Name: nuclear spin and nmr-active nuclei
Uploaded: 2024-04-04T10:51:39.000Z
Duration: 1 min 8 s
Channel: JoVE

Todas as partículas atômicas possuem momento angular intrínseco, ou 'spin'. Elétrons, prótons e nêutrons têm um valor de spin de metade.
Ao contrário dos elétrons, prótons e nêutrons nos núcleos podem ter spins mais altos, mas os valores são sempre meio inteiros.
Todas as partículas em um núcleo contribuem para o spin nuclear geral, I.
Nos núcleos, os prótons emparelham com prótons e os nêutrons emparelham com nêutrons. As partículas emparelhadas contribuem com spin líquido zero para o núcleo. Assim, um núcleo com spin diferente de zero tem pelo menos uma partícula desemparelhada.
Se o número de prótons ou nêutrons for ímpar, o spin nuclear é um meio inteiro por causa do próton ou nêutron desemparelhado.
Da mesma forma, se o número de nêutrons e prótons for ímpar, o spin nuclear é um inteiro diferente de zero. 
Embora todos os núcleos com spins diferentes de zero sejam ativos em RMN, os núcleos de spin - com distribuição de carga esférica e campos simétricos - são preferidos para estudos de RMN.

núcleos atômicos: spin nuclear

Atomic Nuclei: Nuclear Spin

Todas as partículas atômicas possuem um momento angular intrínseco, ou “spin”. Os elétrons, prótons e nêutrons têm cada um um valor de spin de ½, embora prótons e nêutrons nos núcleos possam ter spins de meia-ordem superiores devido a fatores energéticos.

Os núcleos atômicos têm um spin nuclear líquido, CoreOrgo itálico i, que pode ter um valor inteiro ou de meia-ordem. Nos núcleos atômicos, os spins dos prótons estão emparelhados entre si, mas não com os nêutrons, e vice-versa. Consequentemente, um número par de prótons não contribui para o valor do spin nuclear, e o mesmo se aplica aos nêutrons.

Os núcleos de carbono-12 e oxigênio-16 contêm números pares de prótons e nêutrons e têm spin nuclear líquido zero (CoreOrgo itálico i = 0). Quando o núcleo contém um número ímpar de prótons (por exemplo, prótio) ou nêutrons (por exemplo, carbono-13), o spin nuclear é de meia-ordem por causa do núcleon desemparelhado. Um núcleo contendo números ímpares de nêutrons e prótons tem um spin inteiro não nulo. Todos os núcleos com spins não nulos é ativos em RMN.

Núcleos com valores de spin maiores que ½ é chamados de núcleos quadrupolares. Mais de dois terços dos núcleos naturalmente ativos de RMN é quadrupolares (exemplos incluem nitrogênio-14, oxigênio-17, enxofre-33, boro-11 e cloro-35). Núcleos quadrupolares têm distribuição de carga não esférica, levando a campos elétricos e magnéticos assimétricos que resultam em sinais amplos e comportamento complexo de RMN. Consequentemente, núcleos com spin de meia-ordem, com distribuição de carga esférica e campos simétricos, é preferidos para estudos de RMN.

Núcleos Atômicos: Spin Nuclear

Todas as partículas atômicas possuem um momento angular intrínseco, ou 'spin'. Elétrons, prótons e nêutrons têm um valor de spin de 1/2, embora ...

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Analytical Chemistry

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Principles of Nuclear Magnetic Resonance

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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 either. That is why carbon-12 and oxygen-16 have zero nuclear spin, or I = 0, because both nuclei contain even numbers of protons and neutrons. When a nucleus has an odd number of protons, such as protium, or an odd number of neutrons, such as carbon-13, the nuclear spin is a half-integer because of the unpaired nucleon. If a nucleus has odd numbers of both protons and neutrons, its spin is an integer and not zero.

Any nucleus with nonzero spin is NMR-active. Nuclei with spin values greater than 1/2 are called quadrupolar nuclei. More than two-thirds of naturally occurring NMR-active nuclei are quadrupolar, including nitrogen-14, oxygen-17, sulfur-33, boron-11, and chlorine-35. These nuclei have non-spherical charge distributions, which create asymmetric electric and magnetic fields and lead to broad signals and more complex NMR behavior. Spin-1/2 nuclei are often preferred for NMR studies because their charge distribution is more spherical and their fields are more symmetric.

Nuclear Spin and NMR-Active 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

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

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

Learn how proton and neutron pairing affects nuclear spin, why some nuclei are NMR-active, and why spin-1/2 nuclei give cleaner NMR signals.

Nuclear Spin and NMR-Active 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.

Nuclear Spin and NMR-Active Nuclei

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