Intramolecular energy transfer unimolecular reaction rate theory

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. 

In Volume 1 of this series P. J. Robinson gave a brilliant and exhaustive review of experimental studies of unimolecular reactions whidi were published to the end of 1973. The present article covers the literature to early 1976. Rather than continuing to present a similarly complete compilation of experimraital data, we shall concentrate on a complementary description of recent progress in unimolecular reaction rate theory, including selected experiments wMdi deal with the more fundamental aspects of unimolecular processes. In the first part of this review the role of intermolecular energy transfer in unimolecular reactions is discussed. Ihe second part considers the truly intramolecular reaction step. Theory and experiments will be reviewed in a way which clearly shows the connection between them. 

Shalashilin and Thompson [46-48] developed a method based on classical diffusion theory for calculating unimolecular reaction rates in the IVR-limited regime. This method, which they referred to as intramolecular dynamics diffusion theory (IDDT) requires the calculation of short-time ( fs) classical trajectories to determine the rate of energy transfer from the bath modes of the molecule to the reaction coordinate modes. This method, in conjunction with MCVTST, spans the full energy range from the statistical to the dynamical limits. It in essence provides a means of accurately... 

Selective excitation experiments on unimolecular reactions have two main aims. First, to test the proposition, inherent in the statistical theories of unimolecular reactions, that intramolecular energy transfer is extremely rapid and therefore the random lifetime assumption [see equation (1.52)] is valid. Secondly, to measure specific rate constants, k e and compare them with theoretical predictions. In the rest of this section, some of the experimental studies which have had greatest success in fulfilling these objectives are reviewed. 

Slater [13] has criticized the RRKM theory, mainly on the grounds that the proposed model assumes that the intramolecular transfer of energy between vibrational degrees of freedom is very rapid compared to the rate of reaction. In fact, all theories of unimolecular reactions, aside from Slater s, involve this particular assumption. Slater assumes that the rate of intramolecular vibrational-energy transfer is very slow. He then performs a normal-mode analysis of the reacting molecule, assuming that reaction occurs when the bond to be broken reaches a certain critical length. A formally correct expression for is obtained, namely. 

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