7.7: 原子核:磁共振

在较低能态下排列的核自旋数略多于在较高能态下排列的核自旋数。在存在外部磁场的情况下，当自旋以 Larmor 频率进动时，过量的种群会导致沿 z 轴定向的净磁化强度。当沿 x 轴施加 Larmor 频率的脉冲或短脉冲无线电波时，频率耦合会引起共振并将过剩种群的核自旋从较低能量状态翻转到较高能量状态，这会导致净磁化强度从 z 轴向 y 轴移动。在撤回施加的辐射时，随着核自旋失去吸收的能量并返回到自旋上升状态，净磁化矢量沿 z 轴返回其方向，并建立平衡。所有 NMR 活性原子核都表现出核磁共振，这构成了 NMR 波谱和成像的基础。

在较低能量状态对齐的少数核自旋代表过剩的人口。

当自旋以 Larmor 频率 ω 绕 B₀ 场进动时，它们的磁矩之和导致围绕 z 轴的净磁化。

当沿 x 轴施加脉冲或短脉冲无线电波时，原子核会吸收与其 Larmor 频率相对应的能量。

频率的耦合引起共振，并将过剩粒子的核自旋从低能量状态翻转到高能量状态，从而将净磁化强度向 y 轴移动。

在撤回辐射脉冲时，核自旋会失去吸收的能量。净磁化矢量移回 z 轴，并建立平衡。

所有 NMR 活性原子核都表现出核磁共振，这构成了 NMR 波谱和成像的基础。

• Nuclear magnetic resonance (Main keyword) - A technique that exploits the magnetic properties of certain atomic nuclei for spectroscopy and imaging.
• Atomic resonance - The phenomenon of an atom's nuclei oscillating at specific frequencies under energy influence.
• Nuclear magnetization - The net magnetization oriented along an axis due to the excess nuclear spin population.
• Larmor frequency - The frequency at which nuclear spins precess in an external magnetic field.
• NMR-active nuclei - Those atomic nuclei which exhibit nuclear magnetic resonance.

• Define Nuclear magnetic resonance - Explain what it is (e.g., nuclear magnetization of atomic nuclei).
• Contrast Nuclear Magnetization vs Atomic Magnetization – Explain key differences (e.g., orientation of nuclei in an external magnetic field).
• Explore Examples – Describe scenario (e.g., application of radio waves at the Larmor frequency causes atomic resonance and change in nuclear spin).
• Explain Process – Magnetic influence on atomic nuclei and the resulting resonance.
• Apply in Context – How nuclear magnetic resonance contributes to spectroscopy and imaging.

• What is nuclear magnetic resonance and how does it cause atomic resonance?
• How is nuclear magnetization oriented in an external magnetic field?
• What is the role of the Larmor frequency in atomic resonance?

• Students – Understand How nuclear magnetic resonance can provide insight into chemical structures.
• Educators – Provides a clear framework it helps with teaching atomic structure and interactions.
• Research Scientists – Relevance in the study of biomolecular chemistry and medical imaging.
• Physics Enthusiasts – Offer insights into the interaction of magnetic fields with atomic nuclei.