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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 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...

Frost Circles for Different Conjugated Systems

Frost Circles for Different Conjugated Systems

The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.

Structure of Benzene: Molecular Orbital Model

Structure of Benzene: Molecular Orbital Model

According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).

π Molecular Orbitals of 1,3-Butadiene

π Molecular Orbitals of 1,3-Butadiene

Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the number...

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...

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Assessment of Boron Doped Diamond Electrode Quality and Application to In Situ Modification of Local pH by Water Electrolysis

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Particle on a boron disk: ring currents and disk aromaticity in B20(2-).

Truong Ba Tai¹, Remco W A Havenith, Jos L Teunissen

¹Department of Chemistry, University of Leuven , B-3001 Leuven, Belgium.

Inorganic Chemistry

|September 10, 2013

Summary

The B20(2-) cluster displays disk aromaticity due to its electronic structure, analogous to quantum mechanical waves. This boron cluster exhibits unique magnetic properties with both antiaromaticity and aromaticity in different electron channels.

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Area of Science:

Inorganic Chemistry
Theoretical Chemistry
Materials Science

Background:

Boron clusters are known for their unique electronic structures and bonding.
Aromaticity in clusters is a key concept for understanding their stability and properties.
The B20(2-) cluster presents a novel system for exploring electronic delocalization.

Purpose of the Study:

To investigate the electronic structure and bonding of the B20(2-) cluster.
To determine if B20(2-) exhibits aromaticity and understand its magnetic properties.
To establish an analogy between the electronic behavior of B20(2-) and quantum mechanical disk waves.

Main Methods:

Computational chemistry methods were used to model the B20(2-) cluster.
Orbital plots and energy correlations were analyzed to understand electronic structure.
Nucleus Independent Chemical Shift (NICS) values and current density calculations were performed to assess magnetic response.

Main Results:

The B20(2-) cluster exhibits a planar, circular, sheet-like structure.
Its electronic structure closely resembles Bessel functions of a quantum mechanical particle in a disk.
The cluster shows 12 π-electrons, indicating "disk aromaticity," and possesses distinct σ and π electronic channels with unique magnetic responses.

Conclusions:

B20(2-) demonstrates a novel form of "disk aromaticity" analogous to quantum mechanical waves.
The cluster exhibits a complex magnetic response with inner σ-channel antiaromaticity and outer π-channel aromaticity.
This study provides insights into the electronic behavior and potential stability of novel boron clusters.