Configuration change in [14]annulene requires Mobius antiaromatic bond shifting

For both [4n] and [4n + 2]annulenes, cases of configuration change in which the number of trans C=C bonds changes by 1 must involve a Mobius -bond shifting step. The known configuration change in [14]annulene (1, CTCTCTT -> 2, CCTCTCT), for which delta trans = 1, is thus predicted to involve a Mobius antiaromatic transition state for bond shifting. Using unrestricted density-functional theory (DFT), we have located the key transition state for this process, as well as other stationary points involved in the mechanism. CASPT2(14,14)/cc-pVDZ//(U)BH&HLYP/6-311+G** results provide a barrier of 19.3 kcal/mol for the overall process 1 -> 2, in good agreement with the known experimental barrier (21.3 kcal/mol). The bond-shifting transition state has substantial singlet diradical character and is strongly antiaromatic on the basis of its large positive NICS value. [14]Annulene thus represents the first example of a concerted reaction for which a Mobius antiaromatic transition state provides the only possible pathway.