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 C ₂ form over a non-alternating planar D _6h structure deduced from diffraction, infrared (IR) and electronic spectral studies. Here it is shown that computed Raman spectra for the D _6h and C ₂ forms are expected to be very different. However, two equivalent non-D _6h bond-alternate minima of D _3h or C ₂ 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.