Intramolecular energy transfer isomerization theory

MRRKM theory, from RIT, from direct trajectory simulations, and, for reference purposes, from the RRKM theory. In particular, a test of the effect of the RRKM choice of transition state on the predicted rate of isomerization is made by neglecting the contribution of intramolecular energy transfer (Model No. 1). It is seen that the RRKM choice of transition state leads to considerable error the isomerization rate constant predicted is greater than those from the MRRKM theory and RIT by as much as a factor of 4. With intramolecular bottlenecks taken into account, both RIT and the MRRKM theory agree well with trajectory calculations. 

Though not of primary importance to the topic of this chapter, the cyclopropane to propene structural isomerization, it must be noted, has played a prominent role in the development of theory for unimolecular isomerizations and it continues to attract experimental work. The marked dependence of rate on gas pressure and on isotopic substitution exhibited by this isomerizaton provides fundamental insights on intramolecular and vibrational energy transfers and redistributions97"114. 

Several recent studies suggest that the basic assumption of the RRKM theory of rapid intramolecular vibrational-energy transfer is, in fact, essentially correct. For example, several cases have been well documented where the experimental results are contrary to those predicted from physical assumptions of Slater s theory [14]. Butler and Kistiakowsky [15] studied the isomerization of vibrationally excited methylcyclopropane molecules formed by two different chemical reactions (I). 

Unimolecular reactions with thermal, optical, or chemical activation are governed by a competition between intramolecular isomerization, dissociation, or the reverse association (or recombination) processes, and intermolecular energy transfer in collisions. In addition to these traditional unimolecular reactions, many other reaction systems may be considered from a unimolecular point of view when a particular intramolecular event can be separated from preceding or other subsequent processes. Following this more general use of the term, unimolecular reaction rate theory has found a quite general application, and has been harmonized with other theories of reaction dynamics. 

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