Name: nmr relaxation paths in atomic nuclei
Uploaded: 2024-04-04T10:51:41.000Z
Duration: 1 min 28 s
Channel: JoVE

La relaxation dans les systèmes RMN est une décroissance exponentielle de premier ordre et peut se produire par des mécanismes spin-réseau ou spin-spin.
La relaxation spin-réseau ou longitudinale se produit principalement via des interactions magnétiques dipôle-dipôle avec l’environnement, où le noyau excité transfère de l’énergie à un dipôle magnétique proche, généralement un proton qui tombe.
La relaxation spin-réseau rétablit la distribution de Boltzmann, et le temps de relaxation spin-réseau, T1, indique la demi-vie moyenne d’un noyau à l’état excité.
Pour les liquides, les valeurs T1 peuvent varier de 0,01 à 100 secondes, selon le type de noyau, son emplacement dans une molécule, la taille de la molécule et la température.
La relaxation spin-spin ou transversale se produit dans le plan transversal lorsque l’interaction spin-spin entre les noyaux en précession provoque un déphasage.
La relaxation spin-spin est régie par la constante de temps T2, qui est généralement plus courte que T1.
Alors que la relaxation est essentielle pour éviter la saturation et obtenir un signal détectable, des taux de relaxation élevés entraînent un élargissement de la ligne. La demi-vie idéale d’un noyau excité varie de 0,1 à 10 secondes.

noyaux atomiques : types de relaxation nucléaire

Atomic Nuclei: Types of Nuclear Relaxation

1La relaxation nucléaire rétablit le déséquilibre de la population à l’équilibre et peut se produire via des mécanismes spin-réseau ou spin-spin, qui sont des processus de désintégration exponentielle de premier ordre.

Dans la relaxation spin-réseau ou longitudinale, les spins excités échangent de l'énergie avec le réseau environnant lorsqu'ils reviennent au niveau d'énergie inférieur. Parmi les nombreux mécanismes qui contribuent à la relaxation spin-réseau, les interactions magnétiques dipolaires sont importantes. Ici, le noyau excité transfère de l’énergie à un dipôle magnétique proche, généralement un proton en chute libre.

La relaxation spin-réseau se produit pour restaurer l'aimantation longitudinale à sa valeur d'équilibre et est caractérisée par la constante de temps T1, qui indique la durée de vie moyenne d'un noyau dans l'état excité. T1 est également appelé temps de relaxation dipolaire ou dipôle-dipôle et peut aller de 0,01 à 100 secondes pour les liquides. La valeur de T1 dépend de facteurs tels que le type de noyau, l'emplacement d'un noyau dans une molécule, la taille de la molécule et la température.

La relaxation transversale, également appelée relaxation spin-spin, se produit lorsque les noyaux en précession se déphasent, entraînant une désintégration de la magnétisation. La relaxation transversale est influencée par les champs dipolaires statiques et est généralement plus rapide que la relaxation longitudinale. Les temps de relaxation observés dans les expériences RMN typiques vont de 0,1 à 10 secondes. De plus, le temps de relaxation spin-réseau, T1, dépend du champ magnétique appliqué, tandis que T2 en est indépendant.

Si le processus de relaxation est essentiel pour éviter la saturation et obtenir un signal détectable, il affecte également l’intensité des signaux RMN. Généralement, l'intensité du signal RMN est affectée par la relaxation T1, tandis qu'un T2 plus court entraîne des signaux RMN élargis.

Noyaux atomiques : types de relaxation nucléaire

La relaxation dans les systèmes RMN est une décroissance exponentielle de premier ordre et peut se produire par des mécanismes spin-réseau ou spin-spin.
Spin-réseau ...

atomic-nuclei-types-of-nuclear-relaxation

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Analytical Chemistry

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Principles of Nuclear Magnetic Resonance

Principes de la résonance magnétique nucléaire

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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 example is energy transfer from an excited nucleus to a nearby magnetic dipole, usually a tumbling proton.

This type of relaxation restores the longitudinal magnetization to its equilibrium value. It is described by the time constant T1, which shows the average lifetime of a nucleus in the excited state. T1 is also called the dipolar or dipole-dipole relaxation time. In liquids, it can range from 0.01 to 100 seconds.

The value of T1 depends on several factors. These include the type of nucleus, where the nucleus sits in a molecule, the size of the molecule, and temperature. T1 also depends on the applied magnetic field.

Spin-spin relaxation is also called transverse relaxation. It happens when precessing nuclei lose phase with one another, which causes magnetization to decay. Static dipolar fields influence this process, and it is usually faster than longitudinal relaxation.

In typical NMR experiments, relaxation times are often in the 0.1 to 10 second range. T2, the transverse relaxation time, is independent of the applied magnetic field. Relaxation is needed to prevent saturation and to keep a detectable signal. It also affects NMR signal intensity, because T1 influences signal strength while shorter T2 broadens the signals.

NMR Relaxation Paths in Atomic Nuclei

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

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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...

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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

atomic-nuclei-nuclear-relaxation-processes

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...

atomic nuclei: types of nuclear relaxation

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...

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NMR Pulse Angles and Signal Collection

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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

Explore spin-lattice and spin-spin relaxation in NMR, including T1 and T2, how they affect signal intensity, and why relaxation prevents saturation.

NMR Relaxation Paths in Atomic Nuclei | 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.

NMR Relaxation Paths in Atomic Nuclei

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