Kinetic Isotope Effects in the Pyrolysis of Cyclopropane and Cyclobutane

An analysis of the recent kinetic isotope effects in the pyrolysis of cyclopropane and cyclopropane-d6 is shown to be in excellent agreement with the diradical path for the reaction. This is true at the high-pressure limit and possibly near the low-pressure limit for the reaction. Using the same transition state for the isomerization with a structure about half-way between trimethylene and propylene and containing a bridged H atom, leads to a frequency factor for the high-pressure rate constant in excellent agreement with the experimental data. It is also shown, from the examples of the C2H6 and H2O2 pyrolyses that ``secondary'' isotope effects may have temperature coefficients as large as or larger than those associated with ``primary'' isotope effects. Extending the same type of analysis to cyclobutane pyrolysis leads to an excellent a priori calculation of the frequency factor for the high-pressure rate constant and of the kinetic isotope effects in cyclobutane-d8. It is further shown that the apparently discordant kinetic isotope effects reported for[Complex chemical formula]are actually in good agreement with those for cyclo-C4D8. Finally, the use of group additivity relations in kinetics is explored and shown to lead to excellent predictions in the reactions of isotopically labeled species with different isotopic content. Their application to other types of reactions cannot be evaluated at present because of insufficient data.