Ab Initio Calculation of UV–vis Absorption of Parent Mg, Fe, Co, Ni, Cu, and Zn Metalloporphyrins
G. Ganguly, Zdeněk Havlas, Josef Michl
Abstract
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 2+, Zn 2+, Co 2+, Ni 2+, Cu 2+, or FeCl 2+ ions in the 12,500–40,000 cm –1 region. Particular attention was paid to the interaction between the porphyrin ring and the metal 3 d electrons in states of different multiplicities (we used metal 3 d and double d -shell or 3 d ′ orbitals). For this class of compounds, the N -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 ( Q, B, N, and L ) of closed-shell metalloporphyrins. Inclusion of SO coupling was found to have very little effect on excitation energies and oscillator strengths. For FeCl 2+ porphyrin, we treated ligand-to-metal charge-transfer (LMCT; π,d ), metal ligand field ( d,d ), and metal-to-ligand charge-transfer (MLCT; d,π*) transitions within the same framework. The broad and intense spectral features associated with its B (Soret) band are attributed to multiconfigurational LMCT ( d,π*) bands involving strong metal–ligand orbital mixing.