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 i s found to be only 6.4 kcal/mol, Its geometry is of an ion-dipole comp lex along an S(N)2 reaction path. The high-symmetry (D-3h) CF5- specie s is computed to be a transition state of the S(N)2 path and not an en ergy 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 attractions in these species. Theoretical bonding energies are in good agreement with measured ones.