Unimolecular and Bimolecular Chemical Processes

Here we give a brief account of some unimolecular processes other than isomerization. No detailed description of bimolecular processes will be offered, except to remark that (1) the knowledge gained from the unimolecular processes is often useful in interpreting the bimolecular processes and (2) in some cases, the bimolecular processes resemble normal diffusion-influenced reactions in the condensed phase. 

Many molecules of interest to chemistry and biology contain H atoms. For the unimolecular dissociation of the excited states of such molecules, it is instructive to think of elimination of a particle or a group of particles. Thus, we can think of elimination of an electron, an H atom, an H2 molecule, an alkane group (in suitable cases), and so forth. Other factors remaining the same, the rate of the process may 

At least seven modes of dissociation are theoretically possible below the ionization threshold, although their total yield in radiolysis is small (Platzman, 1967). The dissociation products are H, H2, O, and OH, where the first two are in their ground (electronic) states but the last two may be either in ground or excited states. Only two modes of dissociation, H20 -H + O and H20 H + OH, are possible for all excitation energies UV photolysis indicates that the latter process is by far (90%) the most likely. Accordingly, in radiolysis there is a tendency to lump the decay of all excited states of the water molecule into H and OH. 

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