Resonance Raman enhancement of FeIVO stretch in high-valent iron porphyrins: An insight from TD-DFT calculations

Density functional theory (DFT) has been applied to explain the origin of resonance Raman enhancement associated with the FeIVO stretch observed in iron(IV)oxo porphyrins. To accomplish this electronic excitations of the Im–(Por)FeIVO model were computed in the 1.5–4.0 eV spectral range using time-dependent DFT (TD-DFT). All electronic transitions having dominant π → π∗ character were analyzed and assigned in terms of one-electron excitations. It was found that the most intense Soret band has a multi-component character, but the π (a2u) → π∗(dxz, dyz) and π (a1u) → π∗(dxz, dyz) electronic excitations are primarily responsible for observed resonance enhancement of the FeIVO stretch.