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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
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Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is confirmed through isotopic...
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Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n + 2 rule.
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

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Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

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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, respectively.
Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)01:30

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Nucleophilic substitution in aromatic compounds is feasible in substrates bearing strong electron-withdrawing substituents positioned ortho or para to the leaving group. The reaction proceeds via two steps: the addition of the nucleophile and the elimination of the leaving group.
The reaction begins with an attack of the nucleophile on the carbon that holds the leaving group. This results in the delocalization of the π electrons over the ring carbons. The resonance interaction between the...

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Stacking and spreading interaction in N-heteroaromatic systems.

Brijesh Kumar Mishra1, J Samuel Arey, N Sathyamurthy

  • 1Environmental Chemistry Modeling Laboratory, EPFL, 1015 Lausanne, Switzerland.

The Journal of Physical Chemistry. A
|February 20, 2010
PubMed
Summary
This summary is machine-generated.

This study quantifies pi-pi interactions in N-heteroaromatic systems, finding stacked dimers are more stable than benzene dimers. These interactions partly explain DNA and RNA double helix stability.

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Area of Science:

  • Computational chemistry
  • Molecular interactions
  • Biophysics

Background:

  • Pi-pi interactions are crucial in biological systems, particularly in nucleic acid structures.
  • Understanding these interactions in N-heteroaromatic systems provides insights into molecular recognition and stability.

Purpose of the Study:

  • To compute and compare stabilization energies of stacked and hydrogen-bonded N-heteroaromatic dimers.
  • To investigate the factors governing preferred conformations in these systems.
  • To relate findings to the stability of DNA and RNA double helices.

Main Methods:

  • Coupled cluster through the perturbative triples (CCSD(T)) method at the estimated complete basis set (CBS) limit.
  • High-level computational chemistry techniques to model molecular interactions.
  • Analysis of stacked and hydrogen-bonded dimer and trimer geometries and energies.

Main Results:

  • N-heteroaromatic dimers exhibit greater stabilization energies from pi-pi stacking compared to benzene dimers.
  • Stabilization energies for stacked and spread-out (hydrogen-bonded) complexes are comparable.
  • Weak C-H···N interactions stabilize hydrogen-bonded dimers and trimers.

Conclusions:

  • Pi-pi interactions in N-heteroaromatic systems are significant contributors to molecular stability.
  • The findings provide a partial explanation for the structural integrity of DNA and RNA double helices.
  • Computational modeling offers valuable insights into fundamental biological interactions.