title: Theoretical studies on the structures and the aromaticity for condensed cyclobutadienoids series: the combination of Kekulé structures.
abstract: The structures and aromaticity of a series of condensed cyclobutadienoids were investigated by ab initio molecular orbital and density functional methods. A boat-type structure was found to be the most stable for all cyclobutadienoids except butalene, and the structures could also be predicted from a simple combination of asymmetric Kekulé structures. There were found to be three types of stable structure for cyclobutadienoids (C(2n)H(4)). In the case of 2n = 6m (m = 1, 2, ...), the structure consists of a succession of six-membered pi-resonance ring units, while for 2n-4 = 6m, the structure is an assembly of six-membered pi-resonance ring units with two "double-bonds" in the center or at the ends of the structure. In all other cases, the structures are of symmetric Kekule type. The aromaticity for each ring was obtained on the basis of the index of deviation from aromaticity. Although all compounds examined here (except for butalene) showed anti- or nonaromatic nature in the whole molecule, the six-membered ring units in some molecules had an aromatic nature similar to that of butalene.
authors.affiliation: Department of Chemistry, Faculty of Engineering, Gifu University, 1-1, Yanagido, Gifu 501-1193 Japan.
authors.forename: Shogo
authors.initials: S
authors.lastname: Sakai
authors.forename: Taro
authors.initials: T
authors.lastname: Udagawa
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kita