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

In einem Molekül bewirkt ein angelegtes Magnetfeld B0, dass die Elektronendichte, die die Kerne umgibt, zirkuliert und ein lokaler diamagnetischer Strom entsteht, der ein lokales Magnetfeld Blokal induziert, das B0 entgegengesetzt ist.
Das effektive Magnetfeld, das diese Kerne erfahren, Beffektiv, entspricht also B0 minus Blokal, in einem Phänomen, das als lokale diamagnetische Abschirmung bezeichnet wird.
Im Wesentlichen nimmt die lokale B-Dichte mit der Elektronendichte zu, die die Kerne umgibt, was zu einer erhöhten Abschirmung und einer geringeren B-Wirksamkeit führt.
Da die Elektronendichten innerhalb eines Moleküls variieren, ist jeder Kern unterschiedlich stark abgeschirmt und erfährt ein anderes Wirkfeld.
Ein Kern in einer elektronendichten Umgebung ist gut gegen das angelegte Magnetfeld abgeschirmt und erfährt einen niedrigeren B-Effekt.
Die Energie, die erforderlich ist, um seinen Spin umzukehren, ist also geringer als die Energie, die für einen schlecht abgeschirmten Kern in einer elektronenarmen Umgebung erforderlich ist.
Infolgedessen haben abgeschirmte Kerne niedrigere Absorptionsfrequenzen als abgeschirmte Kerne.

diamagnetische abschirmung von kernen: lokaler diamagnetischer strom

Diamagnetic Shielding of Nuclei: Local Diamagnetic Current

Ein angelegtes Magnetfeld bewirkt, dass die im Molekül vorhandenen Elektronen zirkulieren und einen lokalen diamagnetischen Strom innerhalb des Moleküls erzeugen. Der lokale diamagnetische Strom, der aus zirkulierenden Sigma-bindenden Elektronen entsteht, induziert ein Magnetfeld Blocal, das dem angelegten Magnetfeld B0 entgegenwirkt. Das effektive Magnetfeld, dem diese Kerne ausgesetzt sind, ergibt sich aus der Differenz zwischen dem angelegten und dem lokalen Magnetfeld in einem Phänomen, das als lokale diamagnetische Abschirmung bezeichnet wird. Im Wesentlichen nimmt Blocal mit der die Kerne umgebenden Elektronendichte zu, was zu einer erhöhten Abschirmung und einem geringeren Beffektiv führt. Da die Elektronendichte innerhalb eines Moleküls variiert, sind Kerne im selben Molekül unterschiedlich stark abgeschirmt und erfahren unterschiedliche effektive Felder. Ein Kern in einer elektronendichten Umgebung ist gut vom angelegten Magnetfeld abgeschirmt und erfährt ein geringeres Beffektiv. Folglich ist die Energie, die zum Umdrehen seines Spins erforderlich ist, geringer als die, die für einen schlecht abgeschirmten Kern in elektronenarmer Umgebung erforderlich ist. Daher treten abgeschirmte Kerne eher bei niedrigeren Frequenzen in Resonanz als nicht abgeschirmte Kerne. Die Resonanzfrequenzen werden in das NMR-Spektrum eingezeichnet und machen diese Spektren empfindlich für die diamagnetische Abschirmung.

Diamagnetische Abschirmung von Kernen: Lokaler diamagnetischer Strom

Ein angelegtes Magnetfeld bewirkt, dass die im Molekül vorhandenen Elektronen zirkulieren, wodurch ein lokaler diamagnetischer Strom im Molekül aufgebaut wird. Die örtliche ...

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

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Analytical Chemistry

Analytische Chemie

Unit 1

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Principles of Nuclear Magnetic Resonance

Prinzipien der Kernspinresonanz

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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

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.

How Electron Density Shields Nuclei in NMR

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