Oxidation of Atomic Gold Ions: Thermochemistry for the Activation of O2 and N2O by Au+ (1S0 and 3D)

Reaction of Au+ (S-1(0) and D-3) with O-2 and N2O is studied as a function of kinetic energy using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its S-1(0) (5d(10)) electronic ground slate level but with some D-3 and perhaps higher lying excited states. The distribution of states can be altered by adding N2O, which completely quenches the excited states, or CH4 to the flow gases. Cross sections as a function of kinetic energy are measured for both neutral reagents and both ground and excited states of Au+. Formation of AuO+ is common to both systems with the N2O system also exhibiting AuN2+ and AuNO+ formation. All reactions of Au+ (S-1(0)) are observed to be endothermic, whereas the excitation energy available to the D-3 state allows some reactions to be exothermic. Because of the closed shell character of ground state Au+ (S-1(0), 5d(10)), the reactivity of these systems is low and has cross sections with onsets and peaks at higher energies than expected from the known thermochemistry but lower than energies expected from impulsive processes. Analyses of the endothermic reaction cross sections yield the 0 K bond dissociation energy (BDE) in eV of D-0(Au+-O) = 1.12 +/- 0.08, D-0(Au+-N-2) >= 0.30 +/- 0.04, and D-0(Au+-NO) = 0.89 +/- 0.17, values that are all speculative because of the unusual experimental behavior. Combining the AuO+ BDE measured here with literature data also yields the ionization energy of AuO as 10.38 +/- 0.23 eV. Quantum chemical calculations show reasonable agreement with the experimental bond energies and provide the electronic structures of these species.