Weak ion-molecule complexes of F−(CF4)n and CF3−(CF4)n

Gas-phase thermochemical stabilities and structures of cluster ions F−(CF4)n and CF3−(CF4)n have been studied with a high-pressure mass spectrometer and ab initio calculations. The F− … CF4 bonding energy is found to be only 6.4 kcal/mol. Its geometry is of an ion-dipole complex along an SN2 reaction path. The high-symmetry (D3h) CF5− species is computed to be a transition state of the SN2 path and not an energy minimum. The CF3− …CF4 bonding energy is only 3.6 kcal/mol with F2CF− …CF4 coordination. The F … F exchange repulsion hinders the F … C attraction in these species. Th bonding energies are in good agreement with measured ones.