Electronic Excitation Characteristics and Spectral Behavior of Phosphate Anions

Absorption spectra are fundamental bases for the qualitative and quantitative analysis of the target chemical, and the development of an analytical model can be improved by studying its characteristics and rules. In the present study, the electronic excitation characteristics of phosphate anions (H₂PO₄^-, HPO₄^2-, and PO₄^3-) were analyzed based on the charge-transfer spectrum. In addition, the absorption spectra of phosphate anions at the energy level of PBE0/6-311+G (d,p) were recorded based on the time-dependent density functional theory (TD-DFT) method. Different (HPO₄^2-)·(H₂O)_10- molecular clusters were theoretically constructed, and the combined TD-DFT method and independent gradient model revealed that red shift of the maximum absorption wavelength (λ_max) with the increase of phosphate anion concentration (0-10 mM) may be caused by the decrease of hydrogen bond interaction. In addition, the prominent dispersion in the short-wave region mainly resulted in the red shift of λ_max with the increase in optical path length (1-100 mm). Moreover, with the increase in spectral bandwidth (0.4-3.0 nm), λ_max slightly blue-shifted because of the increase in energy through the slit, and repeatability of the corresponding absorbance measurement at λ_max gradually improved. As the spectral bandwidth increased, light monochromaticity became poor, resulting in the decrease of the linearly fitted correlation coefficient of the concentration-absorbance curve. Finally, the multivariate analysis of variance results showed that the optical path length was the most significant factor that influenced the absorption spectral characteristics of phosphate anions. This study provides a basis for the qualitative and quantitative analysis of phosphate anions by using absorption spectra and also renders a theoretical reference for absorption spectroscopy of other chemicals.