在SN2过渡状态下对曲对二次动态同位素效应的影响的计算研究
Faraj Hasanayn1, Andrew Streitwieser, Rasha Al-Rifai
1Department of Chemistry, American University of Beirut, Beirut, Lebanon. fh19@aub.edu.lb
Journal of the American Chemical Society
|February 17, 2005
概括
二次 deuterium 动态同位素效应 (KIEs) 可以区分 SN2 逆转和保留机制. 大的KIEs表明保留,而小的KIEs则表明CH3F与F-.等反应的反转.
科学领域:
- 物理化学 物理化学
- 化学动力学 化学动力学
- 计算化学的计算化学
背景情况:
- 在有机化学中,SN2反应机制至关重要,可以通过反转和保留途径.
- 在实验中区分这些SN2机制可能是具有挑战性的.
- 动态同位素效应 (KIE) 为反应动态提供了一个敏感的探测器.
研究的目的:
- 计算和分析涉及甲基化物 (CH3F) 的SN2反应的二次动态同位素效应 (KIE).
- 确定KIE是否可以作为可靠的可观测物来区分SN2反转和保留机制.
- 在离子分子和离子对反应中研究KIEs.
主要方法:
- 传统的过渡状态理论被用于计算.
- 为CH3F与F-.的反转和保留SN2通路计算了二次KIEs.
- 计算扩展到涉及CH3F和LiF的离子对反应.
主要成果:
- 为反转SN2机制计算了一个小的KIE (0.98在298K),归因于平衡的和值.
- 对于保留SN2机制来说,计算出了一个大的KIE (1.5).
- 在离子对反应中,对于反转和保留路径也观察到很大的KIEs.
- 与大型KIEs相关的过渡结构表现出曲的FCF角度和对敏感的虚构频率.
结论:
- 二次KIE是一种可行的实验可观测物,用于区分SN2反转和保留机制.
- 计算的KIE值为机制歧视提供了理论支持.
- 观察到的大型KIEs,曲过渡结构和敏感的想象频率之间的相关性为反应动态提供了洞察力.
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相关概念视频
02:14SN2 Reaction: Kinetics
Kinetic Studies and Significance
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a reaction.
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a reaction.
02:27SN2 Reaction: Mechanism
The kinetic studies of SN2 reactions suggest an essential feature of its mechanism: it is a single-step process without intermediates. Here, both the nucleophile and the substrate participate in the rate-determining step.
The presence of the more electronegative halogen in the substrate creates a polarized carbon-halide bond. The halide pulls the electron cloud generating an electrophilic center at the carbon atom. Thus, the carbon atom carries a partial positive charge while the halide has a...
The presence of the more electronegative halogen in the substrate creates a polarized carbon-halide bond. The halide pulls the electron cloud generating an electrophilic center at the carbon atom. Thus, the carbon atom carries a partial positive charge while the halide has a...
02:26SN2 Reaction: Transition State
An SN2 reaction of an alkyl halide is a single-step process in which bond formation between the nucleophile and the substrate and bond breaking between the substrate and the halide occurs simultaneously through a transition state without forming an intermediate.
When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken...
When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken...
02:23SN2 Reaction: Stereochemistry
In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
01:08Spin–Spin Coupling Constant: Overview
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
01:06Nuclear Overhauser Enhancement (NOE)
Irradiation of a spin-active nucleus causes an increase or decrease in the signal intensity of neighboring nuclei that are not necessarily chemically bonded or involved in J-coupling. This phenomenon, called the nuclear Overhauser enhancement (NOE), results from through-space interactions between the nuclear spins. The NOE effect decreases with increasing internuclear distance and is generally not observed beyond 4 angstroms. In NOE, dipole-dipole interactions between neighboring spin-active...