Guided ion beam and theoretical studies of the reaction of Ag+ with CS2: Gas-phase thermochemistry of AgS+ and AgCS+ and insight into spin-forbidden reactions

The gas-phase reactivity of the atomic transition metal cation, Ag+, with CS2 is investigated using guided-ion beam mass spectrometry. Endothermic reactions forming AgS+ and AgCS+ are observed but are quite inefficient. This observation is largely attributed to the stability of the closed shell Ag+(S-1,4d(10)) ground state, but is also influenced by the fact that the reactions producing ground state AgS+ and AgCS+ products are both spin forbidden. Analysis of the kinetic energy dependence of the cross sections for formation of these two products yields the 0 K bond energies of D-0(Ag+- S)=1.40 +/- 0.12 eV and D-0(Ag+- CS)=1.98 +/- 0.14 eV. Quantum chemical calculations are used to investigate the electronic structure of the two product ions as well as the potential energy surfaces for reaction. The primary mechanism involves oxidative addition of a CS bond to the metal cation followed by simple Ag - S or Ag - CS bond cleavage. Crossing points between the singlet and triplet surfaces are located near the transition states for bond activation. Comparison with analogous work on other late second-row transition metal cations indicates that the location of the crossing points bears directly on the efficiency of these spin-forbidden processes.