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.

Valence Tautomerism: New Challenges for Electroactive Ligands

Abstract: Valence tautomeric complexes combine redox-active ligands and transition metal ions with two or more accessible oxidation states, exhibiting two nearly degenerated electronic states with localized electronic structures Charge distribution in such electronic isomers has an appreciable sensitivity to the environment so an external perturbation, like photons, temperature and/or pressure, may lead to an intramolecular electron transfer between both redox active units and therefore to a reversible interconversion between the two degenerated electronic states Moreover, since each electronic isomer exhibits different optical, electronic and/or magnetic properties, these complexes are being proposed as candidates for future use in molecular electronic devices and switches Most of the valence tautomeric complexes reported thus far are based on quinone or quinone-type ligands with a series of transition metal ions such as Co, Cu, Ni and Mn Nevertheless, in the last few years, the number of electroactive ligands identified to be active in valence tautomeric complexes is being expanded by including new radical ligands such as polychlorotriphenylmethyl, phenoxyl or tetraphenylporphyrin radicals (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2005)

Interpretation of the Changing the Band Gap of Al2O3 Depending on Its Crystalline Form: Connection with Different Local Symmetries

Abstract: The valence and conduction bands of am- and γ-Al2O3 films grown by the atomic layer deposition technique were studied simultaneously in identical experimental conditions using high-resolution near -edge X-ray absorption fine structure and soft X-ray photoelectron spectroscopy. The valence band maximum was found to be centered at 3.64 ± 0.04 eV for am-Al2O3 and 3.47 ± 0.04 eV for γ-Al2O3. The band gap of Al2O3 was determined to be 7.0 ± 0.1 and 7.6 ± 0.1 eV for measured am- and γ-Al2O3, respectively. The main role in changing the band gap belongs to a shift of the bottom of conduction band depending on Al2O3 crystalline form. The position of the bottom of the conduction band is governed by the charge transfer from Al atom to the oxygen that depends strongly on the Al atom coordination symmetries. A strong p–d hybridization allowed for Td symmetry but forbidden for Oh symmetry plays the decisive role in the formation of the bottom of the conduction band.

Exciton Spectrum of Cadmium Sulfide

Abstract: The reflectance and fluorescent spectra of hexagonal CdS crystals have been measured at 77\ifmmode^\circ\else\textdegree\fi{} and 4.2\ifmmode^\circ\else\textdegree\fi{}K using polarized light in the region of 5000 A. Structure not previously reported has been found in the reflectivity curves which leads to the identification of three exciton series. These can be understood in terms of the splitting of the valence band into three levels at k=0. The observation of excited exciton states and the polarization properties of the excitons make possible: (1) The determination of two of the three exciton binding energies, (2) the determination of the energy splittings of the three valence bands, (3) the verification of the symmetry assignments of the valence and conduction bands, and (4) correlation of the work of others with the present work, showing that the definite intrinsic effects are consistent both with our observations and our interpretations. The fluorescent experiments strongly suggest that the radiative decay of excitons occurs not directly, but from localized impurity exciton states in agreement with theory.

Electronic structure of silicon

Abstract: It is shown that a purely local-pseudopotential calculation is able to accurately reproduce the major optical gaps and cyclotron masses. However, deviations from the experimental results become manifest in photoemission and x-ray charge-density results as we extend our calculations to the lower valence bands. These deviations indicate the necessity of an energy-dependent nonlocal $s$-well potential, a conclusion which is also supported by an analysis of the Heine-Abarenkov pseudopotential scheme. A detailed comparison is made between experimental results obtained from optical, photoemission, x-ray, and cyclotron-resonance measurements, and the results of both the local calculation and an energy-dependent nonlocal calculation. Yang and Coppens's recent determination of the valence charge density in silicon makes it possible to assess the accuracy of the pseudocharge densities for the first time.

Quantum theory of chemical valence concepts

Abstract: Electron density operators and projection operators are fundamental quantities in the general physical interpretation of quantum theory In this series of papers the aim is to explore their relevance to the definition and use of chemical valence concepts Here, the problems involved in defining the charge on an atom in a molecule are discussed, and a new approach is formulated in terms of these operators The approach depends on the result that, if a projection operator P is formed representing some subspace of a molecular Hilbert space, then the probability of occupancy of that subspace is Tr DP, where D is an appropriate electron density operator In particular, the molecular one-electron Hilbert space is considered, and projection operators for atomic orbitals, atoms, pairs of atoms, atoms in threes, and so on, are found The above result allows for the definition of corresponding occupation numbers From these follow definitions of the charge on an atom in a molecule and of occupation numbers for elec