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Atomic Nuclei: Nuclear Spin State Population Distribution01:14

Atomic Nuclei: Nuclear Spin State Population Distribution

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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Atomic Nuclei: Types of Nuclear Relaxation01:28

Atomic Nuclei: Types of Nuclear Relaxation

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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
In spin–lattice or longitudinal relaxation, the excited spins exchange energy with the surrounding lattice as they return to the lower energy level. Among several mechanisms that contribute to spin–lattice relaxation, magnetic dipolar interactions are significant. Here, the excited nucleus transfers...
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Valence Bond Theory02:42

Valence Bond Theory

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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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Spin–Spin Coupling Constant: Overview01:08

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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Atomic Nuclei: Nuclear Spin State Overview01:03

Atomic Nuclei: Nuclear Spin State Overview

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NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of...
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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Site Directed Spin Labeling and EPR Spectroscopic Studies of Pentameric Ligand-Gated Ion Channels
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Teoría funcional de la densidad dependiente del tiempo adaptada al giro

Chima S Chibueze1, Lucas Visscher1

  • 1Department of Chemistry and Pharmaceutical Sciences, Vrije Universiteit, De Boelelaan 1108, 1081 HZ Amsterdam, The Netherlands.

The Journal of chemical physics
|September 4, 2025
PubMed
Resumen
Este resumen es generado por máquina.

Este estudio introduce nuevos métodos de la teoría funcional de la densidad dependiente del tiempo (SFD-TD-DFT) adaptados al espín para calcular con precisión las excitaciones electrónicas bajas en sistemas moleculares de alto espín. Los nuevos métodos de Kohn-Sham de caparazón abierta restringida (ROKS) ofrecen una mayor precisión para las transiciones de giro y giro.

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Área de la Ciencia:

  • Química computacional
  • Mecánica Cuántica
  • Espectroscopia teórica

Sus antecedentes:

  • Los sistemas con degeneración orbital en el nivel de Fermi a menudo exhiben estados de tierra de alto espín.
  • Se puede acceder a excitaciones electrónicas bajas con un giro total más bajo a través de transiciones de giro hacia abajo.
  • El cálculo preciso de estas excitaciones de giro y giro es crucial para comprender las propiedades moleculares.

Objetivo del estudio:

  • Desarrollar y presentar tres enfoques de la teoría funcional de la densidad dependiente del tiempo (SFD-TD-DFT) adaptados al espín.
  • Para calcular las energías de excitación para las transiciones electrónicas desde los estados básicos de alto espín.
  • Investigar el papel de la adaptación del espín y la descripción del núcleo en estos cálculos.

Principales métodos:

  • Desarrollo de tres métodos SFD-TD-DFT basados en la formulación restringida de Kohn-Sham (ROKS) en el marco de la aproximación Tamm-Dancoff (TDA).
  • Utilizó diferentes elementos de acoplamiento de dos electrones en los núcleos de las ecuaciones de trabajo.
  • Se introdujeron métodos ROKS-SFD-TDA totalmente adaptados al espín derivados de la interacción de configuración con excitaciones únicas (SF-CIS) y la ecuación de movimiento ansatz (SF-TDA).

Principales resultados:

  • Una descripción del núcleo no colineal es esencial para el cálculo preciso de las excitaciones de giro y giro.
  • Los métodos totalmente adaptados al espín (SF-CIS y SF-TDA) pueden dar lugar a un doble conteo artificial de los efectos de correlación.
  • El método SF-TDA adaptado a casi espín (Q-SF-TDA) demuestra una estabilidad, eficiencia y rendimiento comparables al SFD-TD-DFT sin restricciones de espín.

Conclusiones:

  • Los métodos ROKS-SFD-TDA desarrollados proporcionan energías de excitación precisas para las transiciones de giro y giro en sistemas de alto giro.
  • El método Q-SF-TDA ofrece un enfoque robusto y eficiente para el estudio de estas excitaciones electrónicas.
  • Estos avances contribuyen a una mejor comprensión teórica de la estructura electrónica y la espectroscopia en sistemas degenerados.