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Properties of Enantiomers and Optical Activity02:24

Properties of Enantiomers and Optical Activity

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It is essential to understand the difference between chiral and achiral interactions and the implications thereof in optical activity and their applications. Just as our feet, which are chiral, interact uniquely with chiral objects, such as a pair of shoes, but identically with achiral socks, enantiomers of a molecule exhibit different properties only when they interact with other chiral media. An example of a significant implication from this facet is the phenomenon known as optical activity,...
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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
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¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

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The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
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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 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.
Removing one hydrogen from the intervening CH2 group...
2.6K
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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Probing the Nonlinear Chiroptical Activity of Multiturn Aromatic Helices Using Linearly Polarized Light.

Rita Borges-Anastácio1,2, Eric Merlet2, Frédéric Adamietz1

  • 1Institut des Sciences Moléculaires, Université de Bordeaux, 351 cours de la Libération, Talence, 33400, France.

Angewandte Chemie (International Ed. in English)
|May 12, 2025
PubMed
Summary

Researchers studied quinoline-based oligoamide foldamers, finding their nonlinear optical (NLO) response depends on helical turns, not just length. This discovery offers new insights into chiral molecular materials.

Keywords:
ChiralityFoldamersHyper‐Rayleigh scatteringNonlinear opticsOptical activity

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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

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Area of Science:

  • Supramolecular Chemistry
  • Nonlinear Optics
  • Chiroptical Spectroscopy

Background:

  • Oligoamide foldamers offer tunable structural properties.
  • Helical chirality influences molecular properties.
  • Nonlinear optical (NLO) techniques probe molecular responses.

Purpose of the Study:

  • Synthesize quinoline-based oligoamide foldamers of varying lengths.
  • Evaluate their chiroptical nonlinear activity using hyper-Rayleigh scattering (HRS).
  • Investigate the relationship between foldamer structure and NLO response.

Main Methods:

  • Synthesis of four quinoline-based oligoamide foldamers (6–28 Å).
  • Analysis of enantiomeric forms using hyper-Rayleigh scattering (HRS).
  • Application of a developed nonlinear chiroptical model.

Main Results:

  • Foldamers exhibited 1.6 to 6 helical turns.
  • HRS successfully measured chiroptical nonlinear activity.
  • Nonlinear optical response showed a nonmonotonic trend with helix elongation, governed by the number of helical turns.

Conclusions:

  • The number of helical turns, not solely length, dictates the NLO response magnitude in these foldamers.
  • A periodic evolution of NLO activity was observed with increasing helical turns.
  • HRS is a valuable tool for studying solution-state chiroptical nonlinear properties.