scholarly journals Helical Electronic Transitions of Spiroconjugated Molecules

2021 ◽  
Author(s):  
Marc Hamilton Garner ◽  
Clemence Corminboeuf
Keyword(s):  
Relative Orientation ◽  
Molecular Orbitals ◽  
Electronic Transitions ◽  
Chiroptical Properties ◽  
Pi Systems ◽  
Pi Conjugation ◽  
Symmetry Protected

<div><div><div><div><p>The two pi-systems of allene can mix into helical molecular orbitals (MOs), yet the helicity is lost in the pi-pi∗ transitions. In spiroconjugated molecules the relative orientation of the two π- systems is different as only half the pi-MOs become helical. Consequently, the helicity of the electronic transitions is symmetry protected and thus helical pi-conjugation can manifest in observable electronic and chiroptical properties.</p></div></div></div></div>

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.

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.

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.

Photoelectron spectra of halogenated ethylenes

1970 ◽  
Vol 315 (1522) ◽  
pp. 323-338 ◽  
Keyword(s):  
Bond Length ◽  
Vibrational Structure ◽  
Molecular Orbitals ◽  
Ionization Potentials ◽  
Photoelectron Spectra ◽  
Electronic Transitions ◽  
Parent Molecule ◽  
Atomic Orbitals

Photoelectron spectra of chloro-, fluoro- and chlorofluoroethylenes have been measured. Ionization potentials in the range 6 to 21 eV have been determined, and vibrational structure associated with many of the electronic transitions has been interpreted in terms of vibrations of the ion and correlated with those of the parent molecule. The various ionization potentials have been associated with specific orbitals such as the C=C π orbital, and molecular orbitals derived from the p atomic orbitals of halogens. Variations in these ionization potentials have been discussed in relation to inductive and conjugative effects of the halogen substituents. Some observed differences between the spectra of the chloro- and fluoroethylenes have been considered in relation to possible mechanisms of the changes in bond length consequent upon ionization.

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>

The chiroptical properties of the strychnine alkaloids: strychnine, β-colubrine, brucine, and their dihydro derivatives

1978 ◽  
Vol 56 (9) ◽  
pp. 1222-1230 ◽  
Author(s):  
Julian W. Show ◽  
Thomas M. Hooker Jr.
Keyword(s):  
Circular Dichroism ◽  
Spectroscopic Properties ◽  
Ultraviolet Absorption ◽  
Electronic Transitions ◽  
Ultraviolet Region ◽  
Circular Dichroism Spectra ◽  
Experimental Conditions ◽  
Chiroptical Properties ◽  
Circular Dichroïsm

Ultraviolet absorption and circular dichroism spectra of the strychnine alkaloids strychnine, β-colubrine, brucine, dihydrostrychnine, dihydro-β-colubrine, and dihydrobrucine have been investigated under various experimental conditions. It appears that at least five electronic transitions are required to account for the spectroscopic properties of these molecules in the ultraviolet region above 190 nm. Transitions, which are tentatively assigned as the 1Lb, 1La, 1Bb, and 1Ba in the notation due to Platt, and an n → π* transition apparently make significant contributions to the spectra of ail of the molecules studied.

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>

Electronic structure and spectroscopy of homoleptic compounds of dimolybdenum using TDDFT

2016 ◽  
Vol 94 (1) ◽  
pp. 20-27
Author(s):  
Pere Vilarrubias
Keyword(s):  
Experimental Data ◽  
Density Functional Theory ◽  
Electronic Structure ◽  
Density Functional ◽  
Molecular Orbitals ◽  
Time Dependent ◽  
Electronic Transitions ◽  
Functional Theory ◽  
Dependent Density ◽  
Good Agreement

Ten compounds of dimolybdenum are studied using density functional theory and time-dependent density functional theory. The energy of the strongest symmetry-allowed bands is calculated. The results are then compared with experimental data, when available. The PW91 functional gives results for geometry and for the energy of the δ→δ* band that show good agreement with experimental data. However, the B3LYP functional gives more realistic values for the whole spectrum when the results are compared with experimental data. Finally, the different values of energy of these bands are explained analyzing the molecular orbitals involved in these transitions. Some ligands can act as an unsaturated system in conjugation with the delta bond, modifying the energies of the electronic transitions.

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