HARTREE-FOCK SYMMETRY-BREAKING IN MAGNESIUM AND NICKEL PORPHYRINS

Since previous studies of free-base porphyrin (H2P) have shown that Ha rtree-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 sy mmetry to the porphyrin framework. Thus, HF geometry optimizations and harmonic frequency analyses are performed for magnesium (MgP) and nic kel porphyrin (NiP) using the 3-21G, 6-311G, and polarized 6-311G(d) b asis 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 H2P. In the case of NIP, a qua litatively correct, high-symmetry geometry is found at the HF/3-21G le vel, but larger basis sets yield a bond-alternating form. The threshol d to symmetry-breaking in porphyrins is thus reduced further in NiP, d ue mainly to 3d bonding interactions which stabilize the porphyrin fra mework.