PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 205 



p. 81) distinguishes between those in which the interaction is primarily 

 physical (e.g., type III) and those in which it is primarily chemical (e.g., 

 type VII). It is important to appreciate, however, that this distinction 

 is often ill defined; thus, type VI may result in chemical change, but the 

 interaction during the collision may be one ordinarily considered to be of 

 physical nature. 



Two general principles influence the probability of collisions of the 

 second kind. They are: first, that the probability is the greater, the 

 smaller the electronic energy that has to be transformed into or from 

 translational energy of heavy entities such as atoms; and second, that 

 the probability is greater, other factors being equal, for that process in 

 which the total resultant spin of the two collision partners is unchanged. 

 The first of these, often called the "resonance rule," is explained by 

 the Franck-Condon principle (Sect. 3-2c), and may be treated in some 

 cases as a special instance of quantum-mechanical resonance; the second, 

 often called the "spin-conservation rule," is an approximate result of a 

 quantum-mechanical calculation which cannot be carried through with 

 great accuracy. The first has been abundantly verified (Massey and 

 Burhop, 1952; Willey, 1937) — for example, by the relative intensities of 

 various lines in sensitized fluorescence. The second has been tested and 

 verified in relatively few cases (Massey and Burhop, 1952; Willey, 1937) ; 

 it is less generally applicable, and has smaller quantitative effect than 

 the first. 



It is an evident truth that collisions of the second kind play a vital role 

 in the mechanisms of many chemical, photochemical, and radiation- 

 chemical reactions — and doubtless in radiobiological reactions as well. 



3-ld. Exchange Reactions.^ One of the most common ways in which an 

 excited atom can initiate chemical change is by the type VII of collision 

 of the second kind just described — often called simply an exchange 

 reaction. This reaction can take place only if the sum of excitation 

 energy of A and dissociation energy of AB is greater than the dissociation 

 energy of BC. The energy excess must be divided between vibrational 

 energy of AB and translational energy of the products. This partition, 

 and its statistical distribution in any one case, must be expected to vary 

 markedly from one instance to another. If the excess energy exceeds the 

 dissociation energy of A B, in a certain fraction of processes the molecule 

 AB will be left in a high enough "vibrational" state so that it will dissociate 

 spontaneously. This is equivalent to process VI, and it is difficult to 

 attain knowledge, either experimentally or theoretically, on the per- 

 centages of process VI that proceed directly or via VII. 



Exchange reactions may have great probability ; cross sections as great 

 as 10"^^ cm- have been reported. [For the definition of the effective 



9 For further details see Massey and Burhop (1952); Willey (1937); Laidler and 

 Shuler (1951); Massey (1949). 



