The Photoelectron Spectrum of [18]Annulene

Abstract

The PE. spectrum of [18]annulene has been measured and correlated with MO‐calculations. The experimental ionization energies can only be explained by computing the electronic states of the cation, that is by taking into account the electron correlation and reorganization in the ionic states. The results allow a discussion of the structure of the neutral molecule; they are consistent with a D6hpoint group of symmetry.

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.…”

“…The same relative insensitivity to structural changes (and the lack of high resolution data) characterizes other spectroscopic methods which have been applied to [18]annulene. [18,19] [14]Annulene: Although the highest possible symmetry of [14]annulene is D 2h , this lower homologue of C 18 H 18 suffers from having more severe nonbonded repulsions among the intra-annular hydrogen atoms. These are ameliorated in nonplanar geometries.…”

Optimization of dispersive liquid–liquid microextraction of Co(II) and Fe(III) as their oxinate chelates and analysis by HPLC: Application for the simultaneous determination of Co(II) and Fe(III) in water samples

“…The 1995 X‐ray reinvestigation at 111 K revealed some disorder in the [18]annulene crystal14 but gave CC bond lengths of 1.385 and 1.405 Å and confirmed the essential D 6 h symmetry 16. Electron correlated MP2 and various DFT computations4, 13, 17 reproduced the X‐ray structure well and supported the experimental conclusions that C 18 H 18 has a “highly delocalized D 6 h structure.” Other indirect evidence based on IR, UV/Vis, fluorescence, and photoelectron spectroscopy18, 19 also was consistent with a D 6 h structure. However, data from such methods are not sensitive to geometry changes.…”

“…The same relative insensitivity to structural changes (and the lack of high resolution data) characterizes other spectroscopic methods which have been applied to [18]annulene 18. 19…”

Conjugated Polymers and Aromaticity

“…The structural alternatives for the other prototypical annulene, [18]annulene, 15/16 , and its related aromaticity have also been extensively studied. Most experiments such as X-ray, , heat of formation, photoelectron spectroscopy, and UV/vis are consistent with a D 6 h structure, 16 . Although earlier theoretical studies − found that the bond alternating D 3 h structure, 15 , might be more stable, later electron correlated calculations ,− indicated that the D 6 h , 16 , structure is the more favorable of the two.…”