Binding Energies of Ag+ and Cd+ Complexes from Analysis of Radiative Association Kinetics

Abstract

Association reactions of Cd+ with benzene and of Ag+ with acetone and several unsaturated hydrocarbons were observed in the Fourier-transform ion cyclotron resonance (FT-ICR) spectrometer. The reactions were presumed to occur by radiative association (RA) involving infrared photon emission, and the kinetics were analyzed to derive bond strengths for the ion−neutral complexes. To supply the structures, infrared frequencies, and infrared intensities required for this analysis, ab initio calculations at the Hartree−Fock (HF) and second-order Moeller−Plesset perturbation theory (MP2) levels were carried out for the reactants and the association complexes, and the results are reported. The RA kinetics analysis yielded values for the binding energies of 1.41 ± 0.2 eV for Cd+(benzene), 1.68 ± 0.2 eV for Ag+(benzene), 1.66 ± 0.2 eV for Ag+(acetone), 1.70 ± 0.2 eV for Ag+(isoprene), and 1.71 ± 0.2 eV for Ag+(2-pentene). The MP2-derived modeling gave higher (and more reliable) binding energies than the HF-derived modeling, but the HF-level modeling was found to provide estimates of useful precision, except for the Ag+(benzene) case. Binding energies were also estimated for the observed Ag+L2 complexes, and within experimental and modeling error the second ligand was found to bind with the same energy as the first. Clustering of six or more acetaldehydes with Ag+ was observed, but it was considered most likely that this reflected fast association with low-abundance polymeric impurities in the acetaldehyde sample.