Kekulé versus Lewis: When Aromaticity Prevents Electron Pairing and Imposes Polyradical Character

Abstract

Some conjugated alternant hydrocarbons, of singlet ground state according to Ovchinnikov's rule, may exhibit strong polyradical character, despite admitting complete pairing of electrons in bond orbitals between adjacent atoms. Typical organizations of this kind are encountered in polycyclic frames supporting two or more extracyclic methylene groups. Lewis bond pairing would require quinonization of six-membered rings, whereas safeguarding aromaticity proves sufficient to impose ground-state open-shell character, that is, the existence of unpaired electrons, providing the number of benzene rings to be quinonized is larger than two. Several examples built as variations around para-polyphenylene frames are examined through unrestricted DFT (UDFT) calculations, using various methods for spin decontamination of wavefunctions, geometries, and singlet-triplet energy gaps. They all illustrate how it is possible to conceive architectures that can be written with a closed-shell bond pairing, although they exhibit a large number of unpaired electrons. The same analyses also apply to systems in which quinonization would not kill but only reduce the number of unpaired electrons.

Why 1,2-quinone derivatives are more stable than their 2,3-analogues?

“…The good performance of the UB3LYP method is confirmed by recent papers [25][26][27][28][29][30]. On the basis of previous results [31], spin contamination corrections were not included.…”

“…It should be noted that Clar structure shown in Scheme 5b is reinforced by all applied aromaticity indices (see Table S1 in Supporting Information). Summarizing, in 2,3-quinones for n > 2, the diradical singlet situation is favored, as in the case of the acenes [25,59,61,63] and in other polycyclic aromatic hydrocarbons [26][27][28][29][30][64][65][66][67][68][69] and graphene nanoflakes [70,71]. Therefore, for 2,3-isomers, only the results of the ground states (singlet closed shell for n ≤ 2 and singlet open shell for n ≥ 3) are presented below.…”

Communication: Proper use of broken-symmetry calculations in antiferromagnetic polyradicals

“…An important feature, here, is that the M s = 1 solution may be significantly lower than the closed-shell solution, as observed, for instance, in Müller's p-dimethylene-p-triphenylene molecules (1 in Chart 1), 18 but the lowest solution is the M s = 0 one and the ground state is a singlet, indeed. 19 Very large hydrocarbons with the same number of atoms of both colors may tend to exhibit a multi-radical character. In such a case the closed-shell mean-field solution is highly unstable and of very high energy.…”

“…Several examples of this practice can be found in Ref. 19. Consider, for instance, the hexaradical hexamethylene-peri-hexabenzocoronene (2, in Chart 1).…”

“…The so-obtained converged M s = 0 solution is indeed the lowest one and confirms the singlet character of this molecule. 19 Notice that one may converge a M s = 1 solution, of higher energy than the M s = 2 one, which corresponds to a strongly different physics, with two unpaired electrons localized on the extracyclic external CH 2 groups linked to a closed-shell fused polycyclic rhombus. Starting from this two-openshell solution does not necessarily reach the lowest M s = 0 solution.…”