Transition state switching

Chesnavich W J, Bass L, Su T and Bowers M T 1981 Multiple transition states in unimolecular reactions a transition state switching model. Application to C Hg" J. Chem. Rhys. 74 2228—46... 

R. A. Marcus My interests in variational microcanonical transition state theory with J conservation goes back to a J. Chem. Phys. 1965 paper [1], and perhaps I could make a few comments. First, using a variational treatment we showed with Steve Klippenstein a few years ago that the transition-state switching mentioned by Prof. Lorquet poses no major problem The calculations sometimes reveal two, instead of one, bottlenecks (transition states, position of minimum entropy along the reaction coordinate) [2], and then one can use a method described by Miller and partly anticipated by Wigner and Hirschfelder to calculate the net dux. 

Transition-state switching may also occur in SACM, either for single-channel potential curves or for groups of channels such as in microcanonical variational versions with adiabatic channel states. 

The validity of the statistical approximation in QET-RRKM-phase-space theories has been tested extensively both experimentally and theoretically. Some of the most incisive tests of statistical theory have been performed with energy-selected ions, prepared as described previously, using methods such as photoelectron-photoion coincidence (PEPICO). Particularly incisive tests of these statistical models have been performed on isomeric C4Hg systems by Baer and Bowers and their co-workers and on C4H by Bowers and collaborators. The latter study provided validation of an important variant of statistical theory called the transition state switching model. 

This behaviour is termed transition state switching its importance in the ISM can be illustrated by reference to 0( P) + aUcene reactions. The reactions involve the formation of an adduct, which can either be colhsionally stabilised or can dissociate to form products. In the case of the reaction with ethene, the latter include CH2CHO + H and CH3 + HCO. The overall rate determining step is adduct formation. 

Figure 1.8 (a) Universal TSS relation for C-C, C-O, C-N, N-O, N-N, and 0-0 dissociation reactions. With kind permission from Springer Science and Business Media,S. Wang et al.. Cat. Lett., 2011, 141, 370-373. (b) Deviations from the universal TSS behavior are observed when the nature of the transition state switches between initial and final states alike. From Ref. 50 with permission from A. Vojvodic et al., J. Chem. Phys., 2011, 134, 244509. Copyright 2011, AIP Publishing LLC. 

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