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Related Concept Videos

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

2.0K
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...
2.0K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

4.3K
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
4.3K
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

4.3K
Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
4.3K
Electrophilic Aromatic Substitution: Overview01:16

Electrophilic Aromatic Substitution: Overview

16.6K
In an electrophilic aromatic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound.
16.6K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

6.7K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
6.7K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

14.9K
Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?  
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n +...
14.9K

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Updated: Mar 29, 2026

Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Aromatic Excimers: Ab Initio and TD-DFT Study.

Maciej Kołaski1, C R Arunkumar1, Kwang S Kim1

  • 1Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, 790-784 Pohang, Korea.

Journal of Chemical Theory and Computation
|November 22, 2015
PubMed
Summary
This summary is machine-generated.

Aromatic molecule excimers are vital in biology and sensing. Theoretical calculations reveal that stacked structures minimize energy for benzene to pyrene excimers in their excited state.

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Area of Science:

  • Computational chemistry
  • Structural biology
  • Photochemistry

Background:

  • Excited dimers (excimers) formed by aromatic molecules play a crucial role in biological systems and chemical sensing.
  • The arrangement of aromatic molecules in biological structures is often dictated by excimer formation.
  • Theoretical studies are essential for understanding the molecular arrangements in structural biology.

Purpose of the Study:

  • To investigate the electronic structure and stability of aromatic excimers.
  • To evaluate the accuracy of various theoretical methods, particularly time-dependent density functional theory (TD-DFT) with different functionals, for describing excimer formation.
  • To determine the minimum energy structures and properties of benzene, naphthalene, anthracene, and pyrene excimers.

Main Methods:

  • Extensive quantum chemical calculations were performed.
  • Methods included Equation of Motion Coupled-Cluster with Singles and Doubles (EOM-CCSD), Resolution of the Identity Coupled-Cluster with Singles and Doubles (RI-CC2), and Complete Active Space Second-Order Perturbation Theory (CASPT2).
  • Time-dependent Density Functional Theory (TD-DFT) was employed with B3LYP, PBE, PBE0, and ωPBEh functionals to assess its reliability.

Main Results:

  • Nearly parallel stacked configurations were identified as the minimum energy structures for all studied aromatic excimers.
  • The reliability of TD-DFT methods was evaluated against higher-level correlated methods for the benzene excimer.
  • Calculations were successfully extended from benzene to larger polycyclic aromatic hydrocarbons like naphthalene, anthracene, and pyrene.

Conclusions:

  • Stacked conformations represent the most stable structures for aromatic excimers in the first singlet excited state.
  • The study provides a basis for estimating layer-to-layer distances in bilayer aromatic systems.
  • The findings contribute to a better understanding of excimer formation in biological and chemical contexts.