Name: nuclear spin alignment and relaxation
Uploaded: 2024-04-04T10:51:41.000Z
Duration: 1 min 23 s
Channel: JoVE

Когда вырожденные ядерные спиновые состояния выравниваются с приложенным магнитным полем или против него в соответствии с распределением Больцмана, их магнитные моменты прецессируют вокруг оси z.
Вспомним, что избыточное население в более низкоэнергетическом состоянии приводит к суммарной намагниченности, M, вдоль оси z, без суммарного вклада поперечных компонент на плоскости xy.
При возбуждении подходящим радиочастотным излучением ядра поглощают энергию.
Случайное распределение магнитных моментов становится слабо фазово-когерентным, что приводит к смещению вектора суммарной намагниченности.
Продолжительное возбуждение может уравнять популяции в верхнем и нижнем спиновых состояниях. Спиновая система насыщается, а сигнал поглощения снижается.
Возбужденные ядерные спины теперь должны подвергнуться релаксации и вернуться к равновесному распределению населения.
Когда скорость расслабления больше или равна скорости возбуждения, избыточное население сохраняется, и наблюдается сигнал.
Во время релаксации когерентность теряется, и суммарная намагниченность восстанавливается до равновесного значения.

Атомные ядра: процессы ядерной релаксации

Atomic Nuclei: Nuclear Relaxation Processes

В отсутствие внешнего магнитного поля спиновые состояния ядра вырождены и хаотично ориентированы. При приложении магнитного поля спины начинают прецессировать и ориентироваться вдоль (более низкая энергия) или против (более высокая энергия) направления поля. В состоянии равновесия в состоянии с более низкой энергией существует небольшое избыточное количество спинов. Поскольку направление магнитного поля фиксировано по оси z, прецессирующие магнитные моменты случайным образом ориентированы вокруг оси z. Это приводит к получению чистой намагниченности M вдоль оси z без чистого вклада поперечных компонентов в плоскости xy.

При возбуждении радиочастотным излучением ядра поглощают энергию, а возбужденные спины приобретают некоторую когерентность. Отсюда следует, что Mx и My больше не равны нулю, Mz уменьшается, а результирующая намагниченность M наклоняется к оси y. При продолжении возбуждения разница населенностей между спиновыми состояниями может уменьшаться вместе с интенсивностью сигнала. Это называется насыщением.

В конце концов, возбужденные ядерные спины возвращаются в равновесное состояние посредством процесса, называемого релаксацией. В ходе релаксации когерентность xy исчезает, а результирующая намагниченность восстанавливается до равновесного значения вдоль оси z.

В отсутствие внешнего магнитного поля ядерные спиновые состояния вырождены и случайно ориентированы. При приложении магнитного поля начинаются вращения...

atomic-nuclei-nuclear-relaxation-processes

Education

JoVE Core

Analytical Chemistry

Аналитическая химия

Unit 1

Раздел 1

Principles of Nuclear Magnetic Resonance

Принципы ядерного магнитного резонанса

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

nuclear spin alignment and relaxation

Nuclear spin alignment and relaxation describe how nuclear spins respond to a magnetic field and then return to equilibrium. Without an external magnetic field, nuclear spin states are degenerate, meaning they have the same energy, and the spins are randomly oriented.

When a magnetic field is applied, the spins begin to precess, or wobble, and orient themselves either along the field in a lower energy state or against it in a higher energy state. At equilibrium, a slight excess of spins occupies the lower energy state. Because the magnetic field defines the z-axis, the precessing magnetic moments are randomly arranged around that axis, so the net magnetization, M, points along the z-axis. The transverse components in the xy plane cancel out and do not add to the net signal.

Radiofrequency radiation can excite the nuclei and make them absorb energy. During this excitation, some of the spins become coherent, which means they start to move together. As a result, Mx and My are no longer zero, Mz decreases, and the net magnetization tips away from the z-axis toward the y-axis.

If excitation continues, the population difference between the spin states can drop further, and the signal intensity can fall. This is called saturation. Eventually, the excited nuclear spins relax back to equilibrium. During relaxation, the xy coherence disappears, and the net magnetization returns to its equilibrium value along the z-axis.

Nuclear Spin Alignment and Relaxation

nuclear magnetic resonance (nmr): overview

NMR Spectra and MRI Imaging

Nuclear magnetic resonance, or NMR, is a phenomenon that certain nuclei can show when they absorb radio frequency energy under the right conditions. Scientists use it in molecular spectroscopy and in medical imaging. In both cases, the sample or the person being studied is placed in a magnetic field and exposed to radio frequency energy. In NMR spectroscopy, the result is a spectrum. This spectrum plots the absorption frequencies of the sample against peak intensity. The pattern gives detailed...

atomic nuclei: nuclear spin

Nuclear Spin and NMR-Active Nuclei

Atomic nuclei have nuclear spin, which comes from the spins of their protons and neutrons. These particle spins are intrinsic angular momentum, and each electron, proton, and neutron has a spin value of 1/2. In nuclei, proton spins pair with other protons, and neutron spins pair with other neutrons. They do not pair with each other, so the final nuclear spin depends on which nucleons are left unpaired. Even numbers of protons do not add to the nuclear spin, and even numbers of neutrons do not...

atomic nuclei: nuclear magnetic moment

Nuclear Spin States and Magnetic Alignment

Atomic nuclei can act like tiny magnets when they have a nonzero spin. Because they are positively charged and rotating, they generate a magnetic field and a magnetic moment. In the absence of an external magnetic field, this magnetic moment points in random directions. When an external magnetic field, B0, is applied, the nuclear magnetic moments can line up with the field or against it. These two directions create 2I + 1 orientations. A hydrogen nucleus is a proton, and it has spin 1/2, so it...

