Analysis of the energy dependence of the cross sections for collision-induced dissociation reactions has permitted the determination of quantitative thermodynamic information for a variety of ionic clusters. As such clusters become larger, the rate at which the decomposition occurs becomes comparable to the instrumental time available for observing the reaction. A method for incorporating statistical theories for energy-dependent unimolecular decomposition in this threshold analysis is reviewed and updated. The revision relies on the fact that for most ionic clusters, the transition state is a loose association of the products that can be located at the centrifugal barrier. This permits a straightforward estimation of the molecular parameters needed in statistical theories for the transition state. Further, we also discuss several treatments of the adiabatic rotations of the dissociating cluster. The various models developed here and previously are compared and used to analyze a series of data for Li+(ROH) complexes, where ROH=methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, s-butanol, and t-butanol. The trends in the bond energies derived by these various models are compared and their accuracy evaluated by comparison with relative values determined by equilibrium methods.

Cross sections for the reactions of Ar+ with H2, D2, and HD to form ArH+ and ArD+ are measured using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory. The apparatus and experimental techniques are described in detail. Cross sections for H2 and D2 are found to be nearly identical over this entire energy range when compared at the same barycentric energy. The total HD cross section is the same as H2 and D2 at low energies, but differs significantly above 4 eV c.m., where product dissociation becomes important. The intramolecular isotope effect for reaction with HD exhibits a reversal at low energy, favoring the deuteride product below ∼0.14 eV c.m., and surprising nonmonotonic behavior at energies above 5 eV c.m. In all these systems, a new feature at higher energies is observed. This is interpreted as the onset of a product channel having an energy barrier of 8±1 eV. The room temperature rate constant derived from the data for the reaction with H2 is (9.5±2)×10−10 cm3 s−1, in good agreement with the literature. Analysis of the data indicates an activation energy of between 2 and 15 meV at room temperature. The results are compared to previous experimental determinations and to theoretical reaction models.

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The sequential bond energies of Cr(CO),+, x = 1-6, are determined by collision-induced dissociation in a guided ion beam tandem mass spectrometer. Values for the 0 K bond energies (in eV) are determined to be D (Cr+-CO) = 0.93 f 0.04, D[(CO)Cr+-CO] = 0.98 f 0.03, D[(C0)2Cr+-CO] = 0.56 f 0.06, D[(CO)&r+-CO] = 0.53 f 0.08, D[(CO)&r+-CO] = 0.64 f 0.03, and D[(CO)&r+-CO] = 1.35 f 0.08. The sum of these bond dissociation energies, 4.99 f 0.14 eV, is in good agreement with literature thermochemistry. The observation that the relative bond strengths vary nonmonotonically with the number of ligands is discussed in terms of spin conservation and ligand field theory. The bond energy for Cr+-Xe is also determined as 0.7 f 0.1 eV and compared with values for other transition metal ion rare gas species.

Threshold collision-induced dissociation of M + L (M + ) Li + , Na + , and K + ; L ) uracil, thymine, and adenine) with xenon is studied using guided ion beam mass spectrometry. In all cases, the primary product formed corresponds to endothermic loss of the intact neutral molecule. The only other product observed is the result of ligand exchange processes to form MXe + . Cross-section thresholds are interpreted to yield 0 and 298 K bond dissociation energies for M + -L after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at the MP2(full)/6-311+G-(2d,2p)//MP2(full)/6-31G* level of theory are used to determine the structures and relative energetics of several conformers of these complexes and to provide molecular constants necessary for the thermodynamic analysis of the experimental data. We find that all of the complexes are very nearly planar. Calculated M + -L bond dissociation energies compare favorably to the experimentally determined bond energies for Na + and K + binding to uracil and thymine, while theoretical values for Li + to all three bases and adenine with all three metal ions are systematically low (by 16 ( 8 kJ/mol). Comparisons with previous values determined by the kinetic method are reasonable, except in the case of Na + (adenine). A key observation in this work is that the metal ions bind most strongly to adenine at the N7 site coupled with chelation to the amino group. The magnitude of the interaction with the amino group is estimated to be sufficient to disrupt hydrogen bonding in A:T (A:U) nucleic acid base pairs for Li + , Na + , and probably transition metal ions and multiply charged ions.