Diethard K. Bohme

Activation of hydrogen and methane by thermalized FeO+ in the gas phase as studied by multiple mass spectrometric techniques

Abstract The ion-molecule reactions of thermalized iron-oxide cation FeO + with dihydrogen and methane have been studied by three different experimental techniques: Fourier transform ion cyclotron resonance (ICR), guided ion beam (GIB), and selected-ion flow tube (SIFT) mass spectrometry. Although these studies agree in a qualitative sense, i.e., FeO + brings about activation of H 2 and CH 4 with quite low efficiencies, there exists a considerable quantitative divergence as far as rate constants and branching ratios are concerned. The sources of error in these three related, but yet different experimental techniques are analyzed and critically reviewed. This error analysis brings the data to internal consistency with each other, once an accurate reference is used for calibration. In general, the rate constants obtained with the SIFT apparatus appear as the most accurate ones, while those obtained under ICR conditions are slightly too large, and the rate constants determined with the GIB instrument are somewhat lower than SIFT. However, the branching ratios for the formation of Fe + and FeOH + in the reaction of FeO + with methane are subject to more subtle effects. In the SIFT apparatus, termolecular stabilization of the intermediates causes differences from the ICR and GIB measurements, which were obtained under single-collision conditions.

Determination of proton affinities from the kinetics of proton transfer reactions. VII. The proton affinities of O2, H2, Kr, O, N2, Xe, CO2, CH4, N2O, and CO

The flowing afterglow and selected ion flow tube techniques are applied in a room temperature study of the kinetics of proton‐transfer reactions of the type XH++Y?YH++X for X or Y=He, H2, O2, Kr, O, N2, Xe, CO2, CH4, CD4, N2O, OH, and CO, and for the deuteration of O2 by D3+. Equilibrium constants are determined and changes in thermodynamic state properties (ΔG°, ΔH°, ΔS°) are derived for the reactions with X/Y=H2/O2, O2/Kr, H2/Kr, H2/N2, O/N2, N2/Xe, Xe/CO2, CO2/CH4, CH4/N2O, and N2O/CO. Proton affinities are reported for O2, H2, Kr, O, N2, Xe, CO2, CH4, N2O, and CO, together with the heats of formation of their protonated species. Also a correlation is presented between the kinetic and thermodynamic results obtained in this study.

Flowing Afterglow Studies of Formation and Reactions of Cluster Ions of O2+, O2−, and O−

The temperature‐controlled flowing afterglow system has been utilized at temperatures below 300°K to measure the rate constants for the association of H2, N2, O2, CO2, N2O, and SO2 to O2+ with helium third body. Rate constants for the association of N2, O2, and CO2 to O2− and for the association of N2 and CO2 to O− have been measured at 200°K. Some of the ion clusters formed in this way reacted via binary neutral interchange reactions of the type A±·B+C → A±·C+B. Rate constants for these processes were obtained at and below 300°K. The observation of these reactions allows an ordering of ion cluster bond energies. The increasing order of binding energies to O2+ is H2, N2, O2, N2O, SO2, and H2O. For the O2− ion clusters the increasing order is N2, O2, H2O, CO2, and NO. In some cases, equilibria are observed allowing a determination of binding energy differences. The three‐body association rate constants increase with increasing binding energy of the ion cluster, in accord with theoretical expectation.

Proton transport in the catalyzed gas-phase isomerization of protonated molecules

Recent theoretical and experimental results for selected ions and ion/molecule reactions are examined which provide insight into fundamental aspects of the isomerization of protonated heteronuclear molecules. The isomerization is viewed in terms of a “proton transport” mechanism in which a “foreign” molecule transports the proton from a high-energy site to a low-energy site of the protonated molecule and thereby catalyzes the isomerization.

Flowing Afterglow Studies of the Reactions of the Rare‐Gas Molecular Ions He2+, Ne2+, and Ar2+ with Molecules and Rare‐Gas Atoms

The flowing afterglow technique has been used to measure rate coefficients and product channels for the reactions of He2+, Ne2+, and Ar2+ with the rare‐gas atoms Ne, Ar, and Kr, and with the molecules NO, O2, CO, N2, and CO2 at 200°K. These reactions are found to proceed principally by asymmetric charge transfer. The experimental results for the reactions of the rare‐gas molecular ions with rare‐gas atoms indicate a correlation between reaction probability and reaction exoergicity analogous to that observed for asymmetric atomic ion–atomic neutral charge‐transfer reactions. The reactions with molecules are found to proceed with unit reaction probability with the exception of the reactions Ar2++O2 and Ar2++NO. The reaction probability for the reactions of the molecular rare‐gas ions with molecules is not very sensitive to an energy resonance criterion. Finally, rate coefficients and reaction channels for the reaction of the mixed rare‐gas molecular ions HeNe+, ArKr+, and ArCO+ with Ne, Kr, and CO, respecti...

