Unimolecular rate constants, isotope effects

Investigation of this reaction isolated in an N2 matrix at 10-20 K has shown that the apparent activation energy is smaller than 0.11 kcal/mol the unimolecular rate constant for N0-03 reactant complexes prepared in this way is 1.4 x 10-5 s-1 at 12 K. Experiments carried out using ozone enriched with 180 have revealed no observable isotope effect. 

Unimolecular Reactions. 1. A Statistical Formulation for the Rate Constants, (b) F. Remade, D. Dehareng, and J. C. Lorquet, J. Phys. Chem., 92, 4784 (1988). Nonadiabatic Unimolecular Reactions. 2. Isotope Effects on the Kinetic Energy Release. 

As pointed out before kuni is a pseudo first order rate constant. Since kuni/[M] is independent of [M], kuni/[M] is a second order rate constant at low pressure. It is significant and important for consideration of isotope effects that this second order rate constant for unimolecular reactions depends only on the energy levels of reactant molecules A and excited molecules A, and on the minimum energy Eo required for reaction. It does not depend on the energy levels of the transition state. There will be further discussion of this point in the following section. 

Experimental rate constants, kinetic isotope effects and chemical branching ratios for the CF2CFCICH3-do, -d, -d2, and -d2 molecules have been experimentally measured and interpreted using statistical unimolecular reaction rate theory.52 The structural properties of the transition states needed for the theory have been calculated by DFT at the B3PW91 /6-31 G(d,p/) level. 

---