Aromaticity and Möbius Antiaromaticity in Monocyclic [11]Annulenium Cations

Abstract

Monocyclic [11]annulenium cations, which are experimentally unknown, have been studied primarily via DFT methods but also with some CCSD(T) validation. We have located six minima: two doubly trans (26, 27), one triply trans (28), one singly trans (29), one quintuply trans (trannulene-type, 33), and one all-cis (31). The first three are aromatic, 33 is modestly aromatic, 29 is nonaromatic, and the last is a Möbius antiaromatic species. We also investigated the fusion of various numbers of three-membered rings (3MRs) to the central 11-membered ring (11MR). We found several planar, all-cis-[11]annulenium ion derivatives as well as another Möbius antiaromatic species (52b); for comparison, we also found planar, antiaromatic all-cis-[12]annulene (60) and [15]annulenium cation (61) derivatives. The (anti)aromatic characterization of these compounds is based mainly on calculated magnetic data for the ground singlet and vertical triplet states, although aromatic stabilization energies (ASE) are also considered. Data for optimized triplets, several of which are Möbius aromatic systems (31t, 52t, 63t, 64t), are also included. Several of these cations are reasonable synthetic targets.

Neutral Möbius Aromatics: Derivatives of the Pyrrole Congener Aza[11]annulene as Promising Synthetic Targets

“…The (CH) 11 + triplet potential surface2,45 has only recently been studied again, and very thoroughly 45. Warner, also employing B3LYP/6‐31G*, and, additionally, CCSD(T)/6‐31G*//B3LYP/6‐31G* computations, validated many of the first authors earlier results 2,45.…”

“…The (CH) 11 + triplet potential surface2,45 has only recently been studied again, and very thoroughly 45. Warner, also employing B3LYP/6‐31G*, and, additionally, CCSD(T)/6‐31G*//B3LYP/6‐31G* computations, validated many of the first authors earlier results 2,45. Most remarkably, the coupled cluster single‐point energies deviated mostly only about 10 % from the B3LYP results for minima and transition states, which we see as an additionalcorroboration for the validity of the computational level we employ in this paper 45…”

Spin States of Zigzag-Edged Möbius Graphene Nanoribbons from First Principles

“…Although the lengths in Table 2 have not reached the crossover yet, data for zigzag-edged Möbius graphene nanoribbon [4,18] indicate that the open-shell singlet's energy is already higher than the triplet state. 6 We plot the spin magnetization density for the quintet of zigzag-edged Möbius graphene nanoribbon [4,18] in Figure 3. It can be seen that both spin up (the majority) and spin down (the minority) electrons are mainly localized at the edge.…”

Carbocations

“…Experimentally unknown monocyclic [11]annulenium ions were investigated by computational methods. 136 Six minima were located, four being aromatic, one non-aromatic, and the sixth a Mobius antiaromatic species. The antiaromatic properties of the cyclopentadienyl cation and its analogue with one silicon were addressed, along with a number of neutral and anionic analogues.…”