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相关概念视频

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

1.0K
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
1.0K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

1.7K
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
1.7K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
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...
1.0K
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

5.1K
When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
5.1K
Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

199
Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...
199
¹H NMR: Pople Notation01:09

¹H NMR: Pople Notation

1.8K
The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...
1.8K

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对于激发状态的特定国家合集群方法.

Yann Damour1, Anthony Scemama1, Denis Jacquemin2,3

  • 1Laboratoire de Chimie et Physique Quantiques (UMR 5626), Université de Toulouse, CNRS, UPS, 31000 Toulouse, France.

Journal of chemical theory and computation
|May 15, 2024
PubMed
概括

我们评估了 ΔCCSD 方法来计算分子激发状态,发现它对双倍激发状态有效,但对其他类型的 EOM-CCSD 准确性通常不如 EOM-CCSD. 国家特定轨道提供了轻微的改进.

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科学领域:

  • 量子化学 是一个量子化学.
  • 计算化学计算化学
  • 理论化学 理论化学

背景情况:

  • 标准的单次和双次激发的运动方程合集群 (EOM-CCSD) 方法与双倍激发的状态作斗争.
  • 使用非奥夫巴乌决定因素的 ΔCCSD 方法为准兴奋状态提供了替代方案,特别是双重兴奋状态.

研究的目的:

  • 为了对 ΔCCSD 方法与 EOM-CCSD 对各种类型的分子激发状态的准确性和一致性进行基准测试.
  • 评估超出双激发状态的 ΔCCSD 的性能,包括闭系统的双倍-双倍过渡和单次激发状态.

主要方法:

  • 对 ΔCCSD 和 EOM-CCSD 计算激发能量的方法进行比较.
  • 利用从任务数据库中获得的276个激发状态的数据集.
  • 在封闭外系统中采用极简主义的两决定因素合集群方法,用于单独激发的状态.

主要成果:

  • ΔCCSD对双激发状态显示有效性,但对其他激发类型的表现通常低于EOM-CCSD.
  • 双倍-双倍过渡的平均绝对误差 (MAE) 是0.10 eV (ΔCCSD) 与0.07 eV (EOM-CCSD) 相比.
  • 单次激发状态的MAE为0.15 eV (ΔCCSD) 与0.08 eV (EOM-CCSD) 相比,具有更高的多配置特征,有助于 ΔCCSD 的精度降低.

结论:

  • CCSD是用于EOM-CCSD失败的双重激发状态的可行方法.
  • 对于大多数其他兴奋状态,EOM-CCSD仍然是更准确和更一致的选择.
  • 国家特定的优化轨道为 ΔCCSD 精度提供了边际改进.