相关概念视频
02:46Halogenation of Alkenes
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Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
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01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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01:14Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene
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Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
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02:38Electrophilic Addition to Alkynes: Halogenation
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Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
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在二维混合化矿中以光驱动的内部化同质化
Wenxin Mao1,2, Enamul Haque3, Stephanie A Bird4
1Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, U.K.
Journal of the American Chemical Society
|January 13, 2026
概括
二维金属化物矿在单个八面体内表现出光驱动的可逆离子异构. 这种内八面体切换调整了光电子特性,为可重新配置的设备提供了新的可能性.
科学领域:
- 材料科学
- 固态化学
- 量子光学
背景情况:
- 二维 (2D) 金属化物矿对量子光发射和神经形态计算具有前景.
- 它们的功能通常与量子封闭和可调节的光电子特性有关.
- 通过利用八面体内不同的化物位点,在子单元细胞层面上实现工程功能.
研究的目的:
- 在单晶二维矿中研究光驱的可逆离子异构.
- 了解八面体内化物切换的机制及其对光电子特性的影响.
- 探索这种现象在先进的光电子应用中的潜力.
主要方法:
- 超光谱成像
- 在现场X射线衍射
- 第一个原理计算 (密度函数理论)
主要成果:
- 在BA2PbBrxI4-x (x=1-3) 中,在单个PbX6(4-) 八面体中证明了可逆的离子同质化.
- 由于八面体内化物位点的切换,观察到光学带隙的调制约0.1 eV.
- 估计可逆电子带隙转移高达0.5 eV,归因于价值带特征的重新分配.
结论:
- 在二维矿中发现结构编码的选择性光异构化机制.
- 展示了可重新配置的光电子设备的新策略.
- 在非易失光学内存和量子光子学中突出了潜在的应用.