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Videos de Conceptos Relacionados

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.
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
Magnetostatic Boundary Conditions01:28

Magnetostatic Boundary Conditions

An electric field suffers a discontinuity at a surface charge. Similarly, a magnetic field is discontinuous at a surface current. The perpendicular component of a magnetic field is continuous across the interface of two magnetic mediums. In contrast, its parallel component, perpendicular to the current, is discontinuous by the amount equal to the product of the vacuum permeability and the surface current. Like the scalar potential in electrostatics, the vector potential is also continuous...
Isothermal Processes01:21

Isothermal Processes

A thermodynamic process that occurs at constant temperature is called an isothermal process. Heat slowly flows into the system or out of the system to maintain thermal equilibrium. Processes involving phase changes like water evaporation into steam or freezing water into ice at a constant temperature are examples of Isothermal Processes.
An ideal gas can also undergo isothermal expansion or compression.
For example, consider 1 mole of an ideal gas inside an isolated cylinder at initial volume V...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...
Magnetic Susceptibility and Permeability01:31

Magnetic Susceptibility and Permeability

In linear magnetic materials, like paramagnets and diamagnets, magnetization is proportional to the magnetic field intensity. The constant of proportionality, a dimensionless number, is called magnetic susceptibility. The value of the susceptibility depends on the type of material.
When diamagnetic materials are placed under an external magnetic field, the moments opposite to the field are induced. Hence, the susceptibility for diamagnets has a minimal negative value of 10-5–10-6. Since...

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Video Experimental Relacionado

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Sediment Core Extrusion Method at Millimeter Resolution Using a Calibrated, Threaded-rod
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Published on: August 17, 2016

La evidencia isotópica acoplada 142Nd-143Nd para la dinámica del manto de Hadeo.

Vickie C Bennett1, Alan D Brandon, Allen P Nutman

  • 1Research School of Earth Sciences, Australian National University, Canberra ACT, 0200 Australia. vickie.bennett@anu.edu.au

Science (New York, N.Y.)
|December 22, 2007
PubMed
Resumen

Las antiguas rocas de Groenlandia revelan la Tierra primitiva.

Área de la Ciencia:

  • Geoquímica y Geología Isotópica.
  • Historia de la Tierra Temprana y Ciencias Planetarias.

Sus antecedentes:

  • Comprender la diferenciación química inicial de la Tierra es crucial para descifrar la formación de los planetas.
  • Los sistemas isotópicos proporcionan herramientas poderosas para rastrear los primeros procesos geológicos.
  • El sistema isotópico del neodimio (Nd), particularmente el 142Nd y el 143Nd, ofrece información sobre la evolución temprana del manto.

Objetivo del estudio:

  • Para fechar directamente la formación de depósitos de silicato químicamente distintos en la Tierra temprana.
  • Investigar la escala y persistencia de las heterogeneidades tempranas del manto.
  • Para rastrear la posterior evolución y remezcla de estos primeros dominios del manto.

Principales métodos:

  • Análisis de excesos acoplados de neodimio-142 (142Nd) y neodimio-143 (143Nd) en rocas de 3.850 millones de años de antigüedad del suroeste de Groenlandia.

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  • Comparación de firmas de 142Nd en rocas antiguas contemporáneas de Groenlandia y Australia Occidental (>3.600 millones de años de antigüedad).
  • Utilizando la desintegración del extinto samario-146 (146Sm) y el de larga vida samario-147 (147Sm) para datar la formación temprana de reservas de silicato.

    • Principales resultados:

    • Los datos isotópicos del neodimio datan directamente la formación de depósitos de silicato químicamente distintos dentro de los primeros 30-75 millones de años de la historia de la Tierra.
    • Las diferencias en las firmas de 142Nd entre las rocas de Groenlandia y Australia Occidental revelan heterogeneidades del manto a gran escala presentes muy temprano en la historia de la Tierra.
    • Estas heterogeneidades persistieron durante al menos los primeros mil millones de años, con variaciones temporales de 142Nd que indican una remezcla incompleta de dominios del manto temprano.

    Conclusiones:

    • El manto de la Tierra era químicamente heterogéneo desde sus primeras etapas, con distintos depósitos de silicato formándose rápidamente.
    • Las firmas isotópicas conservadas en rocas antiguas proporcionan un registro directo de estos primeros procesos de diferenciación.
    • La remezcla incompleta de estos primeros dominios influyó en la posterior evolución geoquímica del manto.