Non-RRKM kinetics

A much more detailed microscopic picture of a unimolecular reaction may be obtained from time-domain experiments in which the number of reactant and/or product molecules is followed in real time [20-26]. At this level of experimental measurement, differences with the RRKM model have been observed. The unimolecular reactions studied in this manner include HjOj 20H [20], NCNO CN + NO [21], HOCO — HO -b CO [22,23], N02 N0-b0 [24,25], and CH3CO CO + CH3 [26]. Pronounced non-RRKM kinetics were observed for the latter two reactions. 

Vande Linde SR, Hase WL. Non-RRKM kinetics in gas-phase Sj 2 nucleophilic substitution. J Phys Chem. 1990 94 6148 50. 

Bunker D L and Hase W L 1973 On non-RRKM unimolecular kinetics molecules in general and CH3NC in particulars Chem. Rhys. 59 4621-32... 

The invocation of non-RRKM behavior in thermal reactions is ordinarily frowned upon by the chemical kinetic community. Here, however, we are dealing with rapid non-adiabatic isomerization of two constitutionally and structurally identical, but energetically different, triplets. It may well be a case in which strong and sometimes peculiar effects (e.g. isotope effects) can be brought about by. .. the non-constancy of the transmission coefficient as a function of kinetic energy [47, 48]. 

Bunker DL, Hase WL (1973) On non-RRKM imimolecular kinetics molecules in general and CH3NC in particular. J Chem Phys 59 4621-4632... 

That surfaces with a linear HCC equilibrium geometry give rise to intrinsic non-RRKM lifetimes is consistent with the fact that the coupling between the HCC bend and the HC and CC stretch vibrational momenta is zero for 0 = 180° and a maximum for 0 = 90°. This can be seen by writing the vibrational kinetic energy in internal coordinates using... 

Simplified Rate Ratio for Non-Equilibrium Activation. Deuterium labeling is widely used and isotope effects may be sufficiently large to alter significantly the kinetic behavior of these systems. The secondary isotope effect, especially, serves to illustrate some recent developments in the formulation and application of the RRKM theory. 

---