Electronic Structure of Free-Base and Transition-Metal Tetraazaporphy rins

Electronic energy levels and charge distribution for metal-free tetraazaporphyrin (H2TAP) and transition-metal tetraazaporphyrins (MTAP, M = Fe and Cu) have been calculated in the one-electron Hartree-Fock-Slater model. Spin densities were obtained for CuTAP and FeTAP. Optical transitions, photoelectron binding energies, and hyperflne fields are presented for comparison with available data and previous theoretical works. Density difference contour maps and a Mulliken atomic orbital population analysis are used to discuss similarities to the related porphine systems. Most bonding features expected on the basis of semiempirical calculations and chemical intuitive arguments are observed in the electron density maps. A transition-state procedure was used to investigate several optical transitions as well as one-electron binding energies. Spectroscopic features are in fair agreement with experiment and with extended-Huckel model results of Gouterman et al.