Electronic structure and classification of electronic transitions in [2.2](3,5)- and [2.2](2,6)-metapyridinophanes

1986 ◽  
Vol 90 (17) ◽  
pp. 3964-3968 ◽  
Author(s):  
Antoni K. Wisor ◽  
Leszek Czuchajowski
Keyword(s):  
Electronic Structure ◽  
Electronic Transitions

scholarly journals Crystal-Field Mediated Electronic Transitions of EuS up to 35 GPa

2021 ◽  
Author(s):  
Virginia Monteseguro ◽  
Jose Antonio Barreda-Argüeso ◽  
Javier Ruiz-Fuertes ◽  
Angelika Rosa ◽  
Holger L. Meyerheim ◽  
...  
Keyword(s):  
Electronic Structure ◽  
Crystal Field ◽  
Local Structure ◽  
Electronic Transitions ◽  
Metallic State ◽  
X Ray ◽  
Half Metal ◽  
Consistent Model ◽  
X Ray Absorption ◽  
Pressure Evolution

Abstract An advanced experimental and theoretical model to explain the correlation between the electronic and local structure of Eu2+ in two different environments within a same compound, EuS, is presented. EuX monochalcogenides (X: O, S, Se, Te) exhibit anomalies in all their properties around 14 GPa with a semiconductor to metal transition. Although it is known that these changes are related to the 4f75d0 → 4f65d1 electronic transition, no consistent model of the pressure-induced modifications of the electronic structure currently exists. We show, by optical and x-ray absorption spectroscopy, and by ab initio calculations up to 35 GPa, that the pressure evolution of the crystal field plays a major role in triggering the observed electronic transitions from semiconductor to the half-metal and finally to the metallic state.

Colour centres in irradiated diamonds. I

1957 ◽  
Vol 241 (1227) ◽  
pp. 433-454 ◽  
Keyword(s):  
Absorption Spectrum ◽  
Electronic Structure ◽  
Theoretical Calculation ◽  
Energy Levels ◽  
Electronic Transitions ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
Colour Centres ◽  
Quantitative Results ◽  
Isolated Neutral

A theoretical calculation of the energy levels, and hence absorption spectrum, of an isolated vacancy in an otherwise perfect diamond lattice has been made. The concept of a defect molecule is introduced. This enables familiar molecular orbital theory to be applied in calculating the electronic structure of the defect. The quantitative results suggest that the observed band at 2·0 eV causing irradiated diamonds to appear blue, is due to spin and orbitally allowed electronic transitions of symmetry 1 E → 1 T 2 in the neighbourhood of isolated neutral vacancies.

scholarly journals Helical Electronic Transitions of Spiroconjugated Molecules

2021 ◽  
Author(s):  
Marc Hamilton Garner ◽  
Clemence Corminboeuf
Keyword(s):  
Optical Properties ◽  
Electronic Structure ◽  
Electron Density ◽  
Relative Orientation ◽  
Molecular Orbitals ◽  
Electronic Transitions ◽  
Linear Combinations ◽  
Electronic And Optical Properties ◽  
Symmetry Protected ◽  
Opposite Helicity

The two perpendicularly oriented π-systems of allene mix into helical molecular orbitals (MOs) when the symmetry of the molecule is reduced. However, the π-π<sup>∗</sup> transitions of allenes are linear combinations of two excitations that always consist of both helicities; consequently, the electronic transitions are not helical. Here, we examine the electronic structure of spiroconjugated molecules, which have the same parent symmetry as allene but with different relative orientation of the two π-systems. We show how the π-mixing in spiropentadiene is analogous to the helical π-mixing in allene. However, in spiroconjugated systems only half the π-MOs become helical. Due to this difference, the π-π<sup>∗</sup> transitions in substituted spiropentadiene come in near-degenerate pairs where the helicity is symmetry protected, and consequently there is no significant mixing between excitations involving MOs of opposite helicity. This inherent helicity of the π-π<sup>*</sup> transitions is verified by computation of the change of electron density. These transitions have big rotatory strengths where the sign correlates with the helicity of the transition. The electronic helicity of spiroconjugated molecules thus manifests itself in observable electronic and optical properties.

Correlation Between Optical Activity and the Helical Molecular Orbitals of Allene and Cumulenes

2020 ◽  
Author(s):  
Marc Hamilton Garner ◽  
Clemence Corminboeuf
Keyword(s):  
Electronic Structure ◽  
Optical Activity ◽  
Optical Rotation ◽  
Molecular Orbitals ◽  
Optically Active ◽  
Electronic Transitions ◽  
Frontier Molecular Orbitals ◽  
Chiral Molecules ◽  
Axial Chirality ◽  
Rotation Dispersion

<div><div><div><p>Helical frontier molecular orbitals (MOs) appear in disubstituted allenes and even-n cumulenes. Chiral molecules are optically active, but while these molecules are single-handed chiral, π-orbitals of both helicities are present. Here we computationally examine whether the optical activity of chiral cumulenes is controlled by the axial chirality or the helicity of the electronic structure. We exploit hyperconjugation with alkyl, silaalkyl, and germaalkyl substituents to adjust the MO helicity without altering the axial chirality. For the same axial chirality, we observe an inversion of the helical MOs contribution to the electronic transitions and a change of sign in the electronic circular dichroism and optical rotation dispersion spectra. While the magnitude of the chiroptical response also increases, it is similar to that of chiral cumulenes without helical π-orbitals. Overall, Helical π-orbitals correlate with the big chiroptical response in cumulenes, but are not a prerequisite for it.</p></div></div></div>

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