The significance of energy relaxation of intermediates in complex organic mechanisms

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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.