atomic nuclei: nuclear spin state overview

atomic-nuclei-nuclear-spin-state-overview

Zeeman Splitting in Nuclear Spin States

NMR-active nuclei have spin states that describe the orientation of their nuclear magnetic moments. Without a magnetic field, these moments point in random directions, and the spin states are degenerate, meaning they have the same energy. When an external magnetic field is applied, the nucleus can only take on 2I + 1 orientations, where I is the nuclear spin. A proton with spin 1/2 has two possible orientations. A quadrupolar nucleus with spin 1 has three possible orientations. The...

atomic nuclei: nuclear spin state population distribution

atomic-nuclei-nuclear-spin-state-population-distribution

Nuclear Spin Populations and NMR Signal

Nuclear spin populations help explain why NMR signals appear. In a magnetic field, nuclei can occupy a lower energy spin-up state or a higher energy spin-down state. Near absolute zero, most nuclei stay in the lower energy state. As temperature rises, thermal collisions spread the spins more evenly between the two states. The balance between the spin −1/2 and spin +1/2 populations is described by the Boltzmann distribution equation. In this equation, ΔE is the energy difference, k is the...

atomic nuclei: larmor precession frequency

atomic-nuclei-larmor-precession-frequency

Magnetic Field and Nuclear Precession

Magnetic field strength controls nuclear precession in NMR. A spinning nucleus has a magnetic moment, and that moment turns around an external magnetic field directed along the z-axis. This motion is called Larmor precession, and its angular frequency is the Larmor frequency. The Larmor frequency is directly proportional to the applied magnetic field, B0. The proportionality constant is the gyromagnetic ratio, gamma. This value is fixed for each nucleus because every nucleus has its own ratio...

atomic nuclei: magnetic resonance

Nuclear Spins and Resonance in a Magnetic Field

Nuclear spins can line up in a magnetic field and create nuclear magnetic resonance. In the lower energy state, slightly more spins are aligned than in the higher energy state. That small difference gives the nuclei a net magnetization along the z axis.

atomic nuclei: nuclear relaxation processes

Nuclear spin alignment and relaxation describe how nuclear spins respond to a magnetic field and then return to equilibrium. Without an external magnetic field, nuclear spin states are degenerate, meaning they have the same energy, and the spins are randomly oriented. When a magnetic field is applied, the spins begin to precess, or wobble, and orient themselves either along the field in a lower energy state or against it in a higher energy state. At equilibrium, a slight excess of spins...

atomic nuclei: types of nuclear relaxation

atomic-nuclei-types-of-nuclear-relaxation

NMR Relaxation Paths in Atomic Nuclei

Nuclear relaxation in atomic nuclei helps restore the balance between energy levels in NMR systems. It occurs through two main paths: spin-lattice relaxation and spin-spin relaxation. Both are first-order exponential decay processes. Spin-lattice relaxation is also called longitudinal relaxation. In this process, excited spins give energy to the surrounding lattice as they move back to the lower energy level. Magnetic dipolar interactions are an important part of this mechanism. A common...

nmr spectrometers: overview

How NMR Spectrometers Record Signals

NMR spectrometers record signals from samples that contain NMR-active nuclei. The system uses a strong magnet, a radiofrequency transmitter, and a detector connected to a computer console. These parts work together to measure and display the spectrum of the sample. Early NMR instruments were continuous-wave spectrometers. In a frequency-sweep method, the magnetic field stays fixed while the rf signal is swept. In a field-sweep method, the radiofrequency signal stays fixed while the magnetic...

nmr spectrometers: radiofrequency pulses and pulse sequences

nmr-spectrometers-radiofrequency-pulses-and-pulse-sequences

NMR Pulse Angles and Signal Collection

NMR pulse angles and signal collection explain how radiofrequency energy starts a measurement. A pulse is a short burst of radio waves spread across a range of frequencies. It excites the nuclei in the sample at the same time. When a radiofrequency pulse is applied along the x-axis, nuclei absorb energy that matches their Larmor frequencies and reach resonance. The net magnetization vector moves away from the z-axis and toward the transverse plane. This rotation angle is called the flip angle.

nmr spectrometers: resolution and error correction

nmr-spectrometers-resolution-and-error-correction

NMR Peak Shape and Magnetic Field Shimming

NMR peak shape depends on the signal created after magnetic nuclei resonate and relax. The detected free induction decay is approximately exponential. When that decay is Fourier transformed, it becomes a Lorentzian peak in the frequency domain. A Lorentzian peak in an NMR spectrum is described by its amplitude, full width at half maximum, and position. Its width is governed by the spin-spin relaxation time alone. In real experiments, the sample and probe can make the magnetic field...

diamagnetic shielding of nuclei: local diamagnetic current

diamagnetic-shielding-of-nuclei-local-diamagnetic-current

How Electron Density Shields Nuclei in NMR

Electron density around a nucleus affects how strongly it is shielded in NMR spectroscopy. When an applied magnetic field acts on a molecule, the electrons begin to circulate. This creates a local diamagnetic current within the molecule. The circulating sigma-bonding electrons produce a local magnetic field, B local, that opposes the applied field, B0. The nucleus then feels an effective magnetic field, which is the difference between the applied field and the local field. This effect is...

key terms and concepts

See how nuclear spins align in a magnetic field, absorb radiofrequency energy, reach saturation, and relax back to equilibrium in simple NMR terms.

Nuclear Spin Alignment and Relaxation | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

Nuclear Spin Alignment and Relaxation

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