Ab Initio Calculation of UV-vis Absorption of Parent Mg, Fe, Co, Ni, Cu, and Zn Metalloporphyrins.

Relativistic restricted active space (RAS) second-order multireference perturbation theory (MRPT2) methods, incorporating spin-orbit (SO) coupling perturbatively via state interaction (SO-MRPT2/RASSCF), were used to reproduce the absorption spectra of parent metalloporphyrins containing the Mg, Zn, Co, Ni, Cu, or FeCl ions in the 12,500-40,000 cm region. Particular attention was paid to the interaction between the porphyrin ring and the metal 3 electrons in states of different multiplicities (we used metal 3 and double -shell or 3' orbitals). For this class of compounds, the -electron valence state perturbation theory (NEVPT2) method is superior to the complete active space perturbation theory (CASPT2) and successfully reproduces the energies of all four characteristic transitions (, , , and ) of closed-shell metalloporphyrins. Inclusion of SO coupling was found to have very little effect on excitation energies and oscillator strengths. For FeCl porphyrin, we treated ligand-to-metal charge-transfer (LMCT; π), metal ligand field (), and metal-to-ligand charge-transfer (MLCT; ,π*) transitions within the same framework. The broad and intense spectral features associated with its (Soret) band are attributed to multiconfigurational LMCT (,π*) bands involving strong metal-ligand orbital mixing.