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π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.8K
In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
1.8K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

2.6K
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...
2.6K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

6.2K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
6.2K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

12.8K
Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?  
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n +...
12.8K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

3.6K
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
3.6K
Molecular Orbital Theory II03:51

Molecular Orbital Theory II

26.9K
Molecular Orbital Energy Diagrams
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使用多引用扰乱理论提高量子选择配置交互计算的准确性:对芳香分子的应用.

Soichi Shirai1, Shih-Yen Tseng2, Hokuto Iwakiri2

  • 1Toyota Central Research and Development Laboratories, Incorporated, 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan.

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概括

量子选择配置交互 (QSCI) 使用量子设备来选择重要的电子配置用于经典计算. 这种混合方法提高了量子化学的准确性,特别是在纳夫他林和四烯等芳香分子中.

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科学领域:

  • 量子计算是一种量子计算.
  • 计算化学是一种计算化学.
  • 理论化学是一种理论化学.

背景情况:

  • 量子选择配置交互 (QSCI) 是量子化学的混合量子-经典算法.
  • QSCI使用经典计算的量子设备识别了关键的电子配置,旨在管理大配置空间并减轻噪声.
  • 目前的QSCI限制包括量子位噪声影响小活跃空间的准确性.

研究的目的:

  • 展示一个计算方案,以提高QSCI计算的准确性.
  • 将该方案应用于芳香分子的地面和激发状态计算.
  • 调查进一步提高准确度的方法.

主要方法:

  • 在古典计算机上使用多引用扰动理论开发了一个计算方案.
  • 使用QSCI波函数作为干扰计算的参考.
  • 将该方法应用于纳夫他林和四烯,用于基底和激发状态分析.

主要成果:

  • 乱理论的结合显著提高了QSCI计算的准确性.
  • 该方法已成功应用于计算纳夫他林和四二烯的地面和兴奋状态.
  • 研究了使用QSCI选择的配置来扩展参考空间,以进一步提高准确性.

结论:

  • 开发的计算方案提高了量子化学QSCI的准确性.
  • 这种混合方法对分子电子结构的更准确计算有希望.
  • 进一步的研究可以探索参考空间扩展,以获得更高的精度.