Density functional theory investigation of porphyrin diacid: electronic absorption spectrum and conformational inversion

The ground-state structure, electronic spectrum and conformational inversion of porphyrin diacid (H4P2+) have been studied with the density functional theory. In accord with available crystallographic data of related compounds, the most stable structure of H4P2+ molecule is the one bearing a saddle-distorted porphyrin ring (D2d symmetry), with the four pyrrole rings tilted up and down alternatively with respect to the porphyrin mean plane. Electronic absorption spectrum of H4P2+ was studied by time-dependent DFT method, and the calculations were compared with the solution experimental results. The computations correctly predict excitation energies and oscillator strengths at least for the first and second excited states. The conformational inversion between the two conjugate D2d structures was studied, and a multi-step process was proposed. The structures of transition states and intermediates as well as the inversion barriers were calculated.