Entropy effects in gas phase ion-molecule association reactions

The kinetics of gas phase reactions of the ions C5H5M+ (M = V, Co, Ni and R u) with oxygen (Me2CO, Me2O, MeOH, Me2CHOH, H2O) and nitrogen (NH3, NH2Me, NHMe2, NMe3) donor ligands have been studied by Ion Trap Mass Spectrometry. The kinetic data confirm the previously found dependence of the log of the rate constants upon the ligand ionization energy. To get some insight on t he origin of the existing barrier to the reaction some experiments were per formed at various temperatures; it was found that the kinetic constants inc reased when the temperature decreased. The results are compatible with an e ntropy effect on the free energy of activation as also shown by some calcul ations performed for the reaction of the C5H5Fe+ ion with water. (C) 2001 E lsevier Science B.V. All rights reserved.