Effect of Pressure on the Isotope Effect in a Unimolecular Gaseous Reaction : Tritium and Carbon-13 Effects in the Isomerization of Cyclopropane

As a new means of studying the effect of pressure on unimolecular gaseous reaction kinetics, the tritium isotope effect in the isomerization of cyclopropane to propylene has been measured. At a pressure of 200 mm, the ratio of rate constants for unlabeled and singly-labeled species is given by k/k′ = (0.86±0.06) exp (385±95/RT). The temperature range was 406–492°C. As the pressure decreases, the isotope effect is lowered and, within experimental error, disappears at 1 mm. Upon the addition of inert gas to cyclopropane at this pressure, the isotope effect is increased to a value corresponding to a higher pressure of reactant. The relation between these results and the calculations of N. B. Slater on the kinetics of this reaction is discussed. The carbon-13 isotope effect was measured at 1 atmos and 492°, and was found to be k/k′=1.0072±0.0006. This may be explained on the basis of a change in either a carbon-carbon or a carbon-hydrogen bond in the transition state.