ベンゼンの環電流のジャンプフローグ効果
Andrea Ligabue1, Alessandro Soncini, Paolo Lazzeretti
1Dipartimento di Chimica, Università degli Studi di Modena e Reggio Emilia, via G. Campi 183, 41100 Modena, Italy.
Journal of the American Chemical Society
|February 28, 2002
まとめ
ベンゼンの磁気性に関する伝統的なリング電流モデルには欠陥がある. 新しい計算により,パイ電子がパラマグネティックな貢献を示し,芳香分子磁気に関する既存の理論に異議を唱えていることが明らかになった.
科学分野:
- 量子化学とは,量子化学である.
- 理論化学 理論化学について
- 固体物理 固体物理学
背景:
- ポールリング,ロンズデール,ロンドン環電流モデルは,ベンゼンの二磁性性を説明する.
- このモデルは,原子軌道の最小の基礎集合に依存しています.
- パラマグネティック貢献の予測におけるモデルの限界は,完全に理解されていません.
研究 の 目的:
- アロマティックシステムのリング電流モデルの固有の欠点を特定する.
- 先進的な計算方法を使用してベンゼンの磁気感受性を調査する.
- 磁場への反応としてパイ電子の振る舞いを解明する.
主な方法:
- リング電流モデルには対称性アグメントが適用された.
- 高品質の結合ハートリーフォーク (CHF) 計算が行われました.
- ベンゼンのパイ電子における誘導電流密度ベクトル場を分析した.
主要な成果:
- 最小ベースセットは,磁気感受性に対するパラマグネットの貢献を予測するのに不十分です.
- ベンゼンのパイ電子は,六角対称性変形を含む,Larmor以外の軌道を示す.
- 外平面感受性コンポーネントに対するパラマグネットの寄与が特定されました.
- 平行電流密度成分により,π電子運動における"ジャンプフローグ効果"が観察されました.
結論:
- 古典的な環電流モデルは,芳香磁気を説明するのに根本的に欠陥があります.
- 拡張ベースセットは,アロマティック・マグネティズムの正確な理論モデルに必要なものです.
- 観測されたパイ電子の振る舞いは,芳香分子における磁気反応の理解を改訂する必要がある.
関連する概念動画
01:07Structure of Benzene: Kekulé Model
In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
01:18Structure of Benzene: Molecular Orbital Model
According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).
01:18Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism
Birch reduction uses solvated electrons as reducing agents. The reaction converts benzene to 1,4-cyclohexadiene. The reaction proceeds by the transfer of a single electron to the ring to form a benzene radical anion. This anion is highly basic—it abstracts a proton from the alcohol to form a cyclohexadienyl radical. Another single electron transfer gives the cyclohexadienyl anion. A proton transfer from the alcohol forms 1,4-cyclohexadiene. Since this reduction occurs via radical anion...
01:37NMR Spectroscopy of Benzene Derivatives
Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling constants depend...
01:13Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene
Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
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...