The degenerate rearrangement processes in i-propyl cation were studied using the isotopically double-labeled 2-propyl-2-13C1-2-dI cation (A), which was obtained from the corresponding chloride using the molecular beam technique. The first step in the rearrangement is a shift of one of the methyl hydrogens to form n-propyl cation (or a species in its vicinity on the energy surface), followed by rotation of the methylene group. Rotation in one direction (Process I) leads to formation of isopropyl cation with an interchange of the methine proton with a methyl proton while rotation in the other direction (Process II) results in the formation of intermediate protonated cyclopropane. Through corner-to-corner proton shifts, the isotopes are completely scrambled before the reverse process returns the ion to isopropyl. Relative rates of Processes I and II were determined on the basis of the experimentally established composition of the mixture of isotopomers obtained in an early stage of scrambling, using Runge-Kutta integration to simulate the kinetics. It was found that Process I was faster (ki/ki = 3.4), meaning, according to the proposed mechanism, that hydrogen scrambles a little more rapidly than carbon
