The influence of fragment size and intermediate barriers on competing near-identical simple cleavage reactions: A variational RRKM study

The fragmentation reactions of almost-symmetrical radical cations exhibit remarkable energy-dependent selectivity. The simple cleavage of the N-propylbutylamine radical cation and its N-methyl analog results in loss of ethyl or propyl radicals. The energy required to expel the ethyl radical (the smaller alkyl fragment) is lower, and ethyl loss is the preferred reaction of ions with low internal energy. The branching ratio is reversed for ions with higher energy, which can be accounted for with variational transition state theory. The competition between the two concurrent cleavage reactions is not governed by product-like orbiting transition states, but by relatively tight transition states; however, these are not equally tight. The transition state for loss of ethyl from higher energy reactant ions lies earlier on the reaction coordinate than does the transition state for loss of propyl, and the latter will in turn be slightly more product-like and hence more loose. k(E) vs E for loss of propyl radicals therefore rises more steeply with increasing internal energy, and this reaction becomes the more rapid process for higher-energy reactants such as the molecular ions formed in mass spectrometers with 70 eV electron ionization. The transition states for cleavage reactions exhibit energy-dependent shifting also when moderate intermediate barriers are present.