title: Homobenzene: homoaromaticity and homoantiaromaticity in cycloheptatrienes.
abstract: Cycloheptatriene (C(s)) is firmly established to be a neutral homoaromatic molecule based on detailed analyses of geometric, energetic, and magnetic criteria. Substituents at the 7 (methylene) position, ranging from the electropositive BH2 to the electronegative F, favor the equatorial conformation but influence the aromaticity only to a small extent. By the same criteria, the planar transition state (C(2v)) for cycloheptatriene ring inversion is clearly antiaromatic. This is attributed to the involvement of the pseudo-2pi-electrons of the CH2 group with the 6pi-electrons of the ring to give an 8pi-electron system. Similarly, the participation of the CH2 groups into C(2v) cyclopentadiene and cyclononatetraene lead to significant 4n + 2 pi electron aromaticity. The cyclization of cycloheptatriene to norcaradiene proceeds via a highly aromatic transition structure, but norcaradiene itself is less aromatic than cycloheptatriene. An annelated cyclopropane ring does not function as effectively as a double bond in promoting cyclic electron delocalization.
authors.affiliation: Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, PR. zhongfangchen@gmail.com
authors.forename: Zhongfang
authors.initials: Z
authors.lastname: Chen
authors.forename: Haijun
authors.initials: H
authors.lastname: Jiao
authors.forename: Judy I
authors.initials: JI
authors.lastname: Wu
authors.forename: Rainer
authors.initials: R
authors.lastname: Herges
authors.forename: S B
authors.initials: SB
authors.lastname: Zhang
authors.forename: Paul von Ragué
authors.initials: Pv
authors.lastname: Schleyer