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Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.

Room-temperature reactions of 46 different atomic cations with N 2 O have been surveyed systematically using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations are produced at about 5500 K in the ICP source and are allowed to decay radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients were measured for the reactions of first-row atomic cations from K + to Se + , of second-row atomic cations from Rb + to Te + (excluding Tc + ), and of third-row atomic cations from Cs + to Bi + . Primary reaction channels were observed corresponding to O-atom transfer, N-atom transfer, and N 2 O addition. Periodicities were observed in overall reaction efficiency, and these are scrutinized in terms of overall exothermicity, the presence of an activation barrier in the reaction coordinate, and the overall conservation of spin. N-atom transfer was observed to compete with O-atom transfer in the reactions of N 2 O with La + (4%), Ti + (22%), Zr + (45%), Nb + (35%), Ta + (40%), and Os + (60%). Up to three N 2 O molecules were observed to add sequentially to selected atomic cations as well as several monoxide and dioxide cations. A second O-atom transfer was observed with the group 4, 5, and 6 transition-metal ions (except Mo + ) as well as the third-row cations Re + , Os + , Ir + , and Pt + . The atomic ions W + , Os + , and Ir + formed trioxides in sequential O-atom transfer reactions, and Os + even formed the tetroxide OsO 4 + . Multicollision-induced dissociation studies with Ar buffer gas indicated thermodynamically controlled dissociation of TiO 2 + , ZrO 2 + , HfO 2 + , VO 2 + , NbO 2 + , TaO 2 + , and WO 2 + by the consecutive detachment of O-atoms while CrO 2 + , ReO 2 + , and PtO 2 + decomposed primarily by loss of O 2 molecules.

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Atomic cations (26), M+, have been shown to lie within a thermodynamic window for O-atom transport catalysis of the reduction of N2O by CO and have been checked for catalytic activity at room temperature with kinetic measurements using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Only 10 of these 26 atomic cations were seen to be catalytic: Ca+, Fe+, Ge+, Sr+, Ba+, Os+, Ir+, Pt+, Eu+, and Yb+. The remaining 16 cations that lie in the thermodynamic window (Cr+, Mn+, Co+, Ni+, Cu+, Se+, Mo+, Ru+, Rh+, Sn+, Te+, Re+, Pb+, Bi+, Tm+, and Lu+) react too slowly at room temperature either in the formation of MO+ or in its reduction by CO. Many of these reactions are known to be spin forbidden and a few actually may lie outside the thermodynamic window. A new measure of efficiency is introduced for catalytic cycles that allows the discrimination between catalytic cations on the basis of the efficiencies of the two legs of the catalytic cycle. Also, a potential-energy landscape is computed for the reduction of N2O by CO catalyzed by Fe+(6D) that vividly illustrates the operation of an ionic catalyst.

Reactions of CH(3)F have been surveyed systematically at room temperature with 46 different atomic cations using an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer. Rate coefficients and product distributions were measured for the reactions of fourth-period atomic ions from K(+) to Se(+), of fifth-period atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of sixth-period atomic ions from Cs(+) to Bi(+). Primary reaction channels were observed corresponding to F atom transfer, CH(3)F addition, HF elimination, and H(2) elimination. The early-transition-metal cations exhibit a much more active chemistry than the late-transition-metal cations, and there are periodic features in the chemical activity and reaction efficiency that maximize with Ti(+), As(+), Y(+), Hf(+), and Pt(+). F atom transfer appears to be thermodynamically controlled, although a periodic variation in efficiency is observed within the early-transition-metal cations which maximizes with Ti(+), Y(+), and Hf(+). Addition of CH(3)F was observed exclusively (>99%) with the late-fourth-period cations from Mn(+) to Ga(+), the fifth-period cations from Ru(+) to Te(+), and the sixth-period cations from Hg(+) to Bi(+) as well as Re(+). Periodic trends are observed in the effective bimolecular rate coefficient for CH(3)F addition, and these are consistent with expected trends in the electrostatic binding energies of the adduct ions and measured trends in the standard free energy of addition. HF elimination is the major reaction channel with As(+), while dehydrogenation dominates the reactions of W(+), Os(+), Ir(+), and Pt(+). Sequential F atom transfer is observed with the early-transition-metal cations, with the number of F atoms transferred increasing across the periodic table from two to four, maximizing at four for the group 5 cations Nb(+)(d(4)) and Ta(+)(d(3)s(1)), and stopping at two with V(+)(d(4)). Sequential CH(3)F addition was observed with many atomic cations and all of the metal mono- and multifluoride cations that were formed.

We report experimental results that demonstrate gas-phase, ionic syntheses of glycine and β-alanine, as well as acetic and propanoic acid, from smaller molecules found in space; in doing so, we infer the formation of these acids in the interstellar environment. We show that ionized glycine and β-alanine are produced in the reactions of hydroxylamine ions, NH 2 OH + , with acetic and propanoic acid respectively. Even more promising in the context of interstellar synthesis are our results that demonstrate the corresponding production of the protonated amino acids from analogous reactions with protonated hydroxylamine. The striking specificity of these syntheses for the β-isomer of alanine suggests that the amino acids of CI (Carbonaceous Ivona) chondrite meteorites are products of interstellar chemistry and supports the hypothesis that these meteorites are of cometary origin.

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