Structures of gas-phase C2F 7 (+) ions

In an ion cyclotron resonance spectrometer, less than 96% of the C7F 7 (+) cation formed on electron ionization of perfluorotoluene reacts with hexamethyldisilazane. In contrast, the C7F 7 (+) from perfluoronorbornadiene or perfluorobicyclo[3.2.O]hepta-2,6-diene is nonreactive with hexamethyldisilazane. Collision-induced dissociation results support this dichotomy, although the evidence is not as clear-cut. The reactive ion is assigned the benzyl structure and the nonreactive ion the tropyl structure, on the basis of analogy with the protio cases. By AM1 calculations, the perfluorobenzyl ion is 25 kcal/mol more stable than the perfluorotropyl ion, the opposite of the situation for the protio analogs (- 12 kcal/mol). Ab initio calculations at the 3-21G level agree with the semiempirical energy difference to within 0.4 kcal/mol; at the more appropriate 6-31G*/MP2 level, the perfluorobenzyl cation is 9.7 kcal/mol more stable than the perfluorotropyl cation.