Hydrogen bonding induced distortion of CO₃ units and kinetic stabilization of amorphous calcium carbonate: results from 2D ¹³C NMR spectroscopy

Systematic correlation in alkaline-earth carbonate compounds between the deviation of the CO₃ units from the perfect D_3h symmetry and their ¹³C nuclear magnetic resonance (NMR) chemical shift anisotropy (CSA) parameters is established. The ¹³C NMR CSA parameters of amorphous calcium carbonate (ACC) are measured using two-dimensional ¹³C phase adjusted spinning sidebands (PASS) NMR spectroscopy and are analyzed on the basis of this correlation. The results indicate a distortion of the CO₃ units in ACC in the form of an in-plane displacement of the C atom away from the centroid of the O₃ triangle, resulting from hydrogen bonding with the surrounding H₂O molecules, without significant out-of-plane displacement. Similar distortion for all C atoms in the structure of ACC suggests a uniform spatial disposition of H₂O molecules around the CO₃ units forming a hydrogen-bonded amorphous network. This amorphous network is stabilized against crystallization by steric frustration, while additives such as Mg presumably provide further stabilization by increasing the energy of dehydration.