一个延伸的形交叉与多种衰变路径相关:在O-基甲中激发状态的分子内质子转移
Annapaola Migani1, Lluís Blancafort, Michael A Robb
1Institut de Química Computacional, Departament de Química, Universitat de Girona, 17071 Girona, Spain. amigani@stark.udg.edu
Journal of the American Chemical Society
|May 14, 2008
概括
-基甲 (OHBA) 的光稳定性是由复杂的衰变通路网络控制的. 它的激发状态分子内质子转移 (ESIPT) 反应性是由平面潜在能量表面上可访问的形交叉点决定的.
科学领域:
- 摄影化学的使用.
- 物理化学 物理化学
- 分子动力学分子动力学
背景情况:
- O-基甲 (OHBA) 是理解光保护机制的关键分子.
- 激发状态的分子内质子转移 (ESIPT) 是许多有机分子中至关重要的光物理过程.
- 像OHBA这样的分子的光稳定性与它们的兴奋状态潜在能量表面和衰变途径密切相关.
研究的目的:
- 为了阐明OHBA的光稳定性与其形交叉之间的关系.
- 为了映射出来自OHBA激发状态的多重衰变路径.
- 了解不同衰变通道如何竞争,影响产品形成.
主要方法:
- 对OHBA的兴奋状态潜在能量表面的理论研究.
- 标识和表征形交叉结构 (HBI,ZEI,TPI) 的方法.
- 分析形交叉的连接性和可访问的衰变路径.
主要成果:
- OHBA表现出一个平坦的兴奋状态潜在能量表面,对形交叉点的障碍很低.
- 一个连续的接连接各种形交叉结构,包括HBI,ZEI和TPI.
- 产品分布是由激发后形交叉上的特定接入点决定的.
结论:
- OHBA的光化学是由延伸的形交叉和竞争的衰变通路控制的.
- 交叉点的复杂网络允许各种光反应,包括分体化,异体化和 adduct 形成.
- 这些发现可以概括为甲基酸盐和酸等相关分子,突出显示出一种共同的光保护机制.
相关概念视频
01:46Oxidative Cleavage of Alkenes: Ozonolysis
In ozonolysis, ozone is used to cleave a carbon–carbon double bond to form aldehydes and ketones, or carboxylic acids, depending on the work-up.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
03:08Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
02:04Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids
Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.
01:09Conjugate Addition to α,β-Unsaturated Carbonyl Compounds
α,β-Unsaturated carbonyl compounds are molecules bearing a carbonyl and alkene functionality in conjugation with each other. The conjugation in the molecule leads to three resonance structures. The hybrid form exhibits two probable electrophilic sites: the carbonyl carbon and the β carbon.
01:27Aldol Condensation with β-Diesters: Knoevenagel Condensation
The Knoevenagel condensation is an aldol-type reaction involving the condensation of aldehydes or ketones with active methylene compounds such as β-diesters to produce substituted olefins.
02:36Regioselectivity and Stereochemistry of Hydroboration
A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.