Studies of reactions involving C2Hx+ ions with HCN using a modified selected ion flow tube

Abstract A modified version of the selected ion flow tube is describe. Its application to the study of the reactions of C2Hx+ ions with HCN eliminates the complications encountered with attempts to study some of these reactions with the flowing afterglow technique. Proton transfer is the only process observed for reactions of C2H3+ and C2H5+ and is the dominant channel for the C2H7+ reaction. C2+ reacts exclusively by condensation while C2H+ reacts, with nearly equal probability, by proton and hydrogen atom transfer. The second-order reactions of all the ions, with the exception of C2H2+, occur with probabilities greater than 65% of their theoretical values. Three-body association is the dominant channel for the C2H2+ reaction, with proton transfer and condensation occurring at less than 1% of the theoretical collision rate. Clustering is the only reaction observed for C2H4+ but, in this case, it is the only known exothermic channel.

Studies of reactions of C3H+ ions in the gas phase at 296±2 K

Rate constants and product distributions have been determined for ion-molecule reactions between C3H+ and H2, D2, CO, N2, O2, CO2, OCS, H2O, H2S, CH3OH, NO, N2O, NH3, ND3, CH4, HCN, CH3CN, C2N2, C2H2, C2D2 and C2H4. The measurements were done using the Selected-Ion Flow Tube or SIFT technique at 296±2 K. The observed reactions exhibit a wide range in reactivity and a large variety of pathways including proton transfer, charge transfer, hydride abstraction, complex formation and association. Many of these reactions provide routes for CH, CC, CO, CN or CS bond formation in the ion and/or neutral product which are of relevance in molecular synthesis by ion chemistry.

Flowing Afterglow Studies of Ion–Molecule Association Reactions

Positive and negative ion–molecule association reactions have been investigated in a flowing afterglow system at 82° and 280°K with helium as a third body over a pressure range 0.1–3.0 torr. Specific rates have been measured for the association reactions Ar++Ar, O++N2, N++N2, O2++O2, N2++N2, and O−+CO2. At low pressures the kinetics of association were termolecular as expected for the ground‐state ions studied. The termolecular rate coefficients were found to decrease significantly with increasing temperature. At 82°K the kinetics of association for both O2++O2 and N2++N2 become bimolecular above helium pressures of about 1.6 and 0.2 torr, respectively. This observation is qualitatively, but not quantitatively, consistent with the simple mechanism usually invoked for association reactions. A consideration of available laboratory data on ion–neutral termolecular associations in which a stable complex is formed indicates a surprisingly systematic variation of the termolecular rate coefficient with the numbe...

Reactions of Atomic Cations with Methane: Gas Phase Room-Temperature Kinetics and Periodicities in Reactivity

Reactions of methane have been measured with 59 atomic metal cations at room temperature in helium bath gas at 0.35 Torr using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations were produced at approximately 5500 K in an ICP source and allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Rate coefficients and product distributions are reported for the reactions of fourth-row atomic cations from K(+) to Se(+), of fifth-row atomic cations from Rb(+) to Te(+) (excluding Tc(+)), of sixth-row atomic cations from Cs(+) to Bi(+), and of the lanthanide cations from La(+) to Lu(+) (excluding Pm(+)). Two primary reaction channels were observed: C-H bond insertion with elimination of H(2), and CH(4) addition. The bimolecular H(2) elimination was observed in the reactions of CH(4) with As(+), Nb(+), and some sixth-row metal cations, i.e., Ta(+), W(+), Os(+), Ir(+), Pt(+); secondary and higher-order H(2) elimination was observed exclusively for Ta(+), W(+), and Ir(+) ions. All other transition-metal cations except Mn(+) and Re(+) were observed to react with CH(4) exclusively by addition, and up to two methane molecules were observed to add sequentially to most transition-metal ions. CH(4) addition was also observed for Ge(+), Se(+), La(+), Ce(+), and Gd(+) ions, while the other main-group and lanthanide cations did not react measurably with methane.

Determination of proton affinity from the kinetics of proton transfer reactions. II. Kinetic analysis of the approach to the attainment of equilibrium

A detailed kinetic analysis of the approach to and attainment of equilibrium in proton transfer reactions proceeding in a flowing afterglow at 300°K has been developed by extending existing analyses for ion molecule reactions to include axial diffusion and back reaction. The analysis has been applied to the experimental data obtained for the reactions H3+ + N2⇄N2H+ + H2 and CO2H+ + CH4⇄CH5+ + CO2 proceeding in a large excess of H2 gas. The latter reaction was investigated independently in both directions. The forward and reverse rate constants, kf and kr, were uniquely determined under nonequilibrium conditions for both reaction systems, and in the case of the CO2H++CH4 system, this was accomplished both in the forward and in the reverse direction. The ratios of rate constants determined under nonequilibrium conditions were found to be equal, within experimental error, to the equilibrium constant, K, determined from the equilibrium concentrations. Furthermore, in the case of the CO2H++CH4 system, for whic...