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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...

Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...

Five-Membered Heterocyclic Aromatic Compounds: Overview

Five-Membered Heterocyclic Aromatic Compounds: Overview

Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom, respectively.

NMR Spectroscopy of Aromatic Compounds

NMR Spectroscopy of Aromatic Compounds

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. Consider...

Aromatic Hydrocarbon Anions: Structural Overview

Aromatic Hydrocarbon Anions: Structural Overview

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 overlap of p...

π Electron Effects on Chemical Shift: Overview

π 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...

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Updated: May 13, 2026

Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

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Tunable molecular plasmons in polycyclic aromatic hydrocarbons.

Alejandro Manjavacas¹, Federico Marchesin, Sukosin Thongrattanasiri

¹IQFR-CSIC, Serrano 119, 28006 Madrid, Spain.

|March 15, 2013

Summary

Chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit tunable molecular plasmons sensitive to charge state. These graphene-like molecules enable new plasmonic devices for advanced optical applications.

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Last Updated: May 13, 2026

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Area of Science:

* Nanoscience and Nanotechnology
* Molecular Plasmonics
* Computational Chemistry

Background:

* Polycyclic Aromatic Hydrocarbons (PAHs) are chemically synthesized molecules.
* Molecular plasmons are collective electron oscillations in molecules.
* Graphene exhibits unique electronic and plasmonic properties.

Purpose of the Study:

* To investigate molecular plasmon resonances in chemically synthesized PAHs.
* To explore the sensitivity of these plasmons to molecular charge state and edge structure.
* To enable the development of novel molecular plasmonic devices.

Main Methods:

* First-principles time-dependent density-functional theory (TDDFT) calculations.
* Tight-binding (TB) approach for larger systems.
* Analysis of molecular charge state and atomic structure effects.

Main Results:

* PAHs exhibit molecular plasmon resonances sensitive to net charge and edge structure.
* A single electron addition/removal switches molecular plasmons on/off.
* Strong plasmon interactions observed between neighboring molecules.

Conclusions:

* Chemically synthesized PAHs act as tunable, imperfection-free plasmonic nanostructures.
* Findings enable molecular-based plasmonic devices for electro-optical modulation, switching, and sensing.
* Potential for single-electron detection and advanced molecular designs.