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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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Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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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 Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

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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,...
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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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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.
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Frost Circles for Different Conjugated Systems01:18

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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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NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

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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.
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Odd-even effect in molecular electronic transport via an aromatic ring.

Tal Toledano1, Haim Sazan, Sabyasachi Mukhopadhyay

  • 1Department of Materials & Interfaces, Weizmann Institute of Science , Rehovot 76100, Israel.

Langmuir : the ACS Journal of Surfaces and Colloids
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Summary

Researchers discovered an odd-even effect in electrical properties of alkyl-phenyl molecule monolayers on silicon. Even-numbered methylene spacers resulted in better electrical conductivity compared to odd-numbered ones.

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Area of Science:

  • Surface Science
  • Organic Electronics
  • Molecular Electronics

Background:

  • Monolayers of organic molecules on semiconductor surfaces are crucial for electronic devices.
  • Controlling molecular orientation is key to tuning interfacial electronic properties.

Purpose of the Study:

  • To investigate the impact of alkyl chain length parity (odd-even effect) on the electrical properties of alkyl-phenyl monolayers on Si(111).
  • To understand the relationship between molecular packing, orientation, and charge transport.

Main Methods:

  • Adsorption of alkyl-phenyl monomers (n=2-5) onto Si-H surfaces.
  • Molecular dynamics simulations to predict molecular packing and orientation.
  • Experimental validation using contact angle, ellipsometry, FT-IR, and XPS.
  • Electrical conductivity measurements.

Main Results:

  • High-quality monolayers with 50-60% binding density were formed.
  • Odd-even alternation in molecular tilt and orientation was observed and matched simulations.
  • Molecules with even methylene spacers exhibited higher perpendicularity to the substrate.
  • Even-numbered monolayers showed significantly better electrical conductivity than odd-numbered ones.

Conclusions:

  • The odd-even effect in molecular orientation directly influences charge transport properties.
  • Alkyl chain parity offers a method to control electron transport barriers in organic interfaces.
  • Findings are relevant for designing and optimizing organic electronic devices.