The structure of [18]-annulene: Computed Raman spectra, zero-point level and proton NMR chemical shifts

[18]-Annulene has been of great interest from the structural point of view of its bond alternation. High-level calculations based on structures selected for agreement with NMR spectra lead to a bond-alternate C2 form over a non-alternating planar D6h structure deduced from diffraction, infrared (IR) and electronic spectral studies. Here it is shown that computed Raman spectra for the D6h and C2 forms are expected to be very different. However, two equivalent non-D6h bond-alternate minima of D3h or C2 geometries are separated by only a small barrier along a motion that involves CC stretching and compression. It is shown here that the zero-point level is above the barrier for this species. In light of that fact, the NMR calculations are reconsidered with inclusion of zero-point level averaging.

► Bond alternation of [18]-annulene at balance point between σ and π forces.
► X-ray diffraction as well as IR and UV/vis spectra indicated absence of alternation.
► Recent theory claims that [18]-annulene has bond alternation based on proton NMR.
► Computed Raman spectra very different for possible forms.
► Zero-point energy shown to be above barrier for interconversion of localized minima.