Thermal Energy Reactions of Size-Selected Hydrated Electron Clusters (H2O)n-

Abstract

The reactions of hydrated electron clusters (H2O)n-, n ∼ 15−30, with several neutral electron scavengers were studied in a selected ion flow tube apparatus at 100 K. The reactions with CO2, O2, and NO primarily produced the solvated charge transfer ions CO2-(H2O)n-1, O2-(H2O)n-5, and NO-(H2O)n-3,n-4, respectively. The reactions with N2O produced both O-(H2O)n-4 and OH-(H2O)n-3,n-4 product ion distributions. The number of neutral water molecules lost from the water clusters during these reactions is strongly correlated to the overall reaction exothermicities. The present measurements yield a value of 0.37 eV for the average effective monomer dissociation energy, D[(H2O)n···(H2O)]. The CO2 reactions proceed at the collision rate, while the NO, N2O, and O2 reaction rates are significantly less than the corresponding collision rates. The reaction efficiencies for the CO2, NO, and O2 reactions can be rationalized on the basis of spin considerations. A proposed electron transfer mechanism is discussed that considers the diabatic free energy curves which correspond to the electron being predominantly associated with either the water cluster or the scavenger molecule. A comparison of the present results with a previous molecular beam reactivity study illustrates the competition between two cluster cooling mechanisms: evaporation and collisional quenching.