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Related Concept Videos

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

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...
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

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.
Aromatic Hydrocarbon Anions: Structural Overview01:18

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 Systems01:18

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.
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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.
Basicity of Heterocyclic Aromatic Amines01:25

Basicity of Heterocyclic Aromatic Amines

Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).

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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
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Published on: August 22, 2018

Do N-heterocyclic aromatic rings prefer π-stacking?

Mridula Guin1, G Naresh Patwari, S Karthikeyan

  • 1Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, India.

Physical Chemistry Chemical Physics : PCCP
|March 2, 2011
PubMed
Summary

Phenylacetylene complexes with N-heterocycles primarily interact via π-stacking with the phenyl ring, not the C-H bond. This study reveals π-stacked complexes are favored, with a C-H...N bond also observed in pyridine interactions.

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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of &#945;-Imino &#947;-Lactones and Alkylidene Pyrazolones
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Published on: February 7, 2019

Area of Science:

  • Physical Chemistry
  • Molecular Spectroscopy
  • Computational Chemistry

Background:

  • Understanding intermolecular interactions is crucial in chemistry.
  • Aromatic systems and N-heterocycles exhibit diverse binding modes.
  • Phenylacetylene serves as a model system for studying C-H bond interactions.

Purpose of the Study:

  • To investigate the complexation of phenylacetylene with triazine, pyrazine, and pyridine.
  • To elucidate the dominant intermolecular interactions in these complexes.
  • To determine the structural and energetic preferences of these complexes.

Main Methods:

  • Infrared-Ultraviolet (IR-UV) double resonance spectroscopy was employed.
  • Quantum chemical calculations, including M06-2X/aug-cc-pVDZ and MP2/aug-cc-pVDZ, were utilized for geometry optimization.
  • Coupled-Cluster Single, Double, and Triple excitations (CCSD(T)) calculations at the Complete Basis Set (CBS) limit were performed for high-accuracy energy evaluation.

Main Results:

  • IR-UV spectra showed minimal perturbation of the acetylenic C-H group in phenylacetylene complexes.
  • The predominant interaction observed was between the π electron density of the phenyl ring and the N-heterocycles.
  • Computational analysis confirmed the formation of π-stacked complexes for all three N-heterocycles.
  • A specific C-H...N hydrogen-bonded complex was identified between pyridine and phenylacetylene.

Conclusions:

  • N-heterocyclic aromatic rings preferentially form π-stacked complexes with phenylacetylene.
  • The acetylenic C-H bond is less involved in direct interactions compared to the phenyl ring's π system.
  • The study highlights the role of π-π interactions in stabilizing complexes involving aromatic systems and N-heterocycles.