The application of quantum mechanics to chemical kinetics

Abstract

Much work has recently been done on the application of quantum mechanics to chemical reactions. In the majority of cases, however, the actual reaction processes have been considered as taking place according to the laws of classical mechanics, quantum-mechanical theory being only employed in calculating the interatomic forces. It has, however, been suggested by various authors that the actual transition processes involved must be treated as non-classical. Some of these authors have claimed that this method of treatment is essential for the true explanation of chemical processes, just as in the case of radioactive disintegration, where it is well established that classical considerations are unable to explain the phenomena observed. It appears, however, to be the general consensus of opinion that for chemical processes the results obtained by a strict quantum-mechanical treatment would differ negligibly from the results of classical mechanics. This opinion appears to be based only on approximate methods of treatment, and no actual figures have been published. The present paper is a contribution to a more exact knowledge of the problem. According to modern views on reaction mechanism, the reacting system passes through a maximum of potential energy in passing adiabatically from the initial to the final state. The energy difference between the initial state and the maximum is the heat of activation for the reaction (E). As a simple type of chemical reaction we may take the system shown in fig. 1. A particle of mass m passes from a to b through a region of varying potential energy V ( x ). Between a and b the potential energy reaches a maximum value E. The energy difference between a and b is Q, the heat of reaction.