The p-fluoro labeling technique is used to demonstrate that the ions formed by the loss of CO from tetracyclones pass through a tetrahedral transition state or intermediate before they fragment.
ISOTOPIC labeling techniques have a well established place in the determination of reaction mechanisms in mass spectr0metry.l As a result of earlier studies on substituent effect^,^.^*^ we have recently suggested that the p-fluoro substituent may be used as a label in a similar fashion to the use of isotopes for the study of compounds with aromatic rings attached to the reaction l enter.^ It was shown that the p-fluoro substituent gives scrambling data quantitatively similar to those obtained by deuterium labeling when the substituted phenyl ring does not interact directly with the reaction site, and has the advantages of commercial availability in many forms, low cost, complete incorporation, and wide separation of the m/e ratios of fragment ions and accompanying metastable peaks.
Applying this technique to the structure of the [M -CO]? ion from the labeled tetracyclone 3,4-bis(p-fluorophenyl)-2,5-diphenylcyclopentadienone (I), we showed that this ion must have a cyclic structure, because Cl,Hl,t is produced from the [M -C o l t ion, as are C,,H,Ft and C14HsF2t. However, the ion cannot be of square symmetry, which would demand a distribution of 1 :2: 1 in the amount of unfluorinated, monofluorinated and difluorinated products formed. The distribution actually observed was 1 : 3.1 : 0.89 for the metastable ions related to the decompositions to these respective products. It was therefore necessary to postulate that distortion from square symmetry, as in a rectangle, or else a new symmetry, as in the tetrahedron, was present. We now report results which in the first place demonstrate that the label distribution found is not unique to the substitution pattern used in I, and more significantly that the structure actually requires interpretation in terms of tetrahedral symmetry.