[7]Circulene: a remarkably floppy polycyclic aromatic C28H14 isomer

A simple model for counter-rotating ring currents in [n]circulenes

“…In agreement with the shallow corrugation potential for 3b ( [14] and see Section 2) and the observation that 3b in solution possesses a timeaveraged planar D 7h structure, even at À90°C, a better fit between theory and experiment is obtained using the symmetry-averaged d C values rather than those computed for the D 7h struc- Fig. 4.…”

“…The geometry of 3 was optimised at the RHF/ 6-31G** level under the constraints of D 7h and C 2 symmetry, respectively, using GAMESS-UK [17]. The geometries, energies, energy difference [DE tot (3a ) 3b) 35.6 kJ mol À1 and imaginary frequencies found for 3a and 3b [3a, 60i (doubly degenerate), 62i and 51i cm À1 , and 3b, 7i cm À1 are in good agreement with those obtained in the previous computational study [14] (see also Table 1). In passing we note that a change of method from RHF/6-31G** to B3LYP/6-31G** does not influence the results.…”

“…A nearby saddle-shaped structure of C 2 symmetry, 3b, lies ca. 35 kJ mol À1 [14] lower in energy and compares closely with the experimental structure obtained from low-temperature X-ray measurements [8], although the precise position of the minimum is difficult to establish because of the extremely low energy of the corrugating modes [14]. The geometry of 3 was optimised at the RHF/ 6-31G** level under the constraints of D 7h and C 2 symmetry, respectively, using GAMESS-UK [17].…”

“…1). Previous ab initio calculations at the RHF level using various basis sets on both 3a (D 7h Þ and 3b (C 2 Þ have indicated that 3 is a floppy molecule with a notoriously shallow corrugation potential [14] (see also [15]). We show here that [7]circulene (3), despite its floppiness, sustains the same global pattern of counter-rotating currents as corannulene (1) and coronene (2), and we provide a graph-theoretical model that accounts for this behaviour of [n]circulenes in general (see also [16]).…”

Electronic structure, aromaticity and spectra of hetero[8]circulenes

“…If the sum of the sector angles of circulene is larger than 3608 (`sector excess'), the macrocycle adopts a corrugated (saddle-shaped) structure. This structure was established by X-ray diffraction for [7]circulene 7,39 (S sect = 60867 = 4208) and the recently synthesized [8]circulene (S sect = 60868 = 4808). This fact provides excellent support for the quantum chemical predictions of the saddleshaped structure of [8]circulene published earlier 40 ± 43 and it is in complete agreement with Wynberg ± Dopper's structural model.…”