Theoretical aspects of unimolecular gas reactions.

Abstract

A brief review is made of the various older theories of unimolecular reactions and then the present situation is examined with particular attention to the theory of N. B. Slater. The assumptions implicit in the different reaction models are made clear and the model of Hinshelwood, Kassel, Rice and Ramsperger (HKRR) is contrasted with that of Slater. Molecular vibration theory is sketched and it is shown how the Wilson FG matrix method may be conveniently adapted to the calculation of the Slater parameters. Both the HKRR and Slater theories are tested on the experimental data for the decompositions of N2O, O3, H2O 2, CH3CH2Cl and more approximately in the case of N2O5 and C2H6. Slater's calculations on the isomerization of cyclopropane are discussed. From the results of these calculations certain deductions are made regarding the nature of a reacting species and especially the possibilities for energy transfer between normal modes. At the same time, a number of inadequacies in the Slater theory, hitherto not appreciated, have become evident and these lead to a critical evaluation of the entire theory. Through the use of an electronic computer in the ethyl chloride calculations there has evolved a systematic approach to the standard problem of adjusting potential constants to produce a desired set of frequencies. Certain considerations in the choice of program and the form in which the problem is submitted to the computer have become apparent and are fully discussed.