Hartree-Fock symmetry-breaking in magnesium and nickel porphyrins

Abstract

Since previous studies of free-base porphyrin (H 2P) have shown that Hartree-Fock (HF) theory yields a qualitatively incorrect, low-symmetry, bond-alternating structure, this paper addresses whether the addition of a central Mg or Ni atom is a perturbation sufficient to restore symmetry to the porphyrin framework. Thus, HF geometry optimizations and harmonic frequency analyses are performed for magnesium (MgP) and nickel porphyrin (NiP) using the 3-21G, 6-311G, and polarized 6-311G(d) basis sets. In the case of MgP, the addition of the central metal atom does not yield a delocalized structure, but the threshold to symmetry-breaking is greatly reduced relative to H 2P. In the case of NiP, a qualitatively correct, high-symmetry geometry is found at the HF/3-21G level, but larger basis sets yield a bond-alternating form. The threshold to symmetry-breaking in porphyrins is thus reduced further in NiP, due mainly to 3d bonding interactions which stabilize the porphyrin framework.