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

Relaxation in NMR-Systemen ist ein exponentieller Zerfall erster Ordnung und kann entweder durch Spin-Gitter- oder Spin-Spin-Mechanismen auftreten.
Spin-Gitter- oder Longitudinal-Relaxation tritt hauptsächlich durch magnetische Dipol-Dipol-Wechselwirkungen mit der Umgebung auf, bei denen der angeregte Kern Energie auf einen nahe gelegenen magnetischen Dipol überträgt, normalerweise ein taumelndes Proton.
Die Spin-Gitter-Relaxation stellt die Boltzmann-Verteilung wieder her, und die Spin-Gitter-Relaxationszeit T1 gibt die durchschnittliche Halbwertszeit eines Kerns im angeregten Zustand an.
Bei Flüssigkeiten können dieT1-Werte zwischen 0,01 und 100 Sekunden liegen, abhängig von der Art des Kerns, seiner Position innerhalb eines Moleküls, der Größe des Moleküls und der Temperatur.
Spin-Spin- oder Transversalrelaxation tritt in der Transversalebene auf, wenn die Spin-Spin-Wechselwirkung zwischen den präzessierenden Kernen eine Dephasierung verursacht.
Die Spin-Spin-Relaxation wird durch die Zeitkonstante T2 bestimmt, die normalerweise kürzer als T1 ist.
Während die Relaxation unerlässlich ist, um eine Sättigung zu verhindern und ein nachweisbares Signal zu erhalten, führen hohe Relaxationsraten zu einer Linienverbreiterung. Die ideale Halbwertszeit für einen angeregten Kern liegt zwischen 0,1 und 10 Sekunden.

atomkerne: arten der kernrelaxation

Atomic Nuclei: Types of Nuclear Relaxation

1Kernrelaxation stellt das Ungleichgewicht der Gleichgewichtspopulation wieder her und kann über Spin-Gitter- oder Spin-Spin-Mechanismen erfolgen, bei denen es sich um exponentielle Zerfallsprozesse erster Ordnung handelt.

Bei der Spin-Gitter- oder Längsrelaxation tauschen die angeregten Spins Energie mit dem umgebenden Gitter aus, während sie auf das niedrigere Energieniveau zurückkehren. Unter den verschiedenen Mechanismen, die zur Spin-Gitter-Relaxation beitragen, sind magnetische dipolare Wechselwirkungen von Bedeutung. Dabei überträgt der angeregte Kern Energie auf einen nahegelegenen magnetischen Dipol, meist ein taumelndes Proton.

Die Spin-Gitter-Relaxation dient der Wiederherstellung des Gleichgewichtswerts der Längsmagnetisierung und wird durch die Zeitkonstante T1 charakterisiert, die die durchschnittliche Lebensdauer eines Kerns im angeregten Zustand angibt. T1 wird auch Dipol- oder Dipol-Dipol-Relaxationszeit genannt und kann für Flüssigkeiten zwischen 0,01 und 100 Sekunden liegen. Der Wert von T1 hängt von Faktoren wie der Art des Kerns, der Position eines Kerns innerhalb eines Moleküls, der Größe des Moleküls und der Temperatur ab.

Querrelaxation, auch Spin-Spin-Relaxation genannt, tritt auf, wenn präzedierende Kerne aus der Phase geraten, was zu einem Magnetisierungsabfall führt. Die Querrelaxation wird durch statische Dipolfelder beeinflusst und ist normalerweise schneller als die Längsrelaxation. Die in typischen NMR-Experimenten beobachteten Relaxationszeiten liegen zwischen 0,1 und 10 Sekunden. Darüber hinaus hängt die Spin-Gitter-Relaxationszeit T1 vom angelegten Magnetfeld ab, während T2 davon unabhängig ist.



Während der Relaxationsprozess wichtig ist, um eine Sättigung zu verhindern und ein nachweisbares Signal zu erhalten, beeinflusst er auch die Intensität der NMR-Signale. Im Allgemeinen wird die Intensität des NMR-Signals durch die T1-Relaxation beeinflusst, wohingegen eine kürzere T2 zu verbreiterten NMR-Signalen führt.

Atomkerne: Arten der Kernrelaxation

Relaxation in NMR-Systemen ist ein exponentieller Zerfall erster Ordnung und kann entweder durch Spin-Gitter- oder Spin-Spin-Mechanismen auftreten.
Spin-Gitter ...

atomic-nuclei-types-of-nuclear-relaxation

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Analytical Chemistry

Analytische Chemie

Unit 1

Einheit 1

Principles of Nuclear Magnetic Resonance

Prinzipien der Kernspinresonanz

KEY TERMS AND CONCEPTS

WICHTIGE BEGRIFFE UND KONZEPTE

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

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

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...

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

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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