アロマティック・システムとアンチアロマティック・システムにおけるプロトンの移転. 移行状態はどの程度アロマティックかアンチアロマティックか? アブ・イニシオ研究です
Claude F Bernasconi1, Philip J Wenzel, Mark L Ragains
1Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA. bernasconi@chemistry.ucsc.edu
Journal of the American Chemical Society
|March 15, 2008
まとめ
この研究では,アロマティック・システムとアンチアロマティック・システムのプロトン伝搬を調査した. 芳香性の高い移行状態は低プロトン伝送障壁を示し,反芳香系はより高い障壁に寄与するストレスを経験しました.
科学分野:
- コンピューティング・ケミストリー
- 物理有機化学 物理有機化学
背景:
- 陽子の移転は化学反応において根本的な役割を果たします.
- アロマティック性は分子安定性と反応性に影響します.
- 移行状態を理解することは,反応機構の鍵です.
研究 の 目的:
- アロマティック・システムとアンチアロマティック・システムにおけるアビニシオ・プロトンの移転を調査する.
- プロトン伝送障壁における芳香度とストレスの役割を分析する.
- 移行状態における電荷移位と芳香性の関係を調べる.
主な方法:
- アブイニシオ電子構造計算.
- 幾何学的パラメータと芳香度指数の分析.
- サイクル・レファレンス・システムと非サイクル・システムの比較
主要な成果:
- アロマティック系 (ベンゼニウムイオン/ベンゼン,サイクロペンタディエネ/サイクロペンタディエニルアニオン) は,低プロトン伝送障壁を持つ非常に芳香的移行状態を示した.
- 抗芳香性サイクロブテニルカチオン/サイクロブタジーン系は,移行状態で低度な抗芳香性を示した.
- アンチアロマティックシステムのプロトン伝送バリアは予想以上に高く,角度およびトルションストレスの貢献を示唆しました.
- 移行状態での電荷移位は,芳香性の発達と相関せず,陽子の移転に遅れていました.
結論:
- トランジション状態の芳香性は,芳香系における陽子伝達障壁を著しく低下させる.
- 角度とトルションのストレスは,抗芳香系における陽子伝送障壁に寄与する.
- 負荷移位と芳香性の発生は,陽子伝送中の異なるプロセスである.
さらに関連する動画
関連する概念動画
01:14π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
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 annulenes. In...
01:16Electrophilic Aromatic Substitution: Overview
In an electrophilic aromatic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound.
01:18Aromatic Hydrocarbon Anions: Structural Overview
Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous overlap of p...
Due to the absence of continuous overlap of p...
01:14NMR Spectroscopy of Aromatic Compounds
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. Consider...
01:18Frost Circles for Different Conjugated Systems
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.
01:20Criteria for Aromaticity and the Hückel 4n + 2 Rule
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 + 2 rule.
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 + 2 rule.