Winfried Wagner-Redeker

The formation and reactivity of HOC+: Interstellar implications

CHO+ ions are made by electron impact on CD3OH in the source (ICR1) of a tandem ion cyclotron resonance spectrometer. These ions are injected into a differentially pumped analysis cell (ICR2) where they are reacted with a number of small molecules. The internal energy distribution in the CHO+ ions is obtained using total reactivity studies with neutral molecules of varying proton affinities. About 40% of the CHO+ ions react with D2 either by proton transfer to form D2H+ or isotopic exchange to form CDO+ ions. A series of experiments are performed that conclusively show these ions are the HOC+ isomer and the exchange is due to the catalytic isomerization reaction HOC++D2→DCO++HD which is approximately 37 kcal/mol exothermic. The product DCO+ ions are vibrationally cool indicating the reaction releases most of its energy as kinetic energy. Absolute rate constants for reactions of CHO+ ions with the neutrals 13CO, 15N2, CO2, O2, D2, and Ar are reported. HOC+ reacts with D2 at about 30% of the collision rate....

Unimolecular and bimolecular reactions in the C6H6+· system. Experiment and theory

Abstract The results of an experimental and theoretical study of unimolecular and bimolecular reactions in the C6H6+· system are reported. The kinetic energy release distributions for the fragmentation of metastable C6H6+· ions were measured and the bimolecular reactions between C6H4+· and H2, cyclic C4H4+· and C2H2, linear C4H4+· and C2H2, and between C2H2+· and linear C4H4 have been studied. The results of these experiments and absolute unimolecular rate constants and product branching ratios from PIPECO studies are compared with the predictions of the transition state switching model form of statistical rate theory. In general, good agreement was found between the experimental results and the calculations. The calculated total unimolecular rate constants are in excellent agreement with the experimental data (for total energies of 15.0–15.7 eV). Below total energies of 16.5 eV the measured and calculated branching ratios are in good agreement. Above 16.5 eV the agreement becomes increasingly poor. These results were interpreted as indicating that above 16.5 eV the rate of fragmentation is no longer slow compared to the rate of isomerization and so products derived from simple bond cleavages are favored over reactions requiring isomerization to different ion structures. The measured kinetic energy distribution for C6H5+ is broader than the calculated distribution; for C4H4+· the measured and calculated distributions are in good agreement, and for C3H3+ the measured distribution is narrower than the calculated distribution. The measured distribution for C6H4+· peaks at large values of kinetic energy, indicating a substantial barrier along the reaction coordinate in this channel. No reaction was observed between cyclic C4H4+· and C2H2 and between C6H4+· and H2. Reaction between linear C4H4+· and C2H2 occurs at a rate slower than the collision rate, and the experimental and theoretical results indicate that the reaction occurs through a long-lived C6H6+·* intermediate complex. Charge transfer between C2H2+· and C4H4 occurs at the collision rate. Most of the charge transfer is direct. However, the experimental and theoretical results indicate that some trajectories sample the deep C6H6+· well.

Kinetics of ion–molecule collision complexes in the gas phase. Experiment and theory

Essential elements of a transition switching model formulation of statistical rate theory are presented. The model is developed for use on rather complex polyatomic potential surface. Data on the energy dependence of the absolute unimolecular rate constants and branching ratios, thermal biomolecular rate constants and branching ratios including isotopic-substitution studies, and kinetic-energy distributions for the lowest-energy pathways are presented for the C4H+˙8, C4H+˙6 and C6H+˙6 reaction systems. Detailed comparisons are made between experiment and the transition-state switching model. Comments are directed toward tests of the fundamental assumptions of statistical rate theory and toward detailed mechanistic interpretation of the specific systems studied.

The reaction of NH+⋅3 with H2S: Dependence on the translational and internal energy of NH+⋅3

The reactions of vibrationally and of kinetically excited NH+⋅3 ions with H2S were investigated using a tandem ion cyclotron resonance spectrometer. NH+⋅3 ions with internal energies ranging from 1–5 eV were generated by charge transfer reactions, whose energy partioning is known. The charge transfer reagents used included Ar+⋅, Kr+⋅, Xe+⋅, N2+⋅, CO+⋅2 , CO+⋅, and O+⋅2 . The ionic products formed in the reaction of excited NH+⋅3 with H2S were NH+4 , H2S+⋅ and H3S+. It is shown that the product distribution is sensitive to the NH+⋅3 internal energy. At low internal energies, NH+4 is the dominant product, while at 5 eV all three reaction products are of comparable intensity. Competition between formation of H2S+⋅ and H3S+ is effective only at NH+⋅3 internal energies above 2 eV. The total rate constant was found to be 6±2×10−10 cm3/s and appeared to be independent of the internal energy of NH+⋅3 . Kinetically excited NH+⋅3 ions were formed using ICR double resonance and the effect on product distribution and...