Thermal energy reactions of size-selected hydrated electron clusters (H2O)(n)(-)

The reactions of hydrated electron clusters (H2O)(n)(-), n similar to 15-30, with several neutral electron scavengers were studied in a selected ion flow tube apparatus at 100 K. The reactions with CO2, O-2, and NO primarily produced the solvated charge transfer ions CO2-(H2O)(n-1), O-2(-)(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 O-2 reaction rates are significantly less than the corresponding collision rates. The reaction efficiencies for the CO2, NO, and O-2 reactions can he 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.