The thermochemical properties of some small clusters such as the (H_2O)_2~(centre dot+) dimer have already been investigated by both experimental and theoretical methods.The recent method to selectively prepare the ammonia-water ionized dimer NH_3,H_2O~(centre dot) (and not its proton transfer isomer NH_4~+,OH~(centre dot)) allowed us to study its chemical reactivity.This study focuses on the charge and proton transfer pathways:Ion-molecule reactions in the cell of an FT-ICR mass spectrometer were carried out with a range of organic compounds.Examination of the reactivity of the NH_3,H_2O~(centre dot) ionized dimer versus ionization energy and proton affinity of the neutral reagents shows a threshold in the reactivity in both instances.This leads to a bracketing of thermochemical properties related to the dimer.From these experiments and in agreement with ab initio calculations,the adiabatic recombination energy of the NH_3,H_2O~(centre dot) dimer was evaluated at -9.38+-0.04 eV.The proton affinity bracketing required the reevaluation of two reference gas-phase basicity values.The results,in good agreement with the calculation,lead to an evaluation of the proton affinity of the NH_2~(centre dot),H_2O dimer at 204.4 +- 0.9 kcal mol~(-1).These two experimental values are respectively related to the ionization energy of NH_3~(centre dot) and to the proton affinity of NH_2~(centre dot) by the difference in single water molecule solvation energies of ionized ammonia,of neutral ammonia,and of the NH_2~(centre dot) radical.
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