Effect of Conformation on Electron Localization and Delocalization in Infinite Helical Chains [X(CH₃)₂]_∞ (X = Si, Ge,...

An intuitive explanation of the effects of conformation (backbone dihedral angle) on electron delocalization in infinite saturated regular helices [(CH₃)₂]_∞Si, [(CH₃)₂Ge]_∞, [(CH₃)₂Sn]_∞, and [(CH₃)₂Pb]_∞ is offered in terms of the simple Ladder C model and confirmed by density functional theory calculations. The effective hole mass, which ranges from near zero to infinity as a function of conformation, is used as a measure of the degree of delocalization and relates to the effects of chain length extension in finite systems. The position of the Fermi level in reciprocal space has a simple counterpart in systems of finite length and is used to characterize the dominant mechanism, σ conjugation (geminal interactions) or σ hyperconjugation (vicinal interactions, through-bond coupling). Constructive or destructive interference of the two mechanisms produces three different delocalization regimes as a function of the backbone dihedral angle and analogy is drawn to polycyclic π-electron systems consisting of fused Hückel or Möbius four-membered rings.