Hydrogen bonds in gas-phase clusters between halide ions and olefins

Abstract

Gas-phase clustering reactions of halide ions (X − = F −, Cl −, Br −, and I −) with ethylene (C 2H 4) and propylene (C 3H 6) were studied with a pulsed electron beam mass spectrometer. Bonding energies of all cluster ions were found to be less than 10 kcal/mol, i.e., no anion-initiated polymerization of C 2H 4 and C 3H 6 took place. For the cluster F −(C 2H 4) n , a small gap in the binding energy is observed between n = 4 and 5 suggesting that the first shell is completed with n = 4. For larger halide ions, the bond energies for the clusters X −(C 2H 4) n were found to be nearly n independent. For Cl −(C 3H 6) n a steep decrease in binding energies was observed between n = 2 and 3 and n = 3 and 4. The structure of the cluster ions was investigated by ab initio calculations. X −(C 2H 4) n complexes were calculated to have hydrogen-bond geometries regardless of the identity of the halide ions, and bidentate (chelate) type geometries of X −(C 3H 6) 1 were found.