The Low‐Temperature UV./VIS. Absorption Spectrum of [18]Annulene

Abstract

SummaryThe UV./VIS. absorption spectrum of [ 18lannulene has been remeasured in 3-methylpentane at room and at liquid nitrogen (glass) temperature and interpreted by the CNDO/S-CI-method. The confrontation of the experimental electronic transitions with the CNDO/S-CI-calculated ones favors a structure with D6h-symmetry, i.e. a structure with delocalized z-bonds.

Aromaticity: The Alternating CC Bond Length Structures of [14]‐, [18]‐, and [22]Annulene

“…[16] Electron correlated MP2 and various DFT computations [4,13,17] reproduced the X-ray structure well and supported the experimental conclusions that C 18 H 18 has a "highly delocalized D 6h structure." Other indirect evidence based on IR, UV/Vis, fluorescence, and photoelectron spectroscopy [18,19] also was consistent with a D 6h structure. However, data from such methods are not sensitive to geometry changes.…”

“…In addition to this NMR spectroscopic evidence, C 2 [18]annulene is appreciably lower in energy than the D 6h form at KMLYP and BHLYP as well as at MPW1PW91 ( Table 2). The same is true at the current cutting edge of theory: single-point energy comparisons using the highly correlated CCSD(T) method (with DZP(C) + DZ(H)) also predict the C 2 form to be more stable than the D 6h structure.…”

“…We establish herein that the preference for bond-alternating structures along the 4n+2 Hückel p-electron series begins at [14]annulene. Additionally, we also show that the next higher homologue, [18]annulene also shows remarkable CÀC bond alternation. Benzene is not the aromatic paradigm!…”

Aromaticity: The Alternating CC Bond Length Structures of [14]‐, [18]‐, and [22]Annulene

“…[3] Shaik, Hiberty, and co-workers have emphasized the strong global distortive tendency of p components towards localized bonding. [6][7][8] This tendency is not overcome in the most symmetrical geometries of the [14]-, [18]-, and [22]annulenes by the modest gain in delocalization energy resulting from bond-length equalization, even when combined with the preference of the s C À C bonds to have the same length. .…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…It has been concluded [267] that further experimental and computational effort should be made in order to confirm the C 2 structure for [18]annulene. Finally, although spectroscopic data do not allow the unequivocal assignment of the [18]annulene structure among geometrical alternatives [20], it is worth noting the better agreement with experimental results of infrared [164] and UV-vis [268] spectroscopy when calculations are made starting from the delocalized D 6h rather than from the localized D 3h structure [268,269]. Taking advantage of the occurrence of the inversion center for D 6h symmetry and of its absence for C 2 symmetry, a careful comparison of the infrared and near-infrared Raman spectra of the [18]annulene crystal at very low temperature appears to be highly recommended in order to solve the structural issue.…”