Dynamics of Metastable Dissociation and Photodissiciation of the Gas-Phase Cluster Ion (OCS:C2H2)+

Article abstract of DOI:10.1021/j100174a055

The dynamics of dissociation of (OCS:C2H2)+ have been examined by using mass-analyzed ion kinetic energy spectrometry (MIKES).Kinetic energy release distributions for the dissociations were determined by analysis of the MIKES peak shapes.Metastable dissociation yields OCS+ and C2H2S+ as the ionic products of the cluster ion bond cleavage and a sulfur ion transfer reaction, respectively.The latter channel is exothermic by more than 25 kcal/ mol, and the kinetic energy release distribution is highly nonstatistical.Ab initio calculations on the ground-state reactants of the S+ - transfer reaction indicate that although the thioketene structure is the lowest energy isomer of C2H2S+, the alternative isomeric species, thiirene molecular ion, is also permitted by the thermochemistry of the reaction.Photodissociation experiments were performed in the photon energy range 2.1-4.5 eV, using a crossed laser beam-ion beam geometry.The photodissociation cross section increases apparently smoothly from about 8 x 10^-20 cm² at 2.1-eV photon energy to 4 x 10^-18 cm² at 3.5 eV.The ionic products formed in this energy range are OCS+ and C2H2+, which appear in an essentially constant ratio of 6:1.Analysis of the angular distributions of the OCS+ and C2H2+ photoproducts indicates that both dissociation channels arise from excitation to a repulsive excited state.Analysis of the kinetic energy release distributions provides an upper limit to the cluster bond dissociation energy of about 1.15 eV and suggests a sulfur-carbon bonding interaction in the cluster ion.These results are supported by ab initio calculations, which indicate a bond dissociation energy of 0.875 eV, and that the most stable form of (OCS:C2H2)+ is with near ?-bonding between The S atom of OCS and one of the carbons of C2H2.