Collision-induced dissociation measurements on Li+(H2O)(n), n=1-6: The first direct measurement of the Li+-OH2 bond energy

Collision-induced dissociation of Li+(H2O)(n,) n = 1-6, with xenon (and with argon for n = 1) is studied as a function of kinetic energy using guided ion beam mass spectrometry. In all cases, the primary and lowest energy dissociation channel observed is endothermic loss of one water molecule. The cross section thresholds are interpreted to yield 0 and 298 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the complexes, and dissociation lifetimes. The experimental bond energies determined here are in good agreement with previous experimental (high-pressure mass spectrometric, HPMS, measurements) and theoretical results for all complexes. In the case of Li+(H2O), the value determined here is actually the first direct measurement of the bond energy. This value lies somewhat below the value reported in the HPMS study which was extrapolated from data for larger clusters. Because the HPMS value has been used extensively to establish an absolute Li+ affinity scale, this discrepancy is discussed in some detail.