Dissociation dynamics of propargyl chloride molecular ion near the reaction threshold: manifestation of quantum mechanical tunneling

The Cl loss from the propargyl chloride molecular ion has been investigated using mass-analyzed ion kinetic energy spectrometry (MIKES), The kinetic e nergy release distribution in the unimolecular dissociation has been determ ined. The potential energy surface for the mechanistic pathway has been cal culated at the B3LYP/6-311G** density functional theory level. The calculat ed potential energy surface suggested that the threshold dissociation of th e propargyl chloride molecular ion produces the C3H3+ ion, only with the cy clopropenium structure, and with the release of a large amount of kinetic e nergy. This is in agreement with experimental results. Also, calculation of the rate constants with statistical rate models predicted that the reactio n observed on a microsecond time scale occurs via tunneling through the rat e-determining isomerization barrier for H-atom transfer. It has been found that a broad lifetime distribution is a manifestation of quantum mechanical tunneling of a precursor prepared under thermal conditions. Reinterpretati on of previous photoelectron-photoion coincidence results taking into accou nt the tunneling effect necessitated raising the critical energy to 0.64 eV from the energy of 0.34 eV reported previously. Copyright (C) 2000 John Wi ley & Sons, Ltd.