Combination reactions producing vibrational excitation

The situation with regard to combination reactions can be summarized briefly. They should produce excited products. The formation of a chemical bond is invariably exothermic. It would come as a great surprise to the kineticist to find a combination reaction that did not show excitation. 

Nevertheless, spectroscopic evidence for this state of affairs is very limited. One example is the H+HX system65. Chemiluminescence of the HX is found. For the diagnostic case with D atoms, the reaction 

These not only show the excitation of the product, but the transfer of energy to the third body. Additional evidence is provided by the combination66-70 

By far, most of the evidence for excitation upon combination comes from an analysis of the kinetics of the reaction. The rate depends upon the pressure of the third body —that is, the newly formed molecule decomposes unless it is deactivated. In theory this deactivation need not remove a large amount of energy on each collision—merely enough to put the new molecule below its dissociation energy. From that point on, the higher efficiency for deactivation versus reacti- 

Combination and addition reactions have been used effectively for the study of excited species. In effect, chemi-excitation reactions have been used for synthesis of reagents of known excitation energy1,72-81. A major effort has been made to use such excited molecules as tools for the exploration of the details of uni-molecular decomposition reactions (see Rabinovitch and Setser82). 

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Chemical effects follow as a result of the transfer of some of the energy of electronic excitation or ion neutralization into vibrational energy sufficient to break bonds. Most of these chemical events result in the formation of free radicals, which may combine with each other or react with other molecules in their vicinity, the ultimate chemical change produced in the system being determined by the proximity of the activated molecules or radicals to each other and the activation energies of possible competing reactions. 

If the reaction passes through formation of a compound nucleus, having a lifetime of many nuclear vibrations, we should expect that the amounts and types of reaction products were independent of the projectile — target combination used to produce a specific compound nucleus and only depend on its excitation energy. This has indeed been found to be the case for some kinds of nuclear reactions, see Ch. 14. 

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