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Basicity of Heterocyclic Aromatic Amines01:25

Basicity of Heterocyclic Aromatic Amines

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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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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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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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IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations01:08

IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations

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Identical bonds within a polyatomic group can stretch symmetrically (in-phase) or asymmetrically (out-of-phase). Similar to hydrogen bonding, these vibrations also influence the shape of the IR peak. Generally, asymmetric stretching frequencies are higher than symmetric stretching frequencies. For example, primary amines exhibit two distinct IR peaks between 3300–3500 cm−1 corresponding to the symmetric and asymmetric N-H stretching, while secondary amines exhibit a single...
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NMR Spectroscopy of Benzene Derivatives01:34

NMR Spectroscopy of Benzene Derivatives

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Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling...
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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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Related Experiment Video

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On-line Analysis of Nitrogen Containing Compounds in Complex Hydrocarbon Matrixes
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Quantitative characterisation of the ring normal modes. Pyridine as a study case.

E Benassi1, H Fan2

  • 1School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China.

Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy
|October 18, 2020
PubMed
Summary

This study quantitatively characterizes pyridine

Keywords:
AnharmonicityDynamic natural bond orbital analysisRing related normal modes

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Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate
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Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate
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Area of Science:

  • Computational chemistry
  • Molecular spectroscopy

Background:

  • Pyridine's vibrational normal modes (NM) are crucial for understanding its molecular behavior.
  • Previous analyses may lack detailed correlation between vibrational modes and electronic structure.

Purpose of the Study:

  • To provide a quantitative characterization of pyridine's ring-related vibrational normal modes (RNMs).
  • To explore the dynamic correlation between RNMs and pyridine's electronic structure.

Main Methods:

  • Quantum Chemical calculations were employed.
  • Vibrational normal modes were decomposed into internal coordinates.
  • Electronic structure was analyzed using frontier Molecular Orbitals (MO), Molecular Electrostatic Potential (MEP) surfaces, and Natural Bond Orbital (NBO) analysis.
  • Vibrational modes were scanned dynamically.

Main Results:

  • Detailed analysis of 7 in-plane ring motions among the 27 total vibrational normal modes.
  • Dynamic mapping of electronic structure changes during specific vibrations.
  • Quantitative data linking specific RNMs to electronic properties.

Conclusions:

  • The study offers a comprehensive, dynamic, and quantitative understanding of pyridine's RNMs.
  • This detailed characterization enhances the comprehension of pyridine's vibrational and electronic properties.