甲与原子在精确的潜在能量表面上的反应动力学
1Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, GA 30322, USA. czako@chem.elte.hu
Science (New York, N.Y.)
|October 25, 2011
概括
研究人员为原子和甲反应开发了一个精确的潜在能量表面. 这个表面准确地预测了化产品的旋转分布,挑战了以前的反应性假设.
科学领域:
- 化学动力学 化学动力学
- 理论化学是一种理论化学.
- 分子动力学分子动力学
背景情况:
- 原子和甲之间的反应是理解多原子反应动态的关键基准.
- 以前的理论模型很难准确地复制这种反应系统的实验结果.
- 特定的分子振动影响反应速率的模式特定反应性是一个复杂的研究领域.
研究的目的:
- 为原子-甲反应开发一个准确的全球潜在能量表面.
- 从理论上研究该系统的选择性反应模式,并与实验结果进行比较.
- 了解振动激发与转化能量在促进反应中的作用.
主要方法:
- 为Cl + CH4系统开发一个准确的全球潜在能量表面.
- 在新的潜在能量表面上进行的准经典轨迹 (QCT) 计算.
- 理论预测与现有实验数据的比较,特别是产品旋转分布.
主要成果:
- 新开发的潜在能量表面与化 (HCl) 产品旋转分布的实验数据有很好的一致性.
- 对于低碰撞能量的Cl + CHD(3) 反应,理论计算证实,激发CH-拉伸振动在驱动反应方面并不比转化能更有效.
- 这一发现与基于众多原子二原子反应研究的预期相矛盾.
结论:
- 精确的潜在能量表面为未来对这一重要的反应进行理论研究提供了可靠的工具.
- 这些结果突显了多原子系统中模式特异反应的复杂性,特别是在晚屏障反应中.
- 进一步的理论和实验研究是有必要的,以充分阐明控制Cl + 甲系统中反应性的因素.
相关概念视频
01:27Radical Substitution: Halogenation of Alkanes and Alkyl Substituents
In the presence of heat or light, alkanes react with molecular halogens to form alkyl halides by a substitution reaction called radical halogenation. This reaction has three steps: initiation, propagation, and termination, as seen in the radical chlorination of methane to produce methyl chloride.
In the initiation step of the reaction, the chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. Propagation occurs in two...
In the initiation step of the reaction, the chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. Propagation occurs in two...
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.
01:34Radical Halogenation: Thermodynamics
The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy between bonds broken and bonds...
01:31Radical Substitution: Allylic Chlorination
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...
01:32Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics
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01:27Inductive Effects on Chemical Shift: Overview
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...