Proton affinities of larger aromatics - naphthalenes

It is interesting to point out that the lowest proton affinity in polyfluorinated naphthalenes is found for ipso protonation (viz. systems 33, 35 and 36). It is a consequence of the out-of-plane shift of fluorine and the accompanying ring puckering. However, this is at the same time a manifestation of the rr-electron fluoro effect put forward by Liebman et al. [45]. It is very well known that multiply fluorinated compounds possess considerably stabilized a-MOs if the systems are planar, the 7r-manifold being almost unaffected [46]. However, in nonplanar systems all MOs at the carbon skeleton are significantly stabilized [45,46] which is exactly the ceise for the ipso protonation. Now, it can be easily shown 

Proton affinities of molecules 25-28 and the corresponding increments as obtained by the MP2(I) model (in kcal/mol)  

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