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Valence (chemistry)

About: Valence (chemistry) is a research topic. Over the lifetime, 24937 publications have been published within this topic receiving 645252 citations. The topic is also known as: valency.


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Journal ArticleDOI
TL;DR: It is found that a moderate ratio of Mn/Co (1:1) favors good low-temperature reducibility and high Oads/Olatt, resulting in superior oxidation performance, although the stability in the existence of water for MOF-Mn1Co1 is not satisfied.

167 citations

Journal ArticleDOI
TL;DR: Nonstandard hybrid density functional calculations with 35% Hartree-Fock-like exchange combined with continuum solvent models are suggested as a practical protocol for the quantum-chemical characterization of organic mixed-valence systems and provides a basis for the evaluation of other computational methods.
Abstract: Structures, dipole moments, electron-transfer barriers, and spin density distributions of a series of mixed-valent bistriarylamin radical cations have been studied systematically by hybrid density functional methods with variable exact-exchange admixture combined with a continuum solvent model. The chosen systems differ in their bridging units and are all relatively close, from both sides, to the class II/III borderline of the Robin-Day classification of mixed-valence systems. Solvent effects are found to have a dramatic influence on the localized vs delocalized character of these cations. While gas-phase calculations or computations in a nonpolar solvent place all systems on the delocalized class III side, a more polar solvent like acetonitrile enables observation of symmetry breaking and charge localization with moderate exact-exchange admixtures in a hybrid functional for the systems on the class II side (with diphenylbutadiyne and diphenylethyne bridges). In contrast, the cations with the shortest bridges (phenylene, biphenylene) are characterized as class III. The comparison of computed intervalence charge-transfer excitation frequencies with experiment confirms the system with the diphenylbutadiyne bridge, and probably the system with the diphenylethyne bridge, to be class II, whereas in the dichloromethane solvent employed for spectroscopic measurements, the two other systems are on the class III side. Nonstandard hybrid density functional calculations with 35% Hartree-Fock-like exchange combined with continuum solvent models are suggested as a practical protocol for the quantum-chemical characterization of organic mixed-valence systems. This approach should allow closer examinations and provides a basis for the evaluation of other computational methods.

167 citations

Journal ArticleDOI
TL;DR: A number of spectroscopic techniques have been used to study the properties of natural and synthetic (pure and doped) zircon (ZrSiO4) as mentioned in this paper.
Abstract: Natural and synthetic (pure and doped) zircon (ZrSiO4) have been studied with a variety of spectroscopic techniques. These techniques are based on different physical phenomena, for instance transitions between spin states of nuclei and electrons, energetic transitions of valence electrons, intra-molecular vibrations, or vibrations of atoms and molecular units in the lattice. All of the diverse spectroscopic techniques, however, have in common that they probe energy differences between a ground and excited states, mostly upon interaction of the mineral with incident radiation. Such interactions are not only determined by the excited elementary particles or molecules themselves but depend greatly on their local environments (i.e. number, type, valence and geometrical arrangement of neighboring atoms). Spectroscopic techniques are thus sensitive to the local structure and provide information on the short-range order. Most research on zircon crystals using spectroscopic techniques was done to study their “real structures,” that is the characterization of deviations from “perfect” zircon. Such features include the incorporation of non-formula elements, structural defects and the presence of inclusions and other impurities. Correspondingly, most of the spectroscopic investigations can be assigned to two major groups. The first group represents studies done to characterize the structural position and local environment of non-formula elements when incorporated in the zircon lattice, and accompanying effects on physical properties. The second group comprises studies subjected to the real structures of “metamict” zircon samples, i.e., changes of the zircon structure caused by the impact of self-irradiation and upon recovery from radiation damage (Ewing et al., this volume). It is most obvious that a spectroscopic bulk or point analysis will first of all yield a spectrum (i.e. a plot of the intensity of the respective physical parameter versus wavelength, frequency or wavenumber), and this is what is used in most studies. In addition, image generation based on …

167 citations

Journal ArticleDOI
TL;DR: The present results strongly support the assignment of a low spin ferric structure to the iron ion in oxyhemoglobin.

167 citations

Journal ArticleDOI
TL;DR: In this paper, the root-mean-square bond valence mismatch (GII) and the chemical plausibility of the structure model can be significantly improved by optimizing the light atoms (oxygen and lithium) positions.
Abstract: Bond valence sums for the ion positions in single-crystal structure data of the garnet-like fast lithium ion conductors Li5La3M2O12 (M = Nb, Ta) exhibit unusually large deviations from the ideal valences. The root-mean-square bond valence mismatch (commonly termed “global instability index” GII) and the chemical plausibility of the structure model can be significantly improved by optimizing the light atoms (oxygen and lithium) positions using a bond valence mismatch minimization procedure in the previously suggested space group I213 or its centrosymmetric counterpart Ia3. Possible pathways for lithium ion migration in Li5La3M2O12 are identified by a bond valence analysis. Li-bond valence mismatch isosurface models for Li+-ion transport pathways are found to be nearly the same in both compounds Li5La3Nb2O12 and Li5La3Ta2O12. The characteristic feature of the three-dimensional Li+-ion pathway network is a nonplanar square of partially occupied Li sites.

166 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,521
20222,997
2021616
2020611
2019584
2018577