The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its 1S0 (5d10) electronic ground state level but with some 3D (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (1S0) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C-H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+ - CH2) = 3.70 +/- 0.07 and D0(Au+ -CH3) = 2.17 +/- 0.24. Ab initio calculations at the B3LYPHW + /6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.

A guided ion beam tandem mass spectrometer is used to study the reactions of atomic 187 Re + with CH 4 and CD 4 and collision-induced dissociation (CID) of ReCH 4 + with Xe. These studies examine the activation of methane by Re + in a low-pressure environment free of ligand supports or other reactive species. In the bimolecular reaction, ReCH 2 + is efficiently produced in a slightly endothermic process and is the only ionic product observed at low energies, whereas at higher energies, ReH + dominates the product spectrum. Other products observed include ReC + , ReCH + , and ReCH 3 + . Modeling of these endothermic reactions yields 0 K bond dissociation energies in eV of D 0 (Re + -C) ) 5.12 ( 0.04, D 0 (Re + -CH) ) 5.84 ( 0.06, D 0 (Re + -CH 2 ) ) 4.14 ( 0.06, D 0 (Re + -CH 3 ) ) 2.22 ( 0.13. Analysis of the behavior of the cross sections suggests that formation of ReH + , ReCH 2 + , and ReCH 3 + occurs via an H-Re + -CH 3 intermediate. CID of ReCH 4 + reveals a bond energy of 0.53 ( 0.15 eV for Re + -CH 4 . The experimental bond energies compare favorably with theoretical calculations at the B3LYP/HW+/6-311++G(3df,3p) level with the exception of the singly bonded species (ReH + , ReCH 3 + ), where the Becke-half-and-half-LYP functional performs much better. Theoretical calculations also elucidate the reaction pathways for each product and provide their electronic structures. Overall we find that the dehydrogenation reaction, which occurs with an efficiency of 86 ( 10%, must involve three facile spin changes (2s + 1 ) 7 f 5 f 3 f 5) indicating that little hint of spin conservation remains in this heavy-metal system. † Part of the special issue "Tomas Baer Festschrift".

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Reaction of Au(+) ((1)S(0) and (3)D) with O(2) and N(2)O 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 (1)S(0) (5d(10)) electronic ground state level but with some (3)D and perhaps higher lying excited states. The distribution of states can be altered by adding N(2)O, which completely quenches the excited states, or CH(4) 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 N(2)O system also exhibiting AuN(2)(+) and AuNO(+) formation. All reactions of Au(+) ((1)S(0)) are observed to be endothermic, whereas the excitation energy available to the (3)D state allows some reactions to be exothermic. Because of the closed shell character of ground state Au(+) ((1)S(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.

The kinetic-energy dependence for the reactions of Co(n)+ (n=2-20) with O2 is measured as a function of kinetic energy over a range of 0 to 10 eV in a guided ion-beam tandem mass spectrometer. A variety of Co(m)+, Co(m)O+, and Co(m)O2+ (m < or = n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Co(n)+ cations with O2 are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Co(n)+-O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O2 chemisorption on the cationic clusters are compared with results from previous work on cationic, anionic, and neutral cobalt clusters.