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Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

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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,...
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Structure of Benzene: Molecular Orbital Model01:18

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

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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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...
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Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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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.
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Aromatic Hydrocarbon Anions: Structural Overview01:18

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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...
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π Molecular Orbitals of 1,3-Butadiene01:24

π Molecular Orbitals of 1,3-Butadiene

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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...
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Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry
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Five-Membered Rings Create Off-Zero Modes in Nanographene.

Peter H Jacobse1,2, Michael C Daugherty3, Kristia Ns Čerņevičs4

  • 1Department of Physics, University of California, Berkeley, California 94720, United States.

ACS Nano
|December 5, 2023
PubMed
Summary

Attaching five-membered rings to graphene nanoribbons creates off-zero modes, enabling electron retention and exploration of magnetic properties. This method allows studying nanoribbon magnetism on gold substrates.

Keywords:
electronic structurefive-membered ringsmagnetic ground statenanographenesopen shellscanning tunneling microscopyzero-modes

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Organic Chemistry

Background:

  • Nanographenes possess tunable electronic structures via zero-energy π-electron states (zero-modes).
  • Coupling zero-modes enables engineering of electronic and magnetic properties, but energy manipulation remains underexplored.

Purpose of the Study:

  • To investigate the energy perturbation of zero-modes by attaching five-membered rings.
  • To explore the potential of these modified states (off-zero modes) for retaining electron occupation and enabling magnetic property studies.

Main Methods:

  • Theoretical investigation of attaching five-membered rings to zigzag edges of nanographenes.
  • Utilizing cyclopentadienyl functionalization to create off-zero modes in 7-atom-wide armchair graphene nanoribbons (7-AGNRs).
  • Scanning tunneling microscopy (STM) experiments on 7-AGNRs physisorbed on Au(111) to probe electronic and magnetic properties.

Main Results:

  • Attaching a five-membered ring converts a zero-mode into an off-zero mode with electron-accepting character.
  • Off-zero modes in 7-AGNRs retain single-electron occupation on Au(111), unlike native end states.
  • Observed a decrease in magnetic coupling between off-zero mode end states with increasing 7-AGNR length.
  • Characterized the ground state evolution from closed-shell to open-shell configurations.

Conclusions:

  • Functionalization with five-membered rings offers a pathway to control the energy and electron occupation of nanographene states.
  • This approach facilitates the study of magnetic interactions in nanographene end states using STM on gold surfaces.
  • The findings provide insights into the length-dependent magnetic coupling and ground state properties of functionalized nanographenes.