Additivity of Proton affinities: Theoretical studies of fluorine‐ and methyl‐substituted benzenes

Ring proton affinities (PAs) in fluorobenzene and toluene were examined by the MP2(fc)/6–31G**//HF/6–31G* + ZPE(HF/6–31G*) model. The calculated PAs are in good accordance with the available experimental evidence, their order being PA(p) > PA(o) > PA(m) > PA(i), where p, o, m and i stand for para, ortho, meta and ipso positions, respectively. The relative values of the proton affinities can be interpreted in terms of the ground‐state charge distribution (initial state effect) and the characteristic π‐bond fixation produced by protonation (final state effect). The influence of the latter is either concerted with the initial charge distribution leading to higher PAs (ortho and para positions) or disconcerted as in meta protonation, which has a lower PA value. Finally, it is shown that PAs in difluorobenzenes and fluorotoluenes are additive and can be reduced to the characteristic PAs of fluorobenzene and toluene with good accuracy.