Entropy and the Absolute Rate of Chemical Reactions. II. Unimolecular Reactions

Abstract

In this paper the considerations of the previous paper have been developed further and compared with the theory of reaction rates as formulated in terms of a specifically defined activated complex by Eyring. The theory has been applied to a discussion of various unimolecular reactions. A number of cases have been treated by considering the reverse bimolecular or trimolecular association and discussing the extent to which rotational degrees of freedom must be frozen out in order for the associations to occur. Other cases have been treated by the activated complex method, which involves discussion of the number of free rotations and the frequency of the vibrations in the complex. It has been shown that it is possible to account for the rates of a considerable number of unimolecular reactions by making reasonable assumptions and that there is a considerable class of unimolecular reactions which conform to what is designated as the ``hypothesis of exact orientation,'' the only necessary assumption being that the rotational degrees of freedom of the fragments which recombine in the reverse reaction must be frozen out just sufficiently so that they correspond as regards their entropy terms to the resulting vibrational degrees of freedom of the molecule formed.