Additivity of the proton affinity in aromatics: Fluorinated naphthalenes

Abstract

Absolute proton affinities (PAs) of fluorinated naphthalenes and their additivity are considered theoretically by utilizing the MP2(fc)/6-31G**//HF/6-31G*+ZPE(HF/6-31G*) model. The hierarchy of the PAs in the parent naphthalene compound (PA)1 > (PA)2 > (PA)8a, where subscripts denote sites of the protonation, is interpreted in terms of the aromaticity defect. Since the fluorine substitution influences PA values in remote parts of the molecular systems, it is concluded that the long range interactions in fluorine derivatives are transmitted via the mobile π-electrons thus giving rise to significant resonance effects. It is also shown that the additivity formula based on the independent substituent approach works very well in polysubstituted naphthalenes as evidenced by a very small average absolute deviation, with one notable exception - protonation at the ipso-position. In this case the out-of-plane bending of the C-F bond leads to considerable puckering of the aromatic ring which is not present in the parent naphthalene. Consequently, the influence of a substituent on the PA at the same carbon center cannot be considered a small perturbation which results in deviations from the strict additivity. Finally, it should be pointed out that low PA values for ipso-protonation are compatible with the (per)fluoro effect. The origin of the additivity is briefly considered. It appears that it is a consequence of cancellation of the many-body effects in the initial (base) and final (conjugated acid) states. The same formula of additivity (mutatis mutandis) should be applicable in other polysubstituted aromatics.