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Atomic Nuclei: Types of Nuclear Relaxation01:28

Atomic Nuclei: Types of Nuclear Relaxation

271
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...
271
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

816
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
816
Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

632
In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
632

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Updated: Jun 14, 2025

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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基于随机阶段近似的高效结构松:对水群的应用.

Muhammad N Tahir1, Honghui Shang2, Jia Li3

  • 1Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.

The journal of physical chemistry. A
|September 6, 2024
PubMed
概括
此摘要是机器生成的。

我们开发了一种更快的方法来计算随机相近似 (RPA) 能量,改进了水集群结构分析. RPA+rSE校正准确地预测了绑定能量,与双重混合功能相竞争.

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Multiscale Sampling of a Heterogeneous Water/Metal Catalyst Interface using Density Functional Theory and Force-Field Molecular Dynamics
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High-Resolution Neutron Spectroscopy to Study Picosecond-Nanosecond Dynamics of Proteins and Hydration Water
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相关实验视频

Last Updated: Jun 14, 2025

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Published on: April 8, 2020

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Multiscale Sampling of a Heterogeneous Water/Metal Catalyst Interface using Density Functional Theory and Force-Field Molecular Dynamics
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科学领域:

  • 计算化学计算化学
  • 量子化学 是一个量子化学.
  • 材料科学 材料科学 材料科学

背景情况:

  • 准确预测分子结构和能量在化学中至关重要.
  • 随机相近似 (RPA) 是一种强大的电子相关联方法,但计算密集.
  • 水集群是理解结和溶解的基本系统.

研究的目的:

  • 为RPA电子相对应能量的分析梯度提供一个高效的实现.
  • 研究中小水群的结构和能量排序.
  • 评估RPA的准确性及其对约束能量的校正.

主要方法:

  • 改进了RPA的分析梯度,使用原子轨道和局部识别分辨率.
  • 水集群的结构放松 (H2O) n,n=21,22,25. 在水集群的结构放松 (H2O) n,n=21,22,25.
  • 基准集融合研究和与双杂交函数的比较.

主要成果:

  • 有效的RPA力计算使水群的结构放松成为可能.
  • 低能异构体的RPA能量排序对基础组的选择敏感.
  • RPA是水集群的基础,在完整的基础设置 (CBS) 极限上加剧.
  • 重新规范的单次激发 (rSE) 校正减轻了低约束,在有限的基础集上显示了过约束.
  • RPA+rSE的准确性可与接近CBS束能量的极限的双混合功能相提并论.

结论:

  • 改进的RPA实施允许对水集群进行高效的结构分析.
  • 基础组的选择显著影响了水集群的预测能源格局.
  • RPA+rSE提供了一个计算上可行的和准确的方法来预测约束能量,特别适用于像水集群这样的系统.