Alkali chloride cluster ion dissociation examined by the kinetic method: heterolytic bond dissociation energies, effective temperatures, and entropic effects.

Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, [Rb-35Cl-Mi]+, where Mi = Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 +/- 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 +/- 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single- and multiple-collision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T-->V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T-->V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using MassKinetics modeling (Drahos and Vékey, J. Mass Spectrom. 2001, 36, 237).