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

Il rilassamento nei sistemi NMR è un decadimento esponenziale del primo ordine e può avvenire sia con meccanismi spin-reticolo che spin-spin.
Il rilassamento spin-reticolo o longitudinale avviene principalmente attraverso le interazioni dipolo-dipolo magnetico con l'ambiente circostante, dove il nucleo eccitato trasferisce energia a un dipolo magnetico vicino, di solito un protone che cade.
Il rilassamento spin-reticolo ripristina la distribuzione di Boltzmann, e il tempo di rilassamento spin-reticolo, T1, indica l'emivita media di un nucleo nello stato eccitato.
Per i liquidi, i valori di T1 possono variare da 0,01 a 100 secondi, a seconda del tipo di nucleo, della sua posizione all'interno di una molecola, delle dimensioni della molecola e della temperatura.
Il rilassamento spin-spin o trasversale si verifica nel piano trasversale quando l'interazione spin-spin tra i nuclei in precessione provoca la sfasatura.
Il rilassamento spin-spin è governato dalla costante di tempo T2, che di solito è più corta di T1.
Mentre il rilassamento è essenziale per prevenire la saturazione e ottenere un segnale rilevabile, alti tassi di rilassamento provocano l'allargamento della linea. L'emivita ideale per un nucleo eccitato varia da 0,1 a 10 secondi.

nuclei atomici: tipi di rilassamento nucleare

Atomic Nuclei: Types of Nuclear Relaxation

1Il rilassamento nucleare ripristina lo squilibrio di equilibrio della popolazione e può avvenire tramite meccanismi spin-reticolo o spin-spin, che sono processi di decadimento esponenziale del primo ordine.

Nel reticolo spin o nel rilassamento longitudinale, gli spin eccitati scambiano energia con il reticolo circostante mentre ritornano al livello energetico inferiore. Tra i numerosi meccanismi che contribuiscono al rilassamento del reticolo spin, le interazioni dipolari magnetiche sono significative. Qui, il nucleo eccitato trasferisce energia a un dipolo magnetico vicino, solitamente un protone rotante.

Il rilassamento del reticolo di spin avviene per riportare la magnetizzazione longitudinale al suo valore di equilibrio ed è caratterizzato dalla costante di tempo, T1, che indica la vita media di un nucleo nello stato eccitato. T1 è anche chiamato tempo di rilassamento dipolare o dipolo-dipolo e può variare da 0,01 a 100 secondi per i liquidi. Il valore di T1 dipende da fattori quali il tipo di nucleo, la posizione di un nucleo all'interno di una molecola, la dimensione della molecola e la temperatura.

Il rilassamento trasversale, chiamato anche rilassamento spin-spin, si verifica quando i nuclei in precessione cadono fuori fase, provocando il decadimento della magnetizzazione. Il rilassamento trasversale è influenzato dai campi dipolari statici ed è solitamente più veloce del rilassamento longitudinale. I tempi di rilassamento osservati nei tipici esperimenti NMR vanno da 0,1 a 10 secondi. Inoltre, il tempo di rilassamento del reticolo di spin, T1, dipende dal campo magnetico applicato, mentre T2 ne è indipendente.

Sebbene il processo di rilassamento sia essenziale per prevenire la saturazione e ottenere un segnale rilevabile, influisce anche sull'intensità dei segnali NMR. Generalmente, l'intensità del segnale NMR è influenzata dal rilassamento T1, mentre T2 più breve si traduce in segnali NMR ampliati.

Nuclei atomici: Tipi di rilassamento nucleare

Il rilassamento nei sistemi NMR è un decadimento esponenziale del primo ordine e può avvenire sia con meccanismi spin-reticolo che spin-spin.
Reticolo di spin ...

atomic-nuclei-types-of-nuclear-relaxation

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Analytical Chemistry

Chimica Analitica

Unit 1

Unità 1

Principles of Nuclear Magnetic Resonance

Principi della Risonanza Magnetica Nucleare

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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