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Atomic Nuclei: Nuclear Relaxation Processes01:23

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Atomic Nuclei: Nuclear Spin State Overview01:03

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NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of one, the...
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Atomic Nuclei: Nuclear Spin State Population Distribution01:14

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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Atomic Nuclei: Nuclear Magnetic Moment00:59

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All atomic nuclei are positively charged. When they have a nonzero spin, they behave like rotating charges. As a consequence of their charge and spin, these nuclei generate a magnetic field (B). This, in turn, gives rise to a magnetic moment (μ), which is randomly oriented in the absence of an external magnetic field. When an external magnetic field (B0) is applied, the magnetic moment vectors can align with the field or against it in 2 + 1 orientations. A hydrogen nucleus, which is just a...
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Irradiation of a spin-active nucleus causes an increase or decrease in the signal intensity of neighboring nuclei that are not necessarily chemically bonded or involved in J-coupling. This phenomenon, called the nuclear Overhauser enhancement (NOE), results from through-space interactions between the nuclear spins. The NOE effect decreases with increasing internuclear distance and is generally not observed beyond 4 angstroms. In NOE, dipole-dipole interactions between neighboring spin-active...
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Atomic Nuclei: Magnetic Resonance01:05

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The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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La eficiencia de la polarización nuclear dinámica aumentada por el giro de ángulo mágico muy rápido

Sachin R Chaudhari1, Dorothea Wisser1, Arthur C Pinon2

  • 1Institut de Sciences Analytiques, Centre de RMN à Très Hauts Champs, Université de Lyon (CNRS/ENS Lyon/UCB Lyon 1), 69100 Villeurbanne, France.

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|July 11, 2017
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Resumen

La polarización nuclear dinámica (DNP) ahora logra una alta sensibilidad (> 100) en campos magnéticos altos (18,8 T) al aumentar las tasas de giro de ángulo mágico (MAS). Este avance mejora la espectroscopia de RMN en estado sólido para el análisis de materiales.

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Área de la Ciencia:

  • Espectroscopia de resonancia magnética nuclear (RMN) en estado sólido
  • Ciencias de los materiales
  • Química Física

Sus antecedentes:

  • La polarización nuclear dinámica (DNP) mejora significativamente la sensibilidad a la RMN en estado sólido.
  • Las mejoras altas de DNP (> 100) suelen limitarse a campos magnéticos más bajos (< 9,4 T).
  • La eficiencia de DNP disminuye sustancialmente en campos magnéticos más altos.

Objetivo del estudio:

  • Para lograr mejoras de alta polarización nuclear dinámica (DNP) en campos magnéticos altos (18,8 T).
  • Investigar la relación entre la eficiencia del DNP y las tasas de giro de ángulo mágico (MAS) en campos altos.
  • Demostrar la utilidad del DNP de alto campo para el análisis de materiales difíciles.

Principales métodos:

  • Los experimentos con DNP del efecto Overhauser en estado sólido se llevaron a cabo a 18,8 T.
  • Las mediciones utilizaron 1,3-bisdifenileno-2-fenilallil disuelto en o-terfenilo.
  • Los experimentos incluyeron giro de ángulo mágico (MAS) a velocidades de hasta 40 kHz.
  • Se desarrolló un modelo de difusión fuente-sumidero para explicar la transferencia de polarización.

Principales resultados:

  • El efecto Overhauser en estado sólido alcanzó mejoras de DNP superiores a 100 a 18,8 T.
  • Se observó un rápido aumento en la mejora de la DNP con el aumento de las tasas de MAS.
  • Se aplicó con éxito el método a la alúmina mesoporosa.
  • Se obtienen espectros de polarización cruzada de 27Al mejorados con DNP de buena resolución.

Conclusiones:

  • El DNP de alto campo magnético (>100 potenciación a 18,8 T) es factible y eficiente.
  • La velocidad de giro del ángulo mágico es un parámetro crítico para optimizar el DNP de alto campo.
  • El modelo desarrollado de difusión fuente-sumidero explica con precisión los mecanismos de transferencia de polarización.
  • Este enfoque avanza significativamente en la RMN en estado sólido para la caracterización de materiales.