Intramolecular Isotope Effects in the HD—Rare Gas Ion—Molecule Reactions

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Abstract
The ion—molecule reactions of HD+—He, HD+—Ne, Ar+–HD, and Kr+–HD have been studied and the intramolecular isotope effects determined over a range of kinetic energies of the reactant ion. The intramolecular isotope effects were found to vary with ion kinetic energy, indicating energy transfer to internal degrees of freedom in the reaction products. Three types of isotope effects are proposed to account for experimental data. An atom or ion transfer mechanism which gives rise to a higher probability of formation of XH+ over XD+ is the dominant factor at high ion kinetic energies. At low energies the unimolecular decomposition of a triatomic intermediate is assumed to form preferentially XD+. For the HD+—He and HD+—Ne reactions evidence is obtained indicating dissociation of the ionic product with an associated isotope effect favoring the XH+.