Madelung Energy Calculations on the Highly Conducting Molecular Metal Nickel Phthalocyanine Iodide
William B. Euler, Inorganic Chemistry, 1984, 23, 2645 – 2650
Abstract
The Madelung energy of the one-dimensional molecular metal NiPcI is calculated as a function of charge transfer and stacking motif along the unique axis. The partial ionicity along the conducting stack is modeled by both uniform, delocalized charges (Hartree-Fock limit) and distributed, localized charges (Wigner limit). It is found that the Madelung energy is minimized at the integral oxidation state ρ = 1 for the Hartree-Fock limit, whereas the Wigner limit is minimum for nonintegral ρ. The most negative Madelung energy is found in the Wigner limit, but only when the anion and cation stacks are highly correlated is there sufficient Madelung energy to balance the energetic cost of charge transfer. The macromolecular stacking axis of NiPcI has alternate rings rotated by 39.5°; however, Madelung energy calculations show that a rotation angle of 0° would be 0.1–0.2 eV/molecule more stable. The observed rotation angle arises from a balance of a variety of forces.