Excited-State Aromaticity Reversals in Möbius Annulenes

Abstract

It is suggested that Möbius annulenes follow a rule similar to Baird's rule such that the 4n and 4nC2 criteria for Möbius electronic ground state aromaticity and antiaromaticity are reversed in the lowest triplet and first singlet excited electronic states. Support comes from an investigation of aromaticity in the ground (S 0 ), lowest triplet (T 1 ) and first singlet excited (S 1 ) electronic states of the Möbiusaromatic cyclononatetraenyl cation, C 9 H C 9 , using isotropic magnetic shielding isosurfaces calculated with state-optimized complete-active-space self-consistent field wave functions constructed from gauge-including atomic orbitals. Examination of these isosurfaces demonstrates that whereas the S 0 state of C 9 H C 9 is aromatic, the T 1 and S 1 states are antiaromatic.

Magnetic shielding paints an accurate and easy-to-visualize portrait of aromaticity

“…deshielded ''halos''' around sp 2 and sp hybridized carbons and other sp 2 hybridized first main row atoms have been observed in not only in conjugated rings, 15,[25][26][27][28] but also in in open-chain conjugated molecules such as ethene, ethyne and s-trans-1,3butadiene. 20 The occurrence of such ''halos'' has been attributed to a specific type of p electron behaviour characteristic of some sp 2 and sp hybridized first main row atoms that is different from traditional ring currents.…”

Visualisation of Chemical Shielding Tensors (VIST) to Elucidate Aromaticity and Antiaromaticity**

“…[27,28] Several of these systems are interesting due to their remarkable capacity to stabilise multiply charged ions [20] making them promising candidates for organic battery electrodes. [24,29,30] Excited-state (triplet) aromaticity [31,32] and Möbius aromaticity are also being investigated, [33][34][35][36] as well as three-dimensional aromaticity in stacked systems, [37] cyclophanes, [38] π-conjugated cages, [39] and borane cages. [40] Finally, homoaromaticity provides yet another fascinating field of non-standard aromaticity [41][42][43][44] with potential applications in mechanoresponsive materials.…”

Visualizing electron delocalization in contorted polycyclic aromatic hydrocarbons

“…16,29 NICS has routinely been computed as single points 1 Å above the middle of each ring. 36,37 Richer pictures of NICS in molecules can be obtained by mapping NICS using 1D, [38][39][40][41] 2D, 37,[42][43][44][45][46][47][48] and 3D grids [49][50][51][52][53][54][55][56][57] of Bq. The latter approach can produce 3D maps of the magnetic properties of molecules as isovalue surfaces.…”