STRUCTURES OF GAS-PHASE C7F7+ IONS

In an ion cyclotron resonance spectrometer, less than 96% of the C7F7 cation formed on electron ionization of perfluorotoluene reacts with hexamethyldisilazane. In contrast, the C7F7+ from perfluoronorbornadie ne or perfluorobicyclo[3.2.0]hepta-2,6-diene is nonreactive with hexam ethyldisilazane. Collision-induced dissociation results support this d ichotomy, although the evidence is not as clear-cut. The reactive ion is assigned the benzyl structure and the nonreactive ion the tropyl st ructure, on the basis of analogy with the protio cases. By AM1 calcula tions, the perfluorobenzyl ion is 25 kcal/mol more stable than the per fluorotropyl 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 mo re appropriate 6-31G/MP2 level, the perfluorobenzyl cation is 9.7 kca l/mol more stable than the perfluorotropyl cation.