Quantum Mechanical Tunneling in the Unimolecular Dissociation of the Propargyl Bromide Molecular Ion

The Br loss of the propargyl bromide molecular ion has been studied using the mass-analyzed ion kinetic energy spectrometry. The kinetic energy release distribution in the unimolecular dissociation has been determined. The potential energy surface for the mechanistic pathway has been investigated by quantum chemical calculations on the B3LYP/6-311G** density functional theory level. Geometries and energies of five isomeric molecular ions, products, and related transition states have been determined. Calculation of the rate constants with statistical rate models predicts that the product C₃H₃⁺ ion has only the cyclopropenium structure and that the observed unimolecular reaction occurs via tunneling through an isomerization barrier for the H-atom transfer.