Effect of Transition State Aromaticity and Antiaromaticity on Intrinsic Barriers of Proton Transfers in Aromatic and Antiaromatic Heterocyclic Systems; An ab Initio Study

An ab initio study of two series of carbon-to-carbon proton transfer reactions is reported. The first series refers to the heterocyclic C4H5X+/C4H4X (X = CH−, NH, S, O, PH, CH2, AlH, BH) systems, and the second to the linear (X = CH−, NH, S, PH, O, CH2, AlH, BH) reference systems . The major objective of this study was to examine to what degree the aromaticity of C4H4X (X = CH−, NH, S, O, PH) and the antiaromaticity of C4H4X (X = AlH, BH) is expressed at the transition state of the proton transfer and how this affects the respective intrinsic barriers. From the differences in the barriers between a given cyclic system and the corresponding linear reference system , ΔΔH⧧ = ΔH⧧(cyclic) − ΔH⧧(linear), it was inferred that in the cyclic systems both aromaticity and antiaromaticity lower ΔH⧧(cyclic). This conclusion was based on the assumption that the factors not associated with aromaticity or antiaromaticity such as resonance, inductive and polarizability effects in the protonated species, and charge delocal...