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The DFT/ROCIS method has been used to model the Mn(acac)2 (I) and Mn(acac)3 (II) L2,3-edges absorption spectra. Non-hybrid (BP86 and PBE), hybrid (B3P, PBE0 and B3LYP) and hybrid meta-GGA (M06) functionals have been considered to test their efficiency in reproducing experimental features. Structural parameters of both I and II are scarcely affected by the functional selection and the same holds as far as the IL2,3-edges absorption spectrum is concerned. At variance to that, the use of the hybrid meta-GGA functional is decisive for a detailed assignment of the IIL2,3-edges spectral features. The role played by the structural arrangement about the central Mn ion appears less important than the Mn oxidation number in determining the overall spectrum appearance and the spectral features’ excitation energies (EEs). DFT/ROCIS results indicate that, both in I and II, the lower EE side of the L3 intensity distribution mainly includes states having the ground state spin multiplicity (S = 5/2 in I and S = 2 in II), whereas states with different spin multiplicity contribute, both in I and II, to the L3 higher EE side. The occurrence of states involving metal to ligand charge transfer transitions and implying low lying empty ligand-based orbitals on the I/IIL3 higher EE sides has been further confirmed herein.

In various mineralizing marine organisms, calcite or aragonite crystals form through the initial deposition of amorphous calcium carbonate (ACC) phases with different hydration levels. Using X-ray PhotoEmission Electron spectroMicroscopy (X-PEEM), ACCs with varied spectroscopic signatures were previously identified. In particular, ACC type I and II were recognized in embryonic sea urchin spicules. ACC type I was assigned to hydrated ACC based on spectral similarity with synthetic hydrated ACC. However, the identity of ACC type II has never been unequivocally determined experimentally. In the present study we show that synthetic anhydrous ACC and ACC type II identified here in sea urchin spines, have similar Ca L2,3-edge spectra. Moreover, using X-PEEM chemical mapping, we revealed the presence of ACC-H2O and anhydrous ACC in growing stereom and septa regions of sea urchin spines, supporting their role as precursor phases in both structures. However, the distribution and the abundance of the two ACC phases differ substantially between the two growing structures, suggesting a variation in the crystal growth mechanism; in particular, ACC dehydration, in the two-step reaction ACC-H2O → ACC → calcite, presents different kinetics, which are proposed to be controlled biologically. Keywords: Sea urchin spine regeneration, Anhydrous amorphous calcium carbonate, Hydrated amorphous calcium carbonate, Ca L2,3-edge spectra, PhotoEmission Electron spectroMicroscopy