Related Concept Videos
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
01:17Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
01:14π 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...
01:27π Electron Effects on Chemical Shift: Overview
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...
01:27¹H NMR: Long-Range Coupling
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
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Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds
Published on: October 18, 2018
Electronic excitations in long polyenes revisited.
Maximilian Schmidt1, Paul Tavan
1Lehrstuhl für BioMolekulare Optik, Ludwig-Maximilians Universität München, Oettingenstr. 67, 80538 München, Germany.
The Journal of Chemical Physics
|April 3, 2012
Summary
We computed excitation energies for polyenes using the OM2 model and multireference configuration interaction (MRCI). Results show OM2/MRCI agrees with PPP models but offers improved geometries for smaller polyenes.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Theoretical Spectroscopy
Background:
- Polyenes are fundamental organic molecules with delocalized π-electron systems.
- Accurate computation of their electronic excitations is crucial for understanding their optical properties.
- Previous models like Pariser-Parr-Pople (PPP) have limitations in describing polyene excitations.
Purpose of the Study:
- To calculate vertical excitation energies and transition dipole moments for low-energy singlet excitations in polyenes (4-22 π-electrons).
- To compare the performance of the OM2 model combined with multireference configuration interaction (MRCI) against established PPP models.
- To investigate the impact of optimized geometries and extended active spaces on excitation energies.
Main Methods:
- Application of the valence shell model OM2 coupled with multireference configuration interaction (MRCI).
- Computation of vertical excitation energies and transition dipole moments for singlet excitations.
- Inclusion of both π-electrons and σ-electrons in the configuration interaction active space for selected calculations.
Main Results:
- OM2/MRCI results closely align with PPP model predictions when using equivalent MRCI procedures and regular geometries.
- Optimized OM2/MRCI geometries provide improved descriptions, especially for smaller polyenes (N ≤ 12), revealing significant deviations from regular geometries.
- Inclusion of σ-electrons reduces excitation energies (up to 0.35 eV for ionic, 0.15 eV for covalent excitations) and shows particle-hole symmetry is only weakly broken in OM2.
Conclusions:
- OM2/MRCI offers a robust method for studying polyene excitations, comparable to PPP but with enhanced geometric descriptions.
- The method accurately predicts the low oscillator strengths for covalent 1B(u)(-) states, unlike ph-symmetric models.
- The 1B(u)(-) state's position shifts from third to second excited singlet state as polyene size increases (N ≥ 14).