大型マクロサイクルにおける芳香安定エネルギーに関する実験的および理論的証拠
Michael Jirásek1, Michel Rickhaus1, Lara Tejerina1
1Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, U.K.
Journal of the American Chemical Society
|January 26, 2021
まとめ
アロマティック安定エネルギー (ASE) は,76π電子を超える大きなポルフィリンナノリングで実験的に検出されました. この発見は,大規模なマクロサイクルシステムにおける芳香性の理解を広げています.
科学分野:
- 有機化学
- 物理化学
- 超分子化学
背景:
- 循環性分子の重要な特徴であるアロマティック性は,熱力学的安定性を高める.
- アロマティック安定エネルギー (ASE) に関する以前の研究は,主に小環システム (最大18π電子) に限られていた.
- より大きなπ結合系におけるASEの実験的検出は,依然として重要な課題でした.
研究 の 目的:
- 大量のπ結合ポルフィリンナノリングにおける芳香的安定エネルギー (ASE) を実験的に検出し,定量化する.
- 伝統的な18π電子の限界を超えたマクロサイクルにおける芳香性の程度を調査する.
- 実験結果と理論的な計算を比較し, [18] アヌレンのような確立された芳香系を比較する.
主な方法:
- イソデミック反応のエネルギー変化 (芳香的<->反芳香的) を計算するためのリドックスポテンシャル解析.
- アロマティック性を乱す形状均衡に対するエネルギーバリアの測定
- 密度関数理論 (DFT) の計算により,実験的観測とASE値を予測する.
主要な成果:
- 76-108π電子のHückel回路を持つカチオンのポルフィリンナノリングにおけるASEの実験検出.
- ポルフィリンナノリングで測定されたASEsは1〜5kJmol−1の範囲であった.
- DFTの計算では,実験データと一致する1〜16kJのmol−1の範囲のASEsを予測した.
結論:
- アロマティック安定化エネルギーは,大きなπ結合ポルフィリンナノリングで実験的に検証可能である.
- この研究は,18π電子を大幅に超えるマクロサイクルへの芳香性の理解を拡大する.
- この発見は,拡張された π システムと大きなマクロサイクルにおける芳香性の基本的な理解に寄与する.
関連する概念動画
01:18Frost Circles for Different Conjugated Systems
3.3K
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.
3.3K
01:18Aromatic Hydrocarbon Anions: Structural Overview
3.3K
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...
Due to the absence of continuous...
3.3K
01:14π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
1.5K
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...
1.5K
01:18Aromatic Hydrocarbon Cations: Structural Overview
3.4K
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
Removing one hydrogen from the intervening CH2 group...
3.4K
01:20Criteria for Aromaticity and the Hückel 4n + 2 Rule
12.1K
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 +...
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 +...
12.1K
02:30Stability of Substituted Cyclohexanes
14.1K
This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
14.1K