Secondary kinetic isotope effects in cation-bound dimers of acetone (C3H6O)M(C3D6O)+ with M = H, Li, Na, K, Rb, Ag, and Cs

The competitive dissociations of mixed cation-bound dimers of unlabeled and perdeuterated acetone, (C3H6O)M(C-2 D6O)(+), with M = H, Li. Na, K, Rb. Ag. and Cs, are investigated by the kinetic method. In all cases, ion dissociation leads to loss of one neutral acetone ligand concomitant with the mono-ligated cations (C3H6O)M+ and (C3D6O)M+, respectively. The branching ratios between these two isotopic fragments can be measured accurately and are assumed to reflect the kinetic isotope effects (KIEs) associated with the binding of unlabeled and perdeuterated acetone to the core cation. Quite surprisingly, however, the apparent KIEs barely depend on collision energy. Moreover, comparison with computed equilibrium isotope effects adds a note of caution in the use of the kinetic method for the determination of isotope effects. (C) 2004 Elsevier B.V. All rights reserved.