Competing simple cleavage reactions: The elimination of alkyl radicals from amine radical cations

Two different alkyl radicals can be expelled when unsymmetrical aliphatic amine radical cations undergo C-C bond cleavage. The branching ratio is strongly dependent on the internal energy of the reactant, even when the competition involves loss of closely related alkyl radicals. In mass spectrometers, the rate of loss of the smaller radical (excepting methyl) always exceeds the rate of loss of the larger close to threshold. The preference is reversed for the more highly energized ions that react in the ion source, demonstrating that the rate of loss of the larger radical rises much more rapidly with increasing internal energy than does the rate of loss of the smaller radical. This result is not easily reconciled with a simple RRKM model, given the expected strong resemblance between the transition states involved, whereas it agrees well with a description based on variational transition state theory. The heats of formation of the products determined with the G3 composite ab initio method show that loss of the smaller radical is without exception the more favorable reaction. The relative rates of the competing C-C bond cleavage reactions of the metastable ions vary with the number of degrees of freedom of the reactant, with the critical energy of the reaction, and with the difference between the heats of formation of the products. The presence of intermediate energy barriers when cleavage occurs at a branching point can give rise to variations in relative rates that are not easily interpreted.