A DFT and TD-DFT study on electronic structures and UV-spectra properties of octaethyl-porphyrin with different central metals(Ni, V, Cu, Co)

Abstract

In this work, the octaethyl-porphyrins with different central metals (M-OEP, M = Ni, VO, Cu, Co) were used to investigate the ground-state molecular structure, electron distribution and UV-spectra properties on molecular level by density functional theory (DFT). The results showed that the calculation structure parameters of metalloporphyrins agreed well with the experimental value. According to the Natural Bond Orbital (NBO) analysis, the charge distribution of different metalloporphyrins was found that the charge values of the central metal M decreased with the order of VO < Ni < Co < Cu, while the bonding strength between M and the coordinating atom N was VO > Ni > Co > Cu. At the same time, the frontier molecular orbital calculations showed that the SOMO energy of VO (OEP) molecules in the open-shell system was higher than that of Co (OEP) and Cu (OEP), which means that its UV absorption characteristic peak would be red-shifted. In addition, the IEFPCM model of Time-dependent Density functional theory (TD-DFT) was further utilized to simulate the four substance in toluene solution: Co (OEP), Ni (OEP), Cu (OEP) and VO (OEP), and the Soret band peaks were calculated respectively as: 382 nm, 383 nm, 391 nm and 401 nm. Furthermore, the quantitative simulation analysis of metalloporphyrins was combined with experimental data. It could be found that the location rules of the four kinds of metalloporphyrins calculated absorption characteristic peaks were consistent with the experimental ones, and the relative errors of each peak were within 3%. These methods used above provide a theoretical path for analyzing and identifying unknown porphyrin compounds in petroleum.