Proton Transfers in Aromatic and Antiaromatic Systems. How Aromatic or Antiaromatic Is the Transition State? An Ab Initio Study

An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5), and the cyclobutenyl cation/cyclobutadiene (C4/C4H4) systems and their respective noncyclic reference systems, that is, ,  and . For the aromatic C6/C6H6 and C5H6/C5systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in t...