Name: how electron density shields nuclei in nmr
Uploaded: 2024-04-04T10:51:42.000Z
Duration: 1 min 14 s
Channel: JoVE

分子内では、印加された磁場B0により、原子核を取り巻く電子密度が循環し、局所的な反磁性電流が発生し、これがB0に対抗する局所的な磁場Bを誘導します。
したがって、これらの原子核が経験する有効磁場、B有効は、局所反磁性シールドと呼ばれる現象で、B0からB局所を引いたものに等しくなります。
基本的に、B局部は原子核を取り巻く電子密度とともに増加し、シールドが増加し、Bの有効性が低下します。
分子内では電子密度が異なるため、各核は異なる程度に遮蔽され、異なる有効磁場を経験します。
電子が密集した環境の原子核は、印加された磁場から十分に遮蔽され、Bの実効値が低くなります。
したがって、そのスピンを反転させるのに必要なエネルギーは、電子の少ない環境でシールドが不十分な原子核に必要なエネルギーよりも少なくなります。
その結果、遮蔽された原子核は、遮蔽された原子核よりも吸収周波数が低くなります。

レッスン:原子核の反磁性シールド: 局所的な反磁性電流

Diamagnetic Shielding of Nuclei: Local Diamagnetic Current

磁場を印加すると、分子内に存在する電子が循環し、分子内に局所的な反磁性電流が生じます。シグマ結合電子を含んだ循環する電子から生じるは、印加磁場 B0 に対抗する磁場 Blocal を誘導します。これらの原子核が受けるB_effectは、局所反磁性シールド効果と呼ばれる現象における印加磁場と局所磁場の差によって与えられます。基本的に、Blocal　は原子核の周囲の電子密度とともに増加し、シールドが増加し、Beffect　が低下します。電子密度は分子内で異なるため、同じ分子内の原子核は異なる程度に遮蔽され、異なる実効場を経験します。電子密度の高い環境にある原子核は、印加された磁場から十分に遮蔽されており、BEffect が低くなります。その結果、スピンを反転させるのに必要なエネルギーは、電子の少ない環境で十分に遮蔽されていない原子核に必要なエネルギーよりも少なくなります。したがって、シールドされた原子核は、シールドされていない原子核よりも低い周波数で共鳴を経験します。共鳴周波数は NMR スペクトルは上にプロットされており、これらのスペクトルはは反磁性シールドの影響を受けやすくなっています。

印加された磁場により、分子内に存在する電子が循環し、分子内に局所的な反磁性電流が設定されます。地元の...

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

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Principles of Nuclear Magnetic Resonance

核磁気共鳴の原理

KEY TERMS AND CONCEPTS

主要な用語と概念

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 called local diamagnetic shielding.

Shielding depends on electron density around the nucleus. A nucleus in an electron-dense area experiences a stronger local field and a lower effective field. A nucleus in an electron-poor area is less shielded and experiences a higher effective field.

These differences change the energy needed to flip nuclear spin. A well-shielded nucleus needs less energy than a poorly shielded nucleus. As a result, shielded nuclei resonate at lower frequencies than deshielded nuclei. Because resonance frequencies appear on the NMR spectrum, NMR signals are sensitive to diamagnetic shielding.

How Electron Density Shields Nuclei in NMR

how-electron-density-shields-nuclei-in-nmr

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

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

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 electron density creates local diamagnetic shielding in NMR, changes the effective magnetic field, and shifts resonance frequencies on the spectrum.

How Electron Density Shields Nuclei in NMR | JoVE K12

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

How Electron Density Shields Nuclei in NMR

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