Abstract: Petroporphyrins are usually porphyrins-coordinated nickel/vanadium compounds. The types and forms of petroporphyrins are very complex. In this work, the geometry structure, electronic structure and spectral properties of of Ni²⁺ and VO²⁺ primary porphyrin (ETIO) type compounds were calculated by using PBE density functional, and the simulated results were in line with experimental results. The ETIO porphyrin ring is coordinated with Ni²⁺ or VO²⁺ via four nitrogen atoms to form molecules with D_4H symmetry and C_4V symmetry, respectively. The V-N bond is weaker than Ni-N bond because of the competition of axial coordination O atoms, therefore, V is more subject to hydrodemetallization (HDM) than Ni during residue hydrotreating. The electronic structure of Ni²⁺/VO²⁺-porphyrin compounds was analyzed. The resuts showed that the porphyrin ring had a remarkable conjugation effect, and the conjugated π orbitals were formed by the porphyrin ring overlaps with the 3d orbitals of Ni or V. Therefore, the interaction between porphyrin and Ni or V is very strong. The valence electron transitions of Ni²⁺-ETIO and VO²⁺-ETIO compounds were calculated by using time-dependent density functional theory (TDDFT), and the characteristic UV-Vis absorption was obtained. The Soret band with strong absorption and the Q band with weak absorption were calculated theoretically, which was consistent with the spectral characteristics reported in literature. These characteristic absorption in UV-Vis spectrum was explained theoretically.

Key words: petroporphyrins, electronic structure, UV-Vis spectrum, TDDFT