title: Why do two π-electron four-membered Hückel rings pucker?
abstract: Notwithstanding their two (i.e., 4n + 2) π electrons, four-membered ring systems, 1-4, favor puckered geometries (1a-4a) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C(4)F(4)(2+)) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH(3) in C(4)(SiH(3))(4)(2+)) enhance hyperconjugation and increase the ring inversion barriers substantially.
authors.affiliation: Center for Computational Chemistry, Department of Chemistry, University of Georgia, Athens, Georgia 30602-2525, USA.
authors.forename: William C
authors.initials: WC
authors.lastname: McKee
authors.forename: Judy I
authors.initials: JI
authors.lastname: Wu
authors.forename: Matthias
authors.initials: M
authors.lastname: Hofmann
authors.forename: Armin
authors.initials: A
authors.lastname: Berndt
authors.forename: Paul von Ragué
authors.initials: Pv
authors.lastname: Schleyer