Metal-ion complexation effects in C 1s-NEXAFS spectra of carboxylic acids-Evidence by quantum chemical calculations

Previous systematic C 1s-NEXAFS studies carried out for humic acid (HA) loaded with polyvalent metal cations (Mn+) reveal spectral features which were postulated to result from metal ion complexation: a strong decrease of the C 1s (COO−) → π*CO transition intensity and the appearance of a new absorption feature at slightly lower energy adjacent to the carboxyl resonance. Although spectroscopic results for the Mn+-PAA (polyacrylic acid) system (selected as model for structurally ill-defined HA) reveal the same spectral features, evidence by an independent approach for interpretation of these features is desirable. It is well established that quantum chemical calculations are capable of reproducing transition features in C 1s core excitation spectra of small organic molecules, e.g., acetate, which is chosen here as a fragment containing the complexing group to model the macromolecular HA and PAA systems. In this study, the RI-ADC(2) approach, as implemented in the TURBOMOLE program package, is applied to calculate vertical core excitation spectra of various metal acetates and the acetate anion. An energy shift (Δf) between the C 1s (COO−) → π*CO transition of the acetate anion and various metal cation acetates is established. Calculated shifts are very similar to the experimentally observed values for the energy difference between the C 1s (COO−) → π*CO peak and the absorption feature appearing after metal ion complexation in Mn+-PAA/PAA. According to our computations, structural changes of the acetate complexes (e.g., the O-C-O bond angle) compared to the free acetate anion are predominantly responsible for the spectral changes observed upon metal ion complexation.