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

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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...
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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...
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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.
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Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

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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 +...
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Frost Circles for Different Conjugated Systems01:18

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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.
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Molecular Orbital Energy Diagrams
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Enhancing Accuracy of Quantum-Selected Configuration Interaction Calculations Using Multireference Perturbation

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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Quantum-selected configuration interaction (QSCI) uses quantum devices to select important electron configurations for classical computation. This hybrid approach enhances accuracy for quantum chemistry, particularly for aromatic molecules like naphthalene and tetracene.

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Area of Science:

  • Quantum computing
  • Computational chemistry
  • Theoretical chemistry

Background:

  • Quantum-selected configuration interaction (QSCI) is a hybrid quantum-classical algorithm for quantum chemistry.
  • QSCI identifies key electron configurations using quantum devices for classical computation, aiming to manage large configuration spaces and mitigate noise.
  • Current QSCI limitations include qubit noise affecting accuracy in small active spaces.

Purpose of the Study:

  • To demonstrate a computational scheme to improve the accuracy of QSCI calculations.
  • To apply this scheme to ground and excited state calculations of aromatic molecules.
  • To investigate methods for further accuracy enhancement.

Main Methods:

  • Developed a computational scheme using multireference perturbation theory on classical computers.
  • Utilized the QSCI wave function as a reference for perturbation calculations.
  • Applied the method to naphthalene and tetracene for ground and excited state analysis.

Main Results:

  • The incorporation of perturbation theory significantly improved the accuracy of QSCI calculations.
  • The method was successfully applied to calculate ground and excited states of naphthalene and tetracene.
  • Investigated extending the reference space using QSCI-selected configurations for further accuracy gains.

Conclusions:

  • The developed computational scheme enhances the accuracy of QSCI for quantum chemistry.
  • This hybrid approach shows promise for more accurate calculations of molecular electronic structures.
  • Further research can explore reference space extension for even greater precision.