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Valence Bond Theory

Valence Bond Theory

Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...

Valence Bond Theory

Valence Bond Theory

Overview of Valence Bond Theory

Valence Bond Theory and Hybridized Orbitals

Valence Bond Theory and Hybridized Orbitals

According to valence bond theory, a covalent bond results when: (1) an orbital on one atom overlaps an orbital on a second atom, and (2) the single electrons in each orbital combine to form an electron pair. The strength of a covalent bond depends on the extent of overlap of the orbitals involved. Maximum overlap is possible when the orbitals overlap on a direct line between the two nuclei.
A σ bond (single bond in a Lewis structure) is a covalent bond in which the electron density is...

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...

π Electron Effects on Chemical Shift: Overview

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An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...

Transition State Theory

Transition State Theory

Transition-state theory, also known as activated-complex theory, provides a molecular-level explanation of reaction rates in both gas-phase and solution-phase reactions. It extends earlier kinetic models by considering the formation of a short-lived, high-energy configuration during a reaction.The progress of a chemical reaction can be represented using a reaction profile, which plots potential energy against the reaction coordinate. As two reactant molecules approach one another, their...

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Creating Virtual-hand and Virtual-face Illusions to Investigate Self-representation

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Optical study of archetypical valence-fluctuating Eu systems.

V Guritanu¹, S Seiro, J Sichelschmidt

¹Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany.

Physical Review Letters

|February 2, 2013

Summary

Optical conductivity studies reveal that europium (Eu) valence-fluctuating systems share similarities with heavy-fermion metals. This suggests crucial hybridization effects in Eu compounds, similar to cerium (Ce) and ytterbium (Yb) systems.

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Area of Science:

Condensed Matter Physics
Materials Science
Spectroscopy

Background:

Valence-fluctuating systems exhibit unique electronic properties due to the mixed occupancy of f-electron shells.
Europium (Eu) compounds are known for their valence-fluctuating behavior, but their electronic properties are less understood compared to Cerium (Ce) or Ytterbium (Yb) analogs.
Understanding these systems is key to developing novel electronic materials.

Purpose of the Study:

To investigate the optical conductivity of EuIr(2)Si(2) and EuNi(2)P(2) in the infrared range.
To gain insights into the electronic properties of valence-fluctuating systems.
To compare the electronic behavior of Eu-based systems with Ce- and Yb-based heavy-fermion and intermediate valence systems.

Main Methods:

Optical conductivity measurements were performed in the infrared energy range.
Analysis focused on spectral features such as Drude response, conductivity suppression, and mid-infrared peaks.
Comparative analysis with existing optical spectra of Ce- and Yb-based systems.

Main Results:

Observation of a renormalized Drude response upon cooling for both EuIr(2)Si(2) and EuNi(2)P(2).
Partial suppression of optical conductivity below 100 meV and emergence of mid-infrared peaks (0.15 eV for EuIr(2)Si(2), 0.13 eV for EuNi(2)P(2)).
Strong similarities found between the optical spectra of these Eu systems and Ce/Yb-based heavy-fermion/intermediate valence systems.

Conclusions:

The electronic properties of valence-fluctuating Eu systems show remarkable parallels with Ce- and Yb-based heavy-fermion metals.
Hybridization between 4f and conduction electrons appears to be a critical factor governing the behavior of these Eu systems, analogous to its role in Ce and Yb systems.
Despite differing phase diagrams and valence temperature dependencies, the underlying electronic mechanisms share common ground.