Protonation of archetypal aromatic and antiaromatic systems - G2 studies of benzene and cyclobutadiene

The problem of protonation of benzene (B) and cyclobutadiene (CB) is addres sed by the ab initio G2 benchmark computations. It appears that the sigma-c omplexes are the most favourable protonated forms in both cases. The proton affinity (PA) of (CB) is higher by 45 kcal/mol than that of B, which is ra tionalized by a decrease in the antiaromatic destabilization of the four-me mbered ring and stabilization by the allylic cation formation upon protonat ion accompanied by the ring puckering, which enables an effective additiona l homoconjugative interaction. The barrier for the hydrogen ring walk in B is estimated to be 8.6 kcal/mol leading to seven equivalent hydrogens on th e NMR time scale. In contrast, this barrier in CB is 43.6 kcal/mol implying that the ring walk is precluded in this cationic system. The ring flipping inversion barrier of the sigma-protonated CB is estimated to be 7 kcal/mol thus being in good accordance with experiment. The striking structural fea tures of sigma and pi-protonated configurations of B and CB involve degrees of pyramidalization and tetrahedral distortions of carbon atoms, which are briefly discussed. Some practical models, alternative to the G2 scheme, de signed for studying the PA of aromatic and antiaromatic compounds are exami ned and recommended. (C) 2000 Elsevier Science B.V. All rights reserved.