Search results for "Valence"
showing 10 items of 2732 documents
Time–carbonyl groups equivalence in photo-oxidative aging of virgin/recycled polymer blends
2004
AbstractThe photo-oxidation behaviour of polymers is strongly dependent on the initial amount of carbonyl groups along the chains. The growing use of recycled post-consumer polymers coming from products used outdoors and then photo-oxidised, both pure and blended with the same virgin polymer, gives rise to an unpredictable behaviour of weathering resistance of products made with these materials. The present work shows that the carbonyl group–exposure time curves can be shifted along the time axis to give a single generalised master plot. It is then possible to predict the formation of the new carbonyl groups by knowing only the initial amount of the same carbonyl groups. The same shift fact…
Gold(iii) tetraarylporphyrin amino acid derivatives: ligand or metal centred redox chemistry?
2015
EPR spectroscopy and DFT calculations show that the site of reduction of porphyrinato gold(iii) complexes depends on the counterions X, the meso substituents R and the solvent.
Exploring the exchange interaction in a mixed valence {CoII4CoIII2} hexanuclear cluster with novel topology
2012
Abstract Reaction between [Co2(μ-OH2)(μ-Piv)2(Piv)2(HPiv)4] and a (salicylidene)ethanolamine ligand under ambient reaction conditions, affords hexanuclear complexes [CoIII2CoII4(Piv)8(HPiv)2(L)2(OH)2] (1–2). Both products have been characterized crystallographically and found to be mixed-valent, containing divalent and trivalent Co centers. Down to 30 K, the variable-temperature magnetic susceptibility data are dominated by the single-ion properties of high spin Co(II) centers with distorted-octahedral coordination geometries. Below this temperature, the effect of intramolecular ferromagnetic exchange interactions becomes apparent. Magnetic data has been analyzed in terms of ground isolated…
Conformation of tert-butoxycarbonylglycyl-dehydroalanyl-glycine methyl ester in the crystalline state and calculated in the gas phase.
2006
tert-Butoxycarbonylglycyl-dehydroalanyl-glycine methyl ester (systematic name: methyl {2-[(tert-butoxycarbonylamino)acetamido]prop-2-enamido}acetate) (Boc0-Gly1-ΔAla2-Gly3-OMe), C13H21N3O6, has been structurally characterized by single-crystal X-ray diffraction and by density functional theory (DFT) calculations at the B3LYP/6–311+G** level. The peptide chain in both the solid-state and calculated structures adopts neither β nor γ turns. All amino acid residues in the tripeptide sequence are linked trans to each other. The bond lengths and valence angles of the amino acid units in the crystal structure and gas phase are comparable. However, the conformation of the third glycyl residue (…
Pressure effect studies in molecular magnetism
2004
We report on temperature dependent magnetic susceptibility and Mossbauer effect studies of the influence of hydrostatic pressure (up to 1.2 GPa) on dynamic electronic structure phenomena in 3d transition metal coordination compounds. The systems under investigation are mononuclear spin crossover compounds of iron (II) and chromium (II), dinuclear complexes of iron (II) exhibiting coexistence of intramolecular antiferromagnetic coupling and thermal spin crossover, 1D, 2D and 3D polynuclear spin crossover complexes of iron (II), a valence tautomeric system of cobalt (II) showing a thermal transition from a high spin [CoII (semiquinone)] to a low spin [CoII (catecholate)] species on lowering t…
Magnetic and Spectroscopic Study on a New Asymmetric Mixed-valence Mn2(II,III) Coordination Compound
2015
A new dinuclear mixed-valence compound [MnIIMnIII(bttpnol)(O2C–C6H4–NO2)2]ClO4 was synthesized by using the asymmetric heptadentate ligand N-(2-hydroxybenzyl)-N,N′,N′-tris(2-pyridylmethyl)-1,3-diaminopropan-2-ol (H2btppnol). One central manganese atom assumes N3O3 and the other N2O4 coordination sphere. Both manganese ions are bridged by the alkoxy-group of the dinucleating ligand and two bidentate carboxylate groups of the nitrobenzoate ligands. The structural data show clearly that MnIII prefers the more oxygen rich donor set. Cyclic voltammetry measurements reveal that the mixed-valence state Mn2(II,III) could be reduced to Mn2(II,II) at E1/2(1) = –0.53 V and oxidized to Mn2(III,III) at …
A theoretical determination of the electronic spectrum of Methylenecyclopropene
1996
The vertical electronic spectrum of methylenecyclopropene, the prototype of the nonalternant hydrocarbons known as fulvenes, has been studied using multiconfigurational second-order perturbation theory. The calculations comprise three valence states and the 3s, 3p, and 3d members of the Rydberg series converging to the first π ionization limit. Vertical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation from the highest occupied σ orbital to a π* orbital, while the other two are π → π* excitations. The third transition gives rise to the most intense feature in the electronic spectrum. The results are rationalized…
Synthesis, characterization and magnetic properties of mixed-valence iron complexes with 2-pyridyl oximes
2018
Two new mixed-valence iron complexes with 2-pyridyl oximes, [Fe(mpko)3Fe(H2O)2(NO3)](NO3)·2H2O (1) (mpko− = methyl(2-pyridyl)ketone oximate) and [{Fe(dpko)3}2Fe](ClO4)·4H2O (2) (dpko− = bis(2-pyridyl)ketone oximate), have been prepared by reaction of FeIII with mpkoH in methanol (1) and FeII with dpkoH in methanol/water (2). Dinuclear FeII(low-spin)FeIII(high-spin) and trinuclear FeII(low-spin)FeIII(high-spin)FeII(low-spin) cations are present in the crystal structure of 1 and 2, respectively. Intermolecular hydrogen bonds in 1 lead to weak antiferromagnetic interactions between pairs of neighboring FeIII centers, which allows observation of single-ion zero-field splitting effects.
Theoretical characterization of the electronic structure of polymeric systems combining disilanylene and /spl pi/-conjugated units
1994
Summary form only given. Polymeric organosilicon systems are in the forefront of research, due to their intriguing properties (semiconducting behavior, photoconduction, non-linear optical properties, and thermo- and piezochromism). The fundamental skeletons of most of these polymers consist of a /spl sigma/-electron network formed by silicon-silicon bonds. Recently, polymeric organosilicon systems containing a regular alternation of a disilanylene unit and carbon /spl pi/-conjugated moiety as phenylene, ethynylene, or ethenylene have been synthesized. The purpose of this contribution is to perform a detailed study of the electronic structure of polymers containing alternated disilanylene an…
Soluble Complexes of Cobalt Oxide Fragments Bring the Unique CO2 Photoreduction Activity of a Bulk Material into the Flexible Domain of Molecular Sci…
2021
The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co3O4 can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence- and conduction-band (VB…