Difluorobenzene cation

At the equilibrium geometries of the neutrals, the symmetry assignments of these cationic doublet states, ordered by ascending vertical ionization potentials, are as follows. The obvious notations Bz+, E-Bz+, 1,2, 1,3, 1,4 and 1,2,3 correspond to the benzene, mono-fluorobenzene, ortho-, meta-, para-difluorobenzene isomers and 1,2,3-trifluorobenzene, respectively. (The numbers refer to the position of the... 

This was the case for the vibrational modes of A2 symmetry in the 1,2-difluorobenzene, B, A2 symmetries in the case of the 1,3-difluorobenzene and 1,2,3-trifluorobenzene cations. As stated in Table 1 the interactions between some electronic states are negligible which appear as zero entries in the off-diagonal terms of (15), (8). But one should note that in case of a 3 x 3 coupling matrix the imperceptible coupling between two electronic states may have an important effect on the coupling of the other two electronic states. 

Among ArH + which coupled readily with N02 in the gas-phase, were cation radicals of benzene, toluene, p-xylene, mesitylene, and fluoroben-zene, all of which couple at about the same rate (1.2-3.7 cm3 molecule-1 s-1). The cation radicals of m- and p-difluorobenzene coupled with N02 about 10 times more slowly. Naphthalene, which was anticipated to couple easily, did so only very slowly. These results may be related to the relative stabilities of the cation radicals and their corresponding a-complexes in the gas phase. 

An interesting rationalization of the mechanistic pathways involved in the duorination of aromatic compounds by high-valency metal fluorides has been provided. The main features of the mechanism [outlined for the conversion of fluorobenzene into /)-difluorobenzene by cobalt(in) fluoride in Scheme 2] are oxidation of the aromatic system to a radical-cation (1) and its subsequent reaction with a fluorine atom from the metal fluoride to give the type of Wheland intermediate [or benzenium ion (c/. p. 362)] (2) which would be formed in a conventional electrophilic process. Such a scheme implies that the relative stabilities of Wheland intermediates might be used to rationalize orientation in this type of reaction in much the same way as for more traditional processes, and some examples of such applications have been provided. Alternative routes from the radical-cation (1) are discussed and possible... 

Figure 3 In para-difluorobenzene, the vibrational structure of the cation was not fully resolved until the Introduction of ZEKE spectroscopy.

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