The significance of energy relaxation of intermediates in complex organic mechanisms

物理有机化学旨在定量研究有机分子之间的反应机理。这些反应通常涉及几个中间体，它们具有一定的寿命范围。反应产物由这些中间体形成;经常存在竞争反应通道，其通过不同的中间体或通过不同的过渡态从相同的中间体发生。反应机理通常通过观察动力学和不同产物的比例进行实验研究;变量包括温度和溶剂。一般认为，中间体是通过与溶剂分子碰撞而形成的，其能量完全弛豫。在这种情况下，它们经历进一步的反应，通过碰撞活化形成产物，以克服能垒。过渡态理论是适用于反应，往往在其热力学公式，通过计算能量和熵的活化。另一种机制有时被应用，其中假设反应发生得如此之快，以至于中间体通过与溶剂的相互作用没有经历能量弛豫。这个项目的目的是调查的重要性，中间的情况下，即反应中发生能量弛豫，但不完整。将使用已广泛用于气相反应的方法。该反应的势能面将使用电子结构方法计算（这已经广泛应用于物理有机化学），并使用为单分子反应开发的方法计算中间体的能量依赖反应速率。主方程模型，然后将被用来调查通过碰撞与溶剂分子和反应的能量弛豫之间的竞争。将使用的主方程方法已在利兹开发，并广泛用于气相反应。该方法允许计算各个步骤和总反应的反应速率。具体的反应将被检查，并将结果与实验进行比较。此外，一些反应参数将被改变（例如中间态和过渡态能量），以检查它们对弛豫和反应之间相互作用的影响。利兹计算机代码将被修改，以便它可以更容易地使用，并通过网络提供给其他人。

Physical organic chemistry seeks to investigate quantitatively the mechanisms of reactions between organic molecules. These reactions generally involve several intermediates, which have a range of lifetimes. Reaction products are formed from these intermediates; there are frequently competing reaction channels which occur either through different intermediates or from the same intermediate via different transition states. The reaction mechanism is generally investigated experimentally from observations of the kinetics and of the ratios of the different products; variables include temperature and the solvent. It is generally assumed that the intermediates are formed with their energies fully relaxed, through collisions with the solvent molecules. In this situation, they undergo further reaction, to form the products, by collisional activation to overcome the energy barriers. Transition state theory is applied to the reactions, often in its thermodynamic formulation, via calculation of energies and entropies of activation. An alternative mechanism is sometimes applied, in which it is assumed that reaction occurs so quickly that the intermediate undergoes no energy relaxation through interaction with the solvent. The aim of this project is to investigate the importance of the intermediate scenario, ie reactions in which energy relaxation occurs, but is incomplete. Methods will be used that have been employed extensively for gas phase reactions. The potential energy surface for the reaction will be calculated using electronic structure methods (this is already widely employed in physical organic chemistry) and energy dependent rates of reaction of the intermediate calculated using methods developed for unimolecular reactions. A master equation model will then be used to investigate the competition between energy relaxation through collision with the solvent molecules and reaction. The master equation methods that will be used have been developed at Leeds and are extensively used there for gas phase reactions. The methodology allows rates of reaction of the individual steps and of the overall reaction to be calculated. Specific reactions will be examined and the results compared with experiment. In addition, some of the reaction parameters will be varied (e.g. intermediate and transition state energies) to examine their effects on the interplay between relaxation and reaction.The Leeds computer code will be modified so that it can be used more easily, and made available to others via the web.