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相关概念视频

Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

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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

Atomic Nuclei: Nuclear Spin State Population Distribution

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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

Atomic Nuclei: Nuclear Magnetic Moment

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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

Atomic Nuclei: Magnetic Resonance

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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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动态核极化效率通过非常快速的魔法角旋转增加

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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概括

动态核极化 (DNP) 现在通过增加神奇角度旋转 (MAS) 速率,在高磁场 (18.8 T) 中实现高灵敏度 (>100). 这一突破增强了材料分析的固态核磁共振光谱.

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科学领域:

  • 固态核磁共振 (NMR) 光谱
  • 材料科学
  • 物理化学

背景情况:

  • 动态核极化 (DNP) 显著提高了固态NMR的灵敏度.
  • 高DNP增强 (>100) 通常仅限于较低的磁场 (<9.4 T).
  • 在较高的磁场下,DNP的效率大大降低.

研究的目的:

  • 在高磁场 (18.8 T) 中实现高动态核极化 (DNP) 增强.
  • 研究DNP效率与高电场中的魔角旋转 (MAS) 速率之间的关系.
  • 证明高场DNP对于分析具有挑战性的材料的实用性.

主要方法:

  • 固态Overhauser效应的DNP实验是在18.8T进行的.
  • 使用在o-terphenyl中溶解的1,3-bisdiphenylene-2-phenylallyl进行测量.
  • 实验涉及到高达40kHz的磁角度旋转 (MAS).
  • 一个源-沉扩散模型被开发来解释极化转移.

主要成果:

  • 在18.8 T时实现了超过100的固态Overhauser效应DNP增强.
  • 随着MAS的增加,DNP增强的速度迅速增加.
  • 成功地将该方法应用于中孔.
  • 获得了良好的DNP表面增强的27Al交叉极化光谱.

结论:

  • 高磁场DNP (>100增强在18.8T) 是可行的和高效的.
  • 魔方旋转率是优化高场DNP的一个关键参数.
  • 开发的源-水槽扩散模型准确地解释了极化转移机制.
  • 这种方法显著提升了材料特征的固态NMR.