Investigating the features in differential absorbance spectra of NOM associated with metal ion binding: A comparison of experimental data and TD-DFT calculations for model compounds

Abstract

In this study, seven model compounds containing typical functional groups (phenolic and carboxylic groups) present in nature organic matter (NOM) were used to ascertain the nature of the characteristic bands in differential absorbance spectra (DAS) of NOM that are induced by metal ion binding. Some similarities were found between the DAS of the examined model compounds, caffeic acid, ferulic acid, sinapic acid, terephthalic acid, isophthalic acid, esculetin and myricetin and those of NOM. The binding of Cu(II) with carboxylic group might produce two peaks, A1 and A2, while the binding of Cu(II) with phenolic group might produce all four Gaussian peaks, from A1 to A4 displayed in the DAS of NOM. The UV-visible spectra predicted using time-dependent density functional theory (TD-DFT)-based methods met well with the experimental DAS of model compounds at different stages of Cu(II) binding. It demonstrates that the features in absorbance spectra are chiefly caused by HOMO (Highest Occupied Molecular Orbital) - LUMO (Lowest Unoccupied Molecular Orbital) transitions in the molecule and that the appearance of peaks in DAS reflects the changes of the molecular orbitals around reactive functional groups in a molecule before and after metal ion binding. The basis of the DAS features of NOM that are induced by metal ion binding could be identified primarily by the frontier molecular orbital theory.

Novel Insights into the Molecular-Level Mechanism Linking the Chemical Diversity and Copper Binding Heterogeneity of Biochar-Derived Dissolved Black Carbon and Dissolved Organic Matter

“…The Gaussian bands with average maxima were located at approximately 230 (A1), 240 (A2), 252 (A3), 265 (A4), 275 (A5), and 296–369 nm (A6–A8 for DOM). These DAS features could be manifestations of ligand-to-metal charge-transfer (LMCT) excitations between copper and reactive functional groups, especially the most common carboxylic/phenolic donors. ,,,− LMCT excitations for photoactive DBC/DOM–copper complexes could lead to the oxidation of carboxylic/phenolic donors and reduction of copper­(II) to copper­(I) . Based on the reported DAS features, the Gaussian bands A2–A5 and A6–A8 were reported to be associated with the LMCT excitations of copper–carboxylic and copper–phenolic complexes, respectively.…”

“…These DAS features could be manifestations of ligand-to-metal charge-transfer (LMCT) excitations between copper and reactive functional groups, especially the most common carboxylic/phenolic donors. ,,,− LMCT excitations for photoactive DBC/DOM–copper complexes could lead to the oxidation of carboxylic/phenolic donors and reduction of copper­(II) to copper­(I) . Based on the reported DAS features, the Gaussian bands A2–A5 and A6–A8 were reported to be associated with the LMCT excitations of copper–carboxylic and copper–phenolic complexes, respectively. The unique presence of A6–A8 in DOM compared with DBC indicated that DOM had more significant LMCT excitations of copper–phenolic complexes, resulting in enhanced transfer bands at relatively longer wavelengths .…”

“…Based on the reported DAS features, the Gaussian bands A2–A5 and A6–A8 were reported to be associated with the LMCT excitations of copper–carboxylic and copper–phenolic complexes, respectively. The unique presence of A6–A8 in DOM compared with DBC indicated that DOM had more significant LMCT excitations of copper–phenolic complexes, resulting in enhanced transfer bands at relatively longer wavelengths . This result was consistent with the higher percentages of tannin-like compounds rich in polyphenolic groups in DOM than in DBC.…”

“…This result was consistent with the higher percentages of tannin-like compounds rich in polyphenolic groups in DOM than in DBC. Furthermore, the bands at ∼250 and 367–371 nm (e.g., A3 and A6–A8) could be chiefly associated with the electronic transitions of the highest occupied and lowest unoccupied molecular orbitals in carboxylic/phenolic-containing molecules (e.g., caffeic acid). , …”

“…According to Noda’s rules, the sequential changes in absorbance wavelengths caused by copper addition generally followed the order (230–280 nm → 230–245 nm) → 280–400 nm, suggesting that copper binding generally first took place with chromophores at <280 nm. These sequential binding properties could be associated with LMCT excitations of different reactive donors (e.g., carboxylic/phenolic groups) at varied copper concentrations. , Compared with the relative changes in DAS peak intensities, 2D-COS with enhanced spectral resolution can effectively reveal the hidden variations and sequential copper-binding responses of absorbance/chromophores. Notably, at the highest copper concentration, possible cation bridging for complexation may occur and cause scattering in the absorbance, while no obvious evidence for possible scattering effects was observed for the synchronous cross-peaks and asynchronous phase differences because the scattering was extremely weak.…”