高圧下でのベンゼン,ナフタレン,アントラセンのポリモルフィズム
まとめ
高圧・高温実験により,ベンゼンとアントラセンの新相が明らかになり,ナフタレンポリモルフが確認されました. これらのポリサイクルアロマティック炭化水素は,無形炭素に分解し,分子サイズが大きくなるにつれて分解温度が低下します.
科学分野:
- マテリアルサイエンス 材料科学
- 高圧物理 高圧物理
- 有機化学 オーガニック・ケミストリー
背景:
- ベンゼン,ナフタレン,アントラセンのようなポリサイクル芳香炭化水素 (PAH) は,基本的な有機分子です.
- 極端な条件下での彼らの行動を理解することは,材料科学と地球化学にとって極めて重要です.
- 以前の研究では,それらの性質が探求されているが,高圧相行動については,さらなる調査が必要である.
研究 の 目的:
- ベンゼン,ナフタレン,アントラセンの高圧,高温相挙動を調査する.
- これらのPAHの新しい高圧ポリモルフを特定する.
- 極端な条件下でこれらの化合物の分解経路と温度を決定する.
主な方法:
- 顕微鏡を用いた光学観測.
- 高圧 (~40kbarまで) を達成するために,ダイヤモンド・アンビル・プレッシャー・セルを適用する.
- サンプルを高温 (~600°Cまで) に加熱する.
主要な成果:
- ベンゼン (ベンゼンIII) とアントラゼン (アントラゼンII) の新しい高圧相が観察されました.
- 高圧ポリモルフナフタレンIIの存在が確認されました.
- この3つの化合物は,赤色の液体に分解し,最終的に無形炭素へと分解した.
- 分解温度は,分子サイズが増加するにつれて低下することが判明しました.
結論:
- ベンゼン,ナフタレン,アントラセンは,高圧時の相変異がはっきりしている.
- これらのPAHは極端な条件下では不安定で,無形炭素に分解します.
- PAHの分子サイズは,高圧下での熱安定性に影響を与えます.
関連する概念動画
NMR 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...
π 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...
Nomenclature of Aromatic Compounds with a Single Substituent
Benzene is the simplest aromatic hydrocarbon or arene. The IUPAC names for simple monosubstituted benzene derivatives are derived by adding the substituent's name as a prefix to the parent benzene. For example, halobenzene, where the halogen could be fluoro (F), chloro (Cl), bromo (Br), and iodo (I).
Frost 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.
Criteria 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.
NMR 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...

