Structure and dynamics of amorphous calