Stationary point structures (or stable structures) and the electronic states of polycyclic conjugated hydrocarbons are estimated by an extended combination method between two Kekulé structures (ECMK). The estimation by the ECMK required two postulation conditions. (i) Stationary point structures can be represented with one Kekulé structure or the combination of two Kekulé structures. (ii) Structures including element Kekulé structures of the structures with lower energy are not candidates for structures with higher energy. The structures and electronic states of 13 compounds composed of six-cyclic and four-cyclic rings were estimated by the ECMK, and corresponded to their geometries and the local aromaticity obtained by complete active space self-consistent field molecular orbital and B3LYP methods. Copyright © 2014 John Wiley & Sons, Ltd.

The mechanisms of the [2 + 2 + 2] cycloaddition reaction of three ethyne molecules were studied by ab initio molecular orbital and density functional methods. The transition states range from that of the concerted mechanism with D_3h symmetry to that of the stepwise mechanism with C₂ symmetry. The transition state structure and the activation energy depend on the basis set and computational method employed in the analysis. The activation energy barrier was determined to be in the range of 36–44 kcal/mol. The activation energy determined by various methods corresponds to the interaction energy, which is related to the electron correlation energy. The best estimation of the activation energy barrier is 41.6 kcal/mol, achieved from the relation between the interaction energy and the activation energy. Copyright © 2013 John Wiley & Sons, Ltd.

The combination method between asymmetric Kekulé structures (CMAK) was expanded to the structure prediction of a series of acenes and phenylenes. The structures predicted by the CMAK correspond to those calculated by the ab initio molecular orbital (MO) and B3LYP density functional methods. The CMAK can characterize the structures. The aromaticity for the whole ring and each ring in the treated compounds was estimated by the index of deviation from aromaticity (IDA). The estimated aromaticity corresponds fairly well to the characterization of the structure predicted by the CMAK. The predicted structures of linear acenes reveal the aromaticity of the central ring and also explain its reactivity. The side ring in [n]phenacene shows high aromaticity. The critical difference in the structure and aromaticity for the central six-membered ring of the isomers benzo[1,2:4,5]dicyclobutene and benzo[1,2:5,6]dicyclobutene was explained. Copyright © 2012 John Wiley & Sons, Ltd.

Cope rearrangement mechanisms and the homoaromaticity of semibullvalene, barbaralane, and 1,5-methanosemibullvalene in the ground and lowest excited states were studied by ab initio methods. In the ground state, the rearrangement reactions of semibullvalene and barbaralane occurred concertedly through the transition states with C_2v symmetry, and the transition states had a homoaromatic nature. In particular, the transition state of barbaralane exhibited the strongest homoaromaticity among the three systems treated here. On the other hand, for 7,8-methanosemibullvalene, the structure with C_2v symmetry was not a transition state but one with a stable energy minimum. The energy minimum structure with C_2v symmetry had a biradical character. The lowest excited states of semibullvalene and barbaralane were the excitation to the σ* anti-orbital, ¹B₂ and ¹B₁ states, and led to near di-allyl states. The lowest excitation state of 1,5-methanosemibullvalene had C_s symmetry and was the A″ state excitation in one side of two allyl parts. Copyright © 2011 John Wiley & Sons, Ltd.