矩阵隔离,光谱表征和m-xylylene的光异构
Patrik Neuhaus1, Dirk Grote, Wolfram Sander
1Lehrstuhl für Organische Chemie II der Ruhr-Universität, D-44780 Bochum, Germany.
Journal of the American Chemical Society
|February 19, 2008
概括
研究人员开发了一种有效的合成m-xylylene,一个反应性二基. 这种化合物被困在中,并使用多种光谱方法进行了表征,为其特性提供了新的见解.
科学领域:
- 有机化学 有机化学
- 频谱学是一种光谱学.
- 计算化学的计算化学
背景情况:
- 以前的m-xylylene (1) 合成是有限的,只能在有机玻璃中产生.
- 之前对m-xylylene的光谱鉴定仅限于光和电子磁共振 (EPR) 光谱.
研究的目的:
- 报告一种新的,高效的m-xylylene (1) 合成方法.
- 用先进的光谱技术来描述二极端的特征.
- 为了研究m-xylylene的可照性,并确定其光产物.
主要方法:
- 有效的合成m-xylylene.
- 在低温 (10 K) 处在基矩阵中捕获二基.
- 使用红外 (IR),紫外线可见 (UV-vis) 和EPR光谱进行表征.
- 用红外光谱和密度函数理论 (DFT) 计算进行光解研究和同位素碳化合物的鉴定.
主要成果:
- 为m-xylylene (1) 建立了一个新的,高效的合成途径.
- 在基矩阵中成功捕获和表征m-烯.
- 光谱数据 (红外线,紫外线,EPR) 提供了全面的识别.
- 对m-xylylene的光解产生了三个同位素碳化合物 (7,9和11).
- 通过将实验性IR光谱与DFT计算结果进行比较来确定异构体.
结论:
- 开发的合成为产生m-xylylene提供了一种更通用的方法.
- 矩阵隔离和先进的光谱学使反应性二极根的详细表征成为可能.
- 了解m-xylylene的光化学对于其处理和应用至关重要.
相关概念视频
01:26Photochemical Electrocyclic Reactions: Stereochemistry
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
01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
01:13¹H NMR: Complex Splitting
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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.
01:17Thermal Electrocyclic Reactions: Stereochemistry
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.
02:33Stereoisomerism of Cyclic Compounds
In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...