N,3-dinitroaniline, both C7H7N3O4) are slightly different because they exhibit different steric hindrances and hydrogen-bonding environments. The aromatic rings are planar. The –N(Me)NO2 and –NO2 groups are not coplanar with the rings. Comparison of the geometric parameters of the ortho, meta and para isomers together with those of N-methyl-N-phenylnitramine suggests that the position of the nitro group has a strong influence on the aromatic ring distortion. The crystal packing is stabilized by weak C—H⋯O hydrogen bonds to the nitramine group.
Crystal structure of 4-[(adamantan-1-yl)amino]naphthalene-1,2-dione
2019
The title compound, an example of a stable 1,2-naphthoquinone, illustrates steric buttressing of the adamantanyl group.
Analysis of the cleaved topaz (001) surface.
2003
We report on the first study of the cleaved (001) topaz surface and the characterization of the chemical composition and atomic arrangement of the surface. We conclude that there is strong evidence for a hydroxyl group termination appropriate for further chemical reactions. The surface itself is easily accessible, atomically flat and suitable for potential technological applications.
<p class="HeadingRunIn"><strong>Considerations on systematics of the Phytoseiidae (Acari: Mesostigmata), with definition of a new species…
2015
The authors debate some aspects of the classification of the Phytoseiidae, especially the subfamily Typhlodrominae. Within this taxon, the rhenanus group is the most numerous species group of Typhlodromus (Anthoseius), with 206 nominal species. Detailed observation of morphological characters of the species in this group showed a considerable variation, suggesting the presence of different natural lineages. The discovery of the new species here described, Typhlodromus (Anthoseius) sandrae Ragusa & Tsolakis n. sp., allowed the definition of the new porathi species group. Definition of the new species group, a dichotomous key as well as the description of the new species are given.
ChemInform Abstract: Synthesis of Sesquiterpenes via Silicon-Guided Rearrangement of Epoxydecalins
2009
A survey of the rearrangement of methyldecalins via C-5 carbocation intermediates, with special attention to the silicon-guided rearrangement of epoxydecalins is provided. A TMS group properly posi...