Topic
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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TL;DR: In this article, the problem of defining the charge on an atom in a molecule is discussed, and a new approach is formulated in terms of electron density operators and projection operators, which are fundamental quantities in the general physical interpretation of quantum theory.
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
272 citations
TL;DR: Using a valence double-zeta polarization basis, full configuration interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation as discussed by the authors.
Abstract: Using a valence double‐zeta polarization basis, full configuration–interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation. At the same geometries and with the same basis set configuration interaction calculations at excitation levels up to hextuples, multireference singles doubles configuration interaction calculations, coupled cluster calculations at excitation levels up to quadruples, Mo/ller–Plesset perturbation theory calculations through order fifteen, and complete active space second‐order perturbation theory calculations are also carried out. The static correlation contribution increase with increasing bond length. The calculations show that the coupled cluster approach has a remarkable ability to describe even relatively large static correlation contributions. The single reference perturbation expansion breaks down for larger OH bond length, while the multireference approach preserves the accu...
272 citations
TL;DR: In this article, the effect of polyhedra distortions on the crystal field splitting of Ti oxides has been investigated in a transmission electron microscope to obtain information on the valence state and site geometry of Ti.
Abstract: Titanium L 3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti2+, Ti3+, and Ti4+. Effects of polyhedra distortions are particularly observed for two oxides with mixed Ti3+-Ti4+ valence state, i.e., the Magneli phases Ti4O7 and Ti5O9. A prominent pre-peak in the Ti L 3 edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti L 3,2 ELNES spectra we propose a new quantification technique for the determination of Ti4+/Ti3+ ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K -edge is sensitive to the covalent bonding in all analyzed oxides.
271 citations
TL;DR: In this article, the ionization energy and electron affinity values of all primary tin and lead-based perovskites were determined using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis.
Abstract: Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.
270 citations
TL;DR: A multicomponent FeCoCrNi alloy with dynamically formed Ni4+ species to offer high catalytic activity via lattice oxygen activation mechanism to offer highly intrinsic activity at low applied potentials is reported.
Abstract: Anodic oxygen evolution reaction (OER) is recognized as kinetic bottleneck in water electrolysis. Transition metal sites with high valence states can accelerate the reaction kinetics to offer highly intrinsic activity, but suffer from thermodynamic formation barrier. Here, we show subtle engineering of highly oxidized Ni4+ species in surface reconstructed (oxy)hydroxides on multicomponent FeCoCrNi alloy film through interatomically electronic interplay. Our spectroscopic investigations with theoretical studies uncover that Fe component enables the formation of Ni4+ species, which is energetically favored by the multistep evolution of Ni2+→Ni3+→Ni4+. The dynamically constructed Ni4+ species drives holes into oxygen ligands to facilitate intramolecular oxygen coupling, triggering lattice oxygen activation to form Fe-Ni dual-sites as ultimate catalytic center with highly intrinsic activity. As a result, the surface reconstructed FeCoCrNi OER catalyst delivers outstanding mass activity and turnover frequency of 3601 A gmetal−1 and 0.483 s−1 at an overpotential of 300 mV in alkaline electrolyte, respectively. Electrocatalytic water oxidation is facilitated by high valence states, but these are challenging to achieve at low applied potentials. Here, authors report a multicomponent FeCoCrNi alloy with dynamically formed Ni4+ species to offer high catalytic activity via lattice oxygen activation mechanism.
270 citations