Collision-induced dissociation dynamics of the [OCS center dot C2H2](+) complex. A combined experimental and theoretical study

Collision-induced dissociation (CID) of the [OCS . C2H2](+) complex ion wit h both Xe and Ar over an energy range of 0 to 10 eV in the center of mass f rame is studied using a guided ion beam tandem mass spectrometer. The cross sections of the ionic products observed (C2H2S+, OCS+, C2H2+, and S+) are analyzed by taking into account reactant energy distributions, multiple col lisions, lifetime effects and competition. A recently devised statistical m odel for the simultaneous analysis of competitive product channels is used to analyze three channels for the first time, with good results. Thresholds for product formation at 0 Kelvin are 0.33 +/- 0,07eV for C2H2S+, 0.95 +/- 0.07 eV for OCS+, 1.22 +/- 0.08 eV for C2H2+, and an upper limit of 4.26 e V for S+. These results are comparable to available Literature thermochemic al data within experimental errors. Competitive shifts are significant, abo ut 0.3 eV for both OCS+ and C2H2+. Ab initio calculations at the QCISD/6-31 1 + G**//MP2/6-311 + G** and CCSD//6-31G*//CCD/6-31G* levels are performed on the system. The reaction coordinates of the potential energy surface of the system is quantitatively mapped using results from CID and ab initio ca lculations. The identity of the C2H2S+ product is suggested to be the cycli c ethylene sulfide cation on the basis of the results of calculations and p revious kinetic energy release measurements. Product branching ratios as a function of energy are analyzed and compared to those determined in previou s photodissociation and bimolecular reaction experiments.