Thermochemical Stabilities and Structures of the Cluster Ions OCS+, S2+, H+(OCS), and C2H5+ with OCS Molecules in the Gas Phase

The gas-phase clustering reactions of OCS+, S2+, H+(OCS), and C2H5+ ions with carbonyl sulfide (OCS) molecules were studied using a pulsed electron-beam high-pressure mass spectrometer and applying density functional theory (DFT) calculations. In the cluster ions OCS+(OCS)n and H+(OCS)(OCS)n, a moderately strong, here referred to as “semi-covalent”, bond was formed with n = 1. However, the nature of bonding changed from semi-covalent to electrostatic with n = 1 → 2. The bond energy of S2+(OCS) was determined experimentally to be 12.9 ± 1 kcal/mol, which is significantly smaller than that of the isovalent S2+(CS2) complex (30.9 ± 1.5 kcal/mol). DFT based calculations predicted the presence of several isomeric structures for H+(OCS)(OCS)n complexes. The bond energies in the C2H5+(OCS)n clusters showed an irregular decrease for n = 1 → 2 and 7 → 8. The nonclassical bridge structure for the free C2H5+ isomerized to form a semi-covalent bond with one OCS ligand, [H3CCH2⋯SCO]+, i.e., reverted to classical structure. However, the nonclassical bridge structure of C2H5+ was preserved in the cluster ions C2H5+(OCS)n below 140 K attributable to the lack of thermal energy for the isomerization. DFT calculations revealed that stability orders of the geometric isomers of H+(OCS)(OCS)n and C2H5+(OCS)n changed with increasing n values.