热化学的同位素反应的层次结构
Steven E Wheeler1, Kendall N Houk, Paul v R Schleyer
1Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA. swheele2@chem.ucla.edu
Journal of the American Chemical Society
|February 3, 2009
概括
计算热化学从新的反应类定义中受益,特别是碳化合物的高同位分离反应 (RC5) 和同位分离反应 (RC4),提供了更高的准确性.
科学领域:
- 计算化学是一种计算化学.
- 热化学 热化学 热化学
- 量子化学是一种量子化学.
背景情况:
- 在计算热化学中,平衡键类型和杂交的化学方程对于提高精度至关重要.
- 现有的"同源性"反应的定义经常被混,并且不相等.
- 对于可靠的热化学计算,需要明确反应类的层次结构.
研究的目的:
- 为了澄清围绕同位素反应定义的广泛混.
- 在计算热化学中为反应类引入新的,一致的定义.
- 为碳化合物建立反应类 (RC1-RC5) 的层次结构.
主要方法:
- 为碳化合物制定一致的反应类 (RC1-RC5) 层次.
- 对等离体键分离反应对等级中的所有类的概括.
- 运用ab initio和密度函数方法来对38种碳化合物的结合分离反应进行应用.
主要成果:
- 证明同位素反应的两个最常见的定义是不等价的.
- 建立一个层次结构:异构 (RC1) 异构 (RC2) 低同构 (RC3) 同构 (RC4) 高同构 (RC5).
- 在RC4和RC5反应中,在各种理论水平上,结合的错误总是<0.4 kcal mol (-1) 的键分离度.
结论:
- 与其他方法相比,高同位分离 (RC5) 和同位分离 (RC4) 键分离反应在热化学计算中提供了更高的准确性.
- 新的层次结构和通用的键分离反应为计算热化学提供了一个强大的框架.
- 通过使用推的方法确定了聚烯和相关化合物形成的精确度.
相关概念视频
Thermochemical Equations
For a chemical reaction (the system) carried out at constant pressure – with the only work done caused by expansion or contraction – the enthalpy of reaction (also called the heat of reaction, ΔHrxn) is equal to the heat exchanged with the surroundings (qp).
Hess's Law
There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes. Some reactions are difficult, if not impossible, to investigate and make accurate measurements for experimentally. And even when a reaction is not hard to perform or measure, it is convenient to be able to determine the heat involved in a reaction without having to perform an experiment.
Thermodynamic Background
The law of mass action states that "the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants." It means that the more 'active mass' or 'concentration' of the reactants present, the faster the reaction will proceed.In a chemical reaction, there are forward and reverse reactions. The forward reaction is the process where the reactants combine to form products. The reverse reaction is the process where the products break down to form the...
Thermal Sigmatropic Reactions: Overview
Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as...
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as...
Enthalpies of Chemical Changes
The enthalpy change of a chemical reaction is the difference between the total enthalpy of the products and the reactants. This energy change for the molecules involved in a chemical reaction can be visualized through an energy diagram, where the lines represent the total enthalpy of the reactants and products, and the gap between them signifies the enthalpy change for the reaction. In an exothermic reaction, the system's enthalpy decreases, whereas it increases in an endothermic reaction.The...
Energy Diagrams, Transition States, and Intermediates
Free-energy diagrams, or reaction coordinate diagrams, are graphs showing the energy changes that occur during a chemical reaction. The reaction coordinate represented on the horizontal axis shows how far the reaction has progressed structurally. Positions along the x-axis close to the reactants have structures resembling the reactants, while positions close to the products resemble the products. Peaks on the energy diagram represent stable structures with measurable lifetimes, while other...


