Guided ion beam tandem mass spectrometry techniques are used to examine promotion of the symmetric bimolecular nucleophilic substitution (SN2) reaction 37Cl- + CH3 35Cl → 35Cl- + CH3 37Cl by translational energy. The translational energy threshold for this process is 45 ± 15 kJ/mol, well above the previously reported potential energy barrier height of 10−13 kJ/mol for the SN2 transition state. The collisionally activated process involves conventional SN2 back-side attack at the carbon atom, but passage over the barrier is impeded by nonstatistical dynamical constraints at collision energies just above the barrier. A significant secondary kinetic isotope effect is observed. The cross section for reaction with CH3Cl is about 20% larger than for the reaction with CD3Cl. At high energies, >410 ± 40 kJ/mol, diatomic Cl2 - products are observed. The guided ion beam apparatus and data analysis procedures are described in detail.

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Reactions of nickel (Ni−n, n=3–10), palladium (Pd−n, n=3–8), and platinum (Pt−n, n=3–7) cluster anions are investigated in a flow tube reactor. Rate coefficients are measured for reactions with N2, O2, CO2, and N2O. Reactions with O2, CO2, and N2O have rates that are greater than 10% of the collision rate for most clusters of four atoms or larger, while N2 reactions generally exhibit much lower reaction efficiencies. All the reactions studied show a strong dependence on cluster elemental composition. Many of the palladium cluster reactions are significantly faster than the corresponding nickel and platinum cluster reactions, while Ni−n and Pt−n have similar rate coefficients. Pt−6 is observed to have anomalously low rate constants for reactions with N2, CO2, and N2O compared to neighboring platinum clusters sizes and the nickel and palladium hexamers. N2, CO2, and O2 reactions are generally association reactions with varying degrees of cluster fragmentation observed. N2O reactions result in sequential addition of O atoms to the cluster. The extent of cluster fragmentation for the various reagents can be correlated with the estimated exothermicities of the adsorption processes.

Cross sections for reaction of mass-selected boron cluster ions (B+n, n=2–24) with N2O are reported for collision energies from 0.1 to 10 eV. The major product channels are addition of a single nitrogen or oxygen atom to the intact cluster ion. For small clusters, there are no activation barriers and cross sections are large, however, as cluster size increases, bottlenecks and activation barriers reduce reactivity substantially. Significant size effects are observed in the product branching distributions. The dominant reaction mechanism at low collision energies is proposed to be complex formation, where only one bond in N2O is broken, followed by desorption of the stable leaving group (N2 or NO). Reactions with boron cluster ions larger than 16 atoms in size have been studied for the first time, and in addition, supporting evidence is given for our previous suggestion that B3 has an anomalously high ionization potential. Comparisons are made with other oxidation reactions of boron cluster ions, and with aluminum and silicon cluster ion reactions with N2O.

This letter reports a collision-induced dissociation study of the stabilities and fragmentation patterns for carbon cluster ions C+n (n = 2–15). For clusters of six or more atoms, the primary fragmentation channel is loss of C3 neutral. Fragmentation thresholds are analyzed to yield dissociation energies, which are found to be substantially higher than the upper limits estimated from photodissociation experiments of Geusic et al. [Z. Phys. D 3, 309 (1986)], but in good agreement with theoretical estimates. The stabilities oscillate strongly, with some evidence of a change in behavior as size increases above 9 atoms. This is the size where the most stable structure is believed to change from linear to cyclic. Photodissociation measurements were also done, and we find that the clusters dissociate efficiently at photon energies far below the CID thresholds. Possible reasons for the anomalous photodissociation results are discussed.