The significance of energy relaxation of intermediates in complex organic mechanisms

物理有机化学旨在定量地研究有机分子之间反应的机制。这些反应通常涉及几种中间体，它们有一定的寿命范围。反应产物由这些中间产物形成；不同中间体或同一中间体经不同过渡态产生的反应通道经常相互竞争。反应机理一般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。一般假定中间体是通过与溶剂分子的碰撞，使其能量完全松弛而形成的。在这种情况下，它们经过进一步的反应，形成产物，通过碰撞激活来克服能量障碍。过渡态理论应用于反应，通常在其热力学公式中，通过计算激活的能量和熵。有时应用另一种机制，其中假设反应发生得如此之快，中间体通过与溶剂的相互作用没有发生能量松弛。这个项目的目的是调查中间情景的重要性，即发生能量松弛的反应，但不完全。将使用已广泛用于气相反应的方法。反应的势能面将使用电子结构方法计算（这已经在物理有机化学中广泛应用），中间反应的能量依赖率将使用为单分子反应开发的方法计算。然后将使用主方程模型来研究通过与溶剂分子碰撞而产生的能量松弛与反应之间的竞争。将使用的主方程方法是在利兹开发的，并在那里广泛用于气相反应。该方法允许计算单个步骤的反应速率和总反应速率。将对具体反应进行检验，并与实验结果进行比较。此外，一些反应参数（如中间态和过渡态能量）将发生变化，以检验它们对弛豫和反应之间相互作用的影响。利兹大学的计算机代码将被修改，以便更容易使用，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.