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.

Infrared Spectra of Isolated Protonated Polycyclic Aromatic Hydrocarbons: Protonated Naphthalene

“…Harmonic frequencies are scaled with a factor of 0.98, which is derived from the recommended procedure [7] in a least squares fit of calculated and experimental frequencies of naphthalene [15] and naphthalene + . [16] In agreement with previous calculations, [17] the potential energy surface (PES) of naphthaleneH + ( Figure 1) exhibits three minima with relative energies 1 < 2 < 3.…”

Contrasting behavior of tetracene and perylene in collision-induced dissociation: a theoretical interpretation

“…7 Preliminary calculations were performed on benzene and dimethyl ether with a number of methods and basis sets following some recently published computational studies of substituted benzene 8,9 and CH 3 OCH 2 C . 10,11 We found the MP2/6-31G ŁŁ //HF/6-31G ŁŁ level to be the most suitable for the present study [the B3LYP method 12 was discarded because it fails to reproduce the experimental PA (see also Refs 13 and 14) and Smith et al 15 have found that large errors occasionally occur], in accord with the work of Eckert-Maksić et al, 9 Kovaček et al 13 and Maksić et al 16 The structures of benzene, C CH 2 -O-CH 3 ion, adducts between these two entities and transition states were fully characterized at the HF/6-31G ŁŁ level. Optimized geometries were then used for single-point energy calculations at the MP2/6-31G ŁŁ level.…”

“…Indeed, for perylene, 1,2 hydride transfers are always possible whereas for tetracene, if C CH 2 OCH 3 is located on one of the four reactive sites, 1,2-hydride transfer would lead to a non-planar cation. Such a cation, with a proton located on a ring junction, is known to be unstable [Kovaček et al 13 found that for naphthalene the protonation on a ring junction is defavored by at least 16 C by a 1,3-proton transfer.…”

Proton affinity of substituted naphthalenes

“…11 Preliminary results obtained for some disubstituted naphthalenes strongly indicate that a similar additivity rule holds in these aromatic systems too. 12,13 The purpose of the present work was twofold: (1) to provide PA increments for monosubstituted naphthalenes involving some of the most important functional groups in organic chemistry, which in turn will describe the variation in the proton affinity as a function of the position within the naphthalene perimeter, and (2) to illustrate the use of the ISA model in estimating the PAs of polysubstituted naphthalenes. The closely related electrophilic reactivity of these compounds will also be briefly discussed.…”