Aromaticity of Pyrene and Its Cyclopentafused CongenersResonance and NICS Criteria. An Ab Initio Valence Bond Analysis in Terms of Kekulé Resonance Structures

Abstract

The effect of cyclopentafusion on the aromatic properties of pyrene and its cyclopentafused congeners has been studied by calculating resonance energies, using the valence bond (VB) method, and nucleus independent chemical shifts using DIGLO. The VB resonance energy is only slightly affected by cyclopentafusion. The resonance interactions between Kekulé resonance structures that lead to six π electron (benzene-like) conjugated circuits have the largest contributions to the resonance energy, in favor of Clar's model. For all compounds these contributions are of similar magnitude. Hence, according to the resonance criterion, all compounds have the same aromatic character. In contrast, the total NICS values show a decrease of aromatic character of the compounds in the series upon the addition of externally fused five-membered rings. However, in line with the resonance criterion, the diamagnetic part of the shielding tensor perpendicular to the molecular framework is nearly constant for all compounds, provided that comparable gauge origins are chosen. Thus, care should be taken by comparing the aromatic character of rings of different molecules by considering only their total NICS values.

An ab Initio Valence Bond Study on Cyclopenta-Fused Naphthalenes and Fluoranthenes

“…Other resonance interactions between structures that lead to π-electron conjugated circuits with more than 6π-electrons do not contribute, in line with the trends found in our previous ab initio VBCI calculations on the cyclopenta-fused pyrenes. 15 In agreement with the results obtained for 1-4 the bond length changes in the C(1)-C(2) bonds (Scheme 2 and Supporting Information) are accompanied by an increase in weights of the Kekulé structures A and A′ [C(1)-C(2): 5, 1.385 Å; 6, 1.342 Å].…”

“…16,18 A corollary is that upon cyclopentafusion the effect of the σ skeleton should be taken into account. The ab initio VBCI results obtained for the cyclopenta-fused pyrene series 15 raise several questions: (1) Is the behavior of the cyclopenta-fused pyrene congeners an exception, or is there in general no correlation between the thermodynamic stability of isomers of CPPAHs and the differences in their resonance energies? (2) To what extent does cyclopenta-fusion exert an effect on the resonance energy (E res ) of a PAH?…”

“…13,14 Furthermore, E res can be decomposed into contributions from "electron delocalization" in various distinct 4n + 2 and 4n π-electron conjugated circuits. 15 In VB theory, a bond is made by the coupling of the spin of two electrons in two (singly occupied) orbitals into a singlet. The atomic orbitals can be chosen either to be fixed and strictly centered on one atom (strictly atomic model) or may be optimized without constraints (cf.…”

Ring-current aromaticity in triplet states of 4n π electron monocycles

“…There has been much discussion in the literature of the degree to which the magnetic and energetic criteria of aromaticity agree [5][6][7][8][9]. In monocycles, open-and closed-shells, the two criteria evidently run in parallel: the aromaticity of triplet states of the 4n p-electron monocycle was predicted by Baird many years ago [14], and the magnetic aromaticity of such systems was later predicted computationally by calculation of nucleus-independent chemical shifts [17].…”

“…Most definitions compare observables with predictions of a parameterised model in procedures which are therefore subject to some ambiguity [5][6][7][8][9]. Depending on the model used, an aromatic molecule may be one that is more stable, less reactive in some directions, has more nearly equal bond lengths, a greater magnetisability anisotropy, or a more negative central nucleus-independent chemical shift (NICS [10]), than is expected.…”