Why CH 3CH 3+· formation competes with h · loss from CCCO C 3H 6O +· isomers

Abstract

How formation of CH 3CH 3 +· competes with H · loss from C 3H 6O +· isomers with the CCCO framework has been a puzzle of gas phase ion chemistry because the first reaction has a substantially higher threshold and a supposedly tighter transition state. These together should make CH 3CH 3 +· formation much the slower of the two reactions at all internal energies. However, the rates of the two reactions become comparable at about 20 kJ mol −1 above the threshold for CH 3CH 3 +· formation. It was recently shown that losses of atomic fragments increase in rate much more slowly with increasing internal energy than do the rates of competing dissociations to two polyatomic fragments. This occurs because fewer frequencies are substantially lowered in transition states for the former type of reaction than for the latter. The resulting lower transition state sums of states cause the rates of dissociations producing atoms as fragments to increase much more slowly than competing processes with increasing energy. Here we show that this is why CH 3CH 3 +· formation competes with H · loss from CH 3CH 2CHO +·. These results further establish that the dependence on energy of the rate of a simple unimolecular dissociation is usually directly related to the number of rotational degrees of freedom in the products, a newly recognized factor in determining the dependence of unimolecular reaction rates on internal energy.