Intrinsic hydrated defects in calcium carbonate: A solid state NMR spectroscopic study

Common biogenic materials constructed with calcium carbonate mineral phases and an organic matrix exhibit delicate structure with unusual mechanical properties, and have inspired ideas for the design and synthesis of biomimetic functional materials. The organic templates play an important role in directing assembly of the mineral/organic composites, and in controlling the nucleation and subsequent crystallization. Consequently, knowledge about the molecular interactions at mineral/organic interfaces is essential for understanding the principles of the organic-matrix-mediated biomineralization process. Double resonance NMR techniques based on nuclear spin dipole-dipole interactions can provide local structural information about the mineral/organic interface. NMR studies of natural biogenic materials have relied on the assumption that H occurs principally in the organic matrix. However, even abiogenic nominally anhydrous calcium carbonate phases can contain significant amounts of H-containing defects, which could interfere with interpretation of spectroscopic results. Therefore, the occurrence of intrinsic hydrated defects in CaCO3 must be considered in studies of biomineralized and biomimetic materials. The purpose of this work is to determine the nature of the H environments in CaCO3 phases.;The research reported in this thesis focuses primarily on synthetic, additive-free calcium carbonate materials, including calcite, aragonite, and an amorphous phase which is now thought to be a common precursor to crystallization in inorganic and biotic systems. The single pulse and double resonance (mostly CP and HETCOR) 13C and 1H NMR spectra were obtained systematically for these phases, giving qualitative information about the H-bearing species involved in the intrinsic defects. These NMR studies provide a baseline for further NMR analysis of biogenic materials or synthesized mineral/organic composites. A correlation based on 13C{1H} cross-polarization kinetics was also established, from which more quantitative geometrical information can be obtained at low 13C/1H abundance. This method was applied to a synthetic calcite/citrate composite, providing evidence for the close association of citrate with carbonate ions in the calcite host.