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Related Concept Videos

¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

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An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
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¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

1.7K
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...
1.7K
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

1.4K
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
1.4K
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

1.1K
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
1.1K
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

617
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
617

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Rift Gaps in Chemical-Wave Network Systems.

Tamar Kanimian1, Lara Haroun1, Rabih Sultan1

  • 1American University of Beirut, Department of Chemistry, P.O. Box 11-0236, 1107 2020 Riad El Solh, Beirut, Lebanon.

Physical Review Letters
|November 1, 2024
PubMed
Summary

In diffusion-precipitation systems with multiple sources, fronts stop, forming gaps. Fractal metal deposits also meander and leave gaps, mimicking chemical Voronoi diagrams.

Area of Science:

  • Physical Chemistry
  • Materials Science
  • Chemical Physics

Background:

  • Diffusion-limited aggregation and precipitation phenomena are fundamental in various natural and synthetic processes.
  • Understanding pattern formation in systems with multiple interacting sources is crucial for controlling material properties.

Purpose of the Study:

  • To investigate pattern formation in diffusion-precipitation systems with multiple sources.
  • To synthesize and analyze fractal metal deposits from multiple reduction centers.
  • To establish a connection between these disparate systems and Voronoi diagrams.

Main Methods:

  • Experimental diffusion-precipitation with multiple diffusion sources.
  • Synthesis of fractal metal deposits from multiple reduction centers.

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  • Analysis of precipitation fronts using entropy and velocity measurements.
  • Characterization of fractal deposits using fractal dimension calculations.

    • Main Results:

    • Precipitation fronts from multiple sources halted, creating distinct rifts or gaps.
    • Synthesized fractal metal deposits meandered but did not overlap, also forming gaps.
    • Quantitative analysis revealed similarities between the two systems.

    Conclusions:

    • The observed structures in both diffusion-precipitation and fractal metal deposition systems are chemical analogs of Voronoi diagrams.
    • The formation of gaps is a common feature driven by diffusion and source interactions.
    • This study provides insights into pattern formation and spatial partitioning in complex chemical systems.