Antiaromaticity in relation to 1,3,5,7-cyclooctatetraene structures

An isodesmic energy analysis has been carried out at the MP 2/6–31G *//HF /3–21G level for the nonplanar ground state ( 1 ) of 1,3,5,7-cyclooctateraene and for two planar forms, one having complete π delocalization ( 2 ) and the other having alternating single and double bonds ( 3 ). 1 is found to have a considerable degree of stabilization, which is attributed to limited π delocalization. The polyene 3 is the more stable of the two planar forms; it is a transition state in the inversion between two possible nonplanar structures. 2 is found to be a triplet at the Hartree–Fock level and is a critical point on an alternate pathway between the two possible arrangements of alternating single and double bonds in 3 . Both 2 and 3 have negative isodesmic energies, indicating the presence of stabilizing factors. Our results for 3 show that an “antiaromatic” system need not necessarily show a net destabilization. © 1994 John Wiley & Sons, Inc.