Observation of conformation-specific pathways in the photodissociation of 1-iodopropane ions

Many molecules can rotate freely around single bonds and thereby interconvert between different conformations, such as gauche and anti 1,2-disubstituted ethane, a classic example of conformational isomerism(1-3). Even though rotation occurs rapidly at room temperature, the product selectivity seen in some reactions has been explained by conformation-dependent reaction mechanisms 4,5 : if reactant molecules differing only in their conformation are located at different positions on the reaction path, they may undergo different reactions. But a direct verification of this effect is difficult, because the energy barrier separating conformational isomers is so low that under ambient conditions reactants with more than one conformation will be present(1). But by using temperatures low enough to suppress the interconversion between different conformations, gauche-1-iodopropane ions and anti-1-iodopropane ions have been selectively generated(6). Here we show that the kinetic energy released during the photodissociation of 1-iodopropane ions depends strongly on the conformation of the ions. Thermodynamic arguments and ab initio calculations indicate that this difference in kinetic energy release results from differences in the reaction mechanism, with gauche-1-iodopropane ions forming 2-propyl ions and anti-1-iodopropane ions forming protonated cyclopropane ions. These findings suggest that the well-known concept of conformation selection forms the basis of a simple scheme for reaction control, thus providing in some cases an attractive alternative for more involved schemes that utilize the phase and pulse shape of laser beams to control chemical reactions(7-10).