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Annelated Semibullvalenes:  A Theoretical Study of How They “Cope” with Strain

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journal contribution
posted on 25.06.1997 by Haijun Jiao, Ruby Nagelkerke, Henry A. Kurtz, Richard Vaughan Williams, Weston Thatcher Borden, Paul von Ragué Schleyer
High levels of ab initio (MP2, CASSCF, CASPT2N) and density functional (Becke3LYP) theory have been used to assess the homoaromatic character of some strained semibullvalenes. Based on geometric, energetic, and magnetic criteria (magnetic susceptibility exaltations and the nucleus independent chemical shifts, NICS), Cs semibullvalene itself is not aromatic, but the C2v transition state for its Cope rearrangement is highly bishomoaromatic. Appropriate annelations destabilize the Cs geometries, and the bishomoaromatic structures are the only minima for several semibullvalenes:  1,5-methano; 2,8:4,6-bisethano and -bismethano; and 2,8-monoethano. In contrast, the 4,6-ethanosemibullvalene is predicted to be localized and not homoaromatic. Inclusion of dynamic electron correlation is very important for computing the geometries and relative energies of the delocalized structures. To aid experimental investigations, the UV and 13C NMR spectra of some of the semibullvalenes have been predicted. Long-wavelength UV absorptions and down-field 13C NMR chemical shifts for C2,8,4,6 are characteristic.