Computational analysis of reaction specificity and characterization of a titanium(iv)-porphyrin reagent for determining hydrogen peroxide

The Ti-TPyP reagent, i.e., an acidic aqueous solution of oxo[5,10,15,20-tetra(4- pyridyl)porphyrinato]titanium(IV) complex, TiO(tpyp), was developed as a highly sensitive and selective spectrophotometric reagent for determining traces of hydrogen peroxide. The reagent exhibited a single absorption peak at 432 nm, and the addition of hydrogen peroxide to the reagent gave rise to a new peak at 450 nm with height being proportional to the added hydrogen peroxide concentration. The Ti-TPyP reagent is characterized by a very high sensitivity and specificity to hydrogen peroxide. Its high sensitivity can be explained to be due to a large light absorptivity of the porphyrin ligand moiety at 400 ∼ 450 nm (the Soret band). On the other hand, its specificity remains unexplained, although a substantial account for the reaction specificity of the reagent should be quite important to secure the reliability of the analytical method. We succeeded to clarify the reaction specificity of the TiO(tpyp) complex to hydrogen peroxide from the viewpoint of the reaction mechanism and molecular orbitals based on ab initio calculations. The results provided a well-grounded argument for the determination of hydrogen peroxide using the Ti-TPyP reagent experimentally. The consistency between the calculated and experimental results was further examined by the UV-visible spectral analysis for individual species involved in the reaction mechanism based on the theoretical calculations using the ZINDO method. We hope that this article should contribute to provide a new way not only to evaluate analytical reagents but also to develop novel analytical reagents.