Onset of Amorphous Structure in CaCO3: Geometric and Electronic Structures of (CaCO3) n (n = 2–7) Clusters by Ab Initio Calculations

Abstract

Electronic structures, vibrational analyses, stabilization energies and amorphicity were analyzed for the most stable configurations of the neutral gas phase clusters of calcium carbonate (CaCO 3 ) n (n = 2-7). Minimum-energy structures were generated through simulated annealing using a mix of molecular dynamics/semiempirical optimization and then full optimization at the ab initio level (RHF/6-31G*). HF-level results are calibrated versus MP2/6-31G* results. Though many clusters are symmetric, the structural arrangements are not crystalline and resemble neither calcite nor aragonite. In general, greater symmetry means a lower stabilization energy. The BSSE-and zero point energy-corrected stabilization energies appear to plateau at about -113 kcal/mol when n = 7. Amorphous clusters start appearing when n = 4 and the lowest-energy minima for n C 5 are of C 1 symmetry. No solvent effect is necessary to induce amorphism.

The effect of Mg2+ incorporation on the structure of calcium carbonate clusters: investigation by the anharmonic downward distortion following method

“…The former type of structure has a relatively lower energy (around 0.5 kcal/mol) than the latter. These two configurations seem to be the same as the clusters that have been reported as the stable structure in the gas phase by RosasGarcia et al 19 . The ADDF method can further explore less stable structures, such as Structures 11 and 12.…”

“…This structural feature leads to the stacking sequence in which Ca ions are aligned by CO3 groups, which can be observed in the tetramer with structure 13. Previously, Roass-García et al has reported that the structure of (CaCO3)n clusters in gas phases is not similar to that of crystalline phases 19 ; however, the structure appearing in the present study is obviously related to the structure of calcite. The present calculation's adoption of the solvent effect may account for these differences.…”

“…1. In this figure, the most stable arrangement is shown as Structure 1, which is also reported in the previous calculation as the most stable configuration of a dimer in a gas phase 19 . Some structures appear in both the present and previous calculations while others are observed in only one or the other.…”

“…Later, Pouget et al used a cryogenic transmission electron microscope to directly observe and confirm the existence of those same clusters 13 . There have also been many attempts to use computer simulation to understand early-stage CaCO3 formation [14][15][16][17][18][19] . For example, Raiteri and Gale used a molecular dynamics (MD) simulation with a newly developed accurate force field to show that the free energy of amorphous calcium carbonate (ACC) should decrease as more ion pairs of calcium carbonate form from the solution, suggesting that initial formation of CaCO3 may be barrierless and non-classical 15 20 performed a first-principles study of a Mg-containing aragonite surface and indicated that Mg 2+ substituted into a Ca site near the surface changes the surface structure.…”

Distinct Short‐Range Order Is Inherent to Small Amorphous Calcium Carbonate Clusters (<2 nm)

“…While it is still not clear why PCDA can stabilize such small ACC clusters, the number of CaCO 3 units seven finds support in recent ab initio simulations where the stabilization energy per monomer appears to reach a plateau at this size. [23] Moreover, control experiments with additional ligands ( Figures S2,3) show that besides PCDA, oleic acid and sodium bis(2 ethylhexyl)sulfosuccinate (NaAOT) can stabilize a small amount of clusters. It is speculated that only fatty acids with long alkyl chains and low chain order effectively stabilize the CaCO 3 clusters, especially when we compare the chemical structures of oleic acid and stearic acid which have the same chain length but different degrees of structural saturation.…”