Entropy effects in gas phase ion-molecule association reactions

The kinetics of gas phase reactions of the ions C5H5M+ (M=V, Co, Ni and Ru) 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 the origin of the existing barrier to the reaction some experiments were performed at various temperatures; it was found that the kinetic constants increased when the temperature decreased. The results are compatible with an entropy effect on the free energy of activation as also shown by some calculations performed for the reaction of the C5H5Fe+ ion with water.