Small‐molecular‐weight (MW) additives can strongly impact amorphous calcium carbonate (ACC), playing an elusive role in biogenic, geologic, and industrial calcification. Here, we present molecular mechanisms by which these additives regulate stability and composition of both CaCO₃ solutions and solid ACC. Potent antiscalants inhibit ACC precipitation by interacting with prenucleation clusters (PNCs); they specifically trigger and integrate into PNCs or feed PNC growth actively. Only PNC‐interacting additives are traceable in ACC, considerably stabilizing it against crystallization. The selective incorporation of potent additives in PNCs is a reliable chemical label that provides conclusive chemical evidence that ACC is a molecular PNC‐derived precipitate. Our results reveal additive‐cluster interactions beyond established mechanistic conceptions. They reassess the role of small‐MW molecules in crystallization and biomineralization while breaking grounds for new sustainable antiscalants.

An anionic dilithium compound: The dianion [(R₂Si)₃Li₄]²⁻ (R=tBu₂MeSi) as salt 2 was synthesized as the first of its kind by dissolving 1 in THF. The structure of 2 was solved by X-ray crystallography and studied by solid-state and solution NMR spectroscopy. Compound 2 reacts as a dilithiosilane.

The ¹³C CPMAS NMR spectra of four crystalline forms of p-aminobenzenesulphonamide (sulphanilamide) were recorded at room temperature. Three of these forms (α, β, and γ) showed doublings in the resonances of the carbon atoms ortho to the amino group, but only a single signal was obtained from those ortho to the asymmetric sulphonamide group. A variabletemperature study allowed the interconversion of the α and β forms to the γ form to be monitored. Changes were also observed in the spectrum of the γ form as the temperature was increased, and were ascribed to the presence of 180° flips of the phenyl rings about their para axis. This interpretation was confirmed by analysis of the broadenings introduced by the assumed motion on the centreband and sidebands in the ¹³C CPMAS NMR spectrum of the exchanging nuclei. Variable-temperature spectra of the γ form were simulated in order to obtain information about the geometry, the rates and the activation parameters involved in the process. These calculations were in good agreement with the experimental data. The possible relevance that the observed doublings and ring motion may have for the mode of action of sulphonamides is also discussed.

In many marine organisms biomineralization–the crystallization of calcium-based ionic lattices–demonstrates how regulated processes optimize for diverse functions, often via incorporation of agents from the precipitation medium. We study a model system consisting of L-aspartic acid (Asp) which when added to the precipitation solution of calcium carbonate crystallizes the thermodynamically disfavored polymorph vaterite. Though vaterite is at best only kinetically stable, that stability is tunable, as vaterite grown with Asp at high concentration is both thermally and temporally stable, while vaterite grown at 10-fold lower Asp concentration yet 2-fold less in the crystal, spontaneously transforms to calcite. ssNMR shows that Asp is sparsely occluded within vaterite and calcite. CP-REDOR NMR reveals that each Asp is embedded in a perturbed occlusion shell of ~8 disordered carbonates which bridge to the bulk. In both the as-deposited vaterites and the evolved calcite, the perturbed shell contains two sets of carbonate species distinguished by their proximity to the amine and identifiable based on 13C chemical shifts. The embedding shell and the occluded Asp act as an integral unit which minimally rearranges even as the bulk undergoes extensive reorganization. The resilience of these occlusion units suggest that large Asp-free domains drive the vaterite to calcite transformation—which are retarded by the occlusion units, resulting in concentration-dependent lattice stability. Understanding the structure and properties of the occlusion unit, uniquely amenable to ssNMR, thus appears a key to explaining other macroscopic properties, such as hardness.