PHOTOCHEMISTRY 19 



Secondary steps are, of course, not restricted to reactions involving 

 radicals; the reactive species may be a relatively unstable molecule or an 

 excited atom or molecule. Tn some cases the excited molecule in a sing- 

 let, fluorescent state may enter into the reaction, as is illustrated by 



Hg*(6^Pi) + H2^HgH-^H, 

 "" ^'cH^ + 02^ peroxide. 



Most photochemical reactions of complex molecules, such as the autooxi- 

 dation of rubrene, which take place by way of an excited state appear to 

 involve a long-lived (possibly triplet) excited state. 



UNIMOLECULAR STEPS 



True unimolecular reaction steps are limited to complex radicals or 

 molecules. They can result in the formation of (1) two radicals, (2) two 

 stable molecules, or (3) a stable molecule and a radical: 



(1) Hg(C2H5)2^HgC2H5 + C.>H,, 



(2) CH3OH -^ HCHO + H2, 



(3) CH.CO -> CH3 + CO. 



Internal rearrangements of complex molecules can be the result of uni- 

 molecular reactions. Examples of this type, which have been studied, 

 include cis-trans isomerizations, racemizations (probably), as well as reac- 

 tions of the following type: 



CHo 



CH2 CHo — > CH:{ — CH = CH2. 



Many first-order reactions which occur in solution involve a molecule of 

 the solvent and are therefore bimolecular rather than unimolecular 

 reactions. 



TERMOLECUI.AR STEPS 



With few possible exceptions, termolecular gas-phase reactions are 

 recombinations of atoms (or radicals), occurring as three-body collisions 

 iiu'olving some other molecule or radical, i.e., 



2Br + No^ Br2 + N2. 



In solution, termolecular reactions involving one or more molecules of the 

 solvent are probably of much more frecjuent occurrence. 



DIFFUSION-CONTROLLED PROCESSES 



The rates of some reaction steps which take place in condensed systems 

 or in gases at moderate or high pressures are controlled by diffusion proc- 

 esses. Most of these reactions involve heterogeneous or microhetero- 

 geneous media. There is an important class of such reactions which occur 



