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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...
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...
Stereoisomerism02:52

Stereoisomerism

Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
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...
Structure of Amines01:19

Structure of Amines

The hybridized nitrogen atom in amines possesses a lone pair of electrons and is bound to three substituents with a bond angle of around 108°, which is less than the tetrahedral angle of 109.5°. However, the C–N–H bond angle is slightly larger at 112°, with a carbon–nitrogen bond length of 147 pm. This carbon–nitrogen bond length of of amines is longer than the carbon–oxygen bond of alcohols (143 pm) but shorter than alkanes’ carbon–carbon bond (154 pm). These aspects are illustrated in Figure...
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.

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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method

Published on: July 19, 2019

Desmotropy, polymorphism, and solid-state proton transfer: four solid forms of an aromatic o-hydroxy Schiff base.

Mirta Rubčić1, Krunoslav Užarević, Ivan Halasz

  • 1Department of Chemistry, Faculty of Science, University of Zagreb, Croatia. mirta@chem.pmf.hr

Chemistry (Weinheim an Der Bergstrasse, Germany)
|March 27, 2012
PubMed
Summary

This study explores four solid forms of a Schiff base, revealing how proton position influences molecular shape and crystal structure. Metastable forms convert to the most stable one, changing color, especially with methanol vapor.

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Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates
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Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates

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Solid-phase Synthesis of [4.4] Spirocyclic Oximes
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Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
05:51

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Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates
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Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates

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Solid-phase Synthesis of [4.4] Spirocyclic Oximes
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Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

Area of Science:

  • Solid-state chemistry
  • Crystallography
  • Spectroscopy

Background:

  • Schiff bases exhibit diverse solid forms.
  • Tautomerism plays a crucial role in molecular conformation and crystal packing.

Purpose of the Study:

  • To investigate the polymorphic behavior of a Schiff base derived from salicylaldehyde and 2-amino-3-hydroxypyridine.
  • To understand the relationship between proton position, molecular conformation, and supramolecular architecture.
  • To establish conditions for the formation and interconversion of different solid phases.

Main Methods:

  • Infrared (IR) spectroscopy
  • X-ray crystallography
  • (13)C cross-polarization/magnetic angle spinning (CP/MAS) NMR spectroscopy
  • Solution and solvent-free experiments
  • Computational studies
  • Temperature-resolved powder X-ray diffraction
  • UV/Vis spectroscopy

Main Results:

  • Four desmotropic solid forms (two polymorphic pairs: enol-imino D1a/D1b and keto-amino D2a/D2b) were identified.
  • Planar molecular conformations with strong intramolecular hydrogen bonds (O-H···N or N-H···O) were observed.
  • Proton migration within the hydrogen bond system dictates distinct molecular conformations and supramolecular arrangements.
  • Metastable solid phases convert to the most stable form (D1a) under specific conditions (methanol vapor, heating, mechanical treatment), accompanied by color changes.
  • All forms rapidly equilibrate in solution, favoring the enol tautomer.

Conclusions:

  • Peripheral groups significantly stabilize metastable tautomers in the solid state.
  • Proton tautomerism is a key factor driving polymorphism and structural diversity in Schiff bases.
  • Understanding these transformations is crucial for controlling solid-state properties and material design.