Journal of the American Chemical Society, Vol.141, No.33, 13101-13113, 2019
Effect of Conformation on Electron Localization and Delocalization in Infinite Helical Chains [X(CH3)(2)](infinity) (X = Si, Ge, Sn, and Pb)
An intuitive explanation of the effects of conformation (backbone dihedral angle) on electron delocalization in infinite saturated regular helices [(CH3)(2)](infinity)Si, [(CH3)(2)Ge](infinity), [(CH3)(2)Sn](infinity), and [(CH3)(2)Pb](infinity) 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, sigma conjugation (geminal interactions) or sigma hyper-conjugation (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 pi-electron systems consisting of fused Huckel or Mobius four-membered rings.