The structure of [18]-annulene: Computed Raman spectra, zero-point level and proton NMR chemical shifts Bruce S. Hudson ⇑ , Damian G. Allis Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100, United States highlights " Bond alternation of [18]-annulene at balance point between r and p 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. article info Article history: Available online 17 May 2012 Keywords: Bond alternation Theoretical calculations Zero-point level Raman spectrum NMR chemical shifts abstract [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 calcula- tions are reconsidered with inclusion of zero-point level averaging. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Annulenes, cyclic (CH) n structures, include the famous even-n species cyclobutadiene, benzene and cyclooctatetraene. The higher [4n + 2] cases of [10,14,18,22] etc., are aromatic species that, in contrast to benzene, may exhibit bond alternation. [18]-Annulene is at or near the boundary between equal bond and bond-alternate larger rings. The current state-of-the-art of reported computations [1] use density functional theory (DFT) methods, such as KMLYP/6- 311 + G ÃÃ , selected on the basis of their ability to predict structures that reproduce experimental proton NMR chemical shifts. The constrained equal bond D 6h , planar bond-alternating D 3h and non- planar C 2 structures are then evaluated by high-level single-point energy calculations (CCSD(T)/DZP(C)+DZ(H)(fc) and CCSD(T)/ DZ(fc)). These two high-level calculations show that a bond- alternate non-planar C 2 structure is lower in energy than the bond-equivalent D 6h form by 2.1 and 2.7 kcal/mol, respectively [1]. The highest energy reported for the C 2 /D 6h difference is 3.2 kcal/mol using the KMLYP functional [1]. In more correlated DFT methods (e.g., B3LYP) and conventional MP2, the D 6h structure is the most stable form. In [1] these are rejected on the basis of the above CCSD calculations and the fact that they predict the wrong proton NMR chemical shifts. In the KMLYP treatment two equivalent planar bond-alternat- ing D 3h structures are separated by the D 6h barrier. The non-planar bond-alternate C 2 form is further stabilized slightly relative to the D 3h form by out-of-plane motions that minimize inner-ring H ... H repulsion. The two equivalent D 3h minima are interchanged by C@C expansion and CAC compression along a mode that has b 2u symmetry in D 6h . Displacement along this mode beyond the D 3h minima results in a rapidly rising potential. It is the importance of a treatment of this aspect of the problem that is emphasized here. Experimental determination of the presence of bond alterna- tion, or its absence, has proven difficult. Diffraction studies show that the content of the average unit cell is best described as having two molecules with D 6h symmetry. This conclusion ‘‘must be wrong’’ [1] on the basis of observed and computed NMR proton chemical shifts. Comparison of IR [2,3] and electronic [4] spectra based on equal bond and bond-alternate structures favor the D 6h structure. The conclusions of these comparisons have been rejected 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.05.016 ⇑ Corresponding author. E-mail address: bshudson@syr.edu (B.S. Hudson). Journal of Molecular Structure 1023 (2012) 212–215 Contents lists available at SciVerse ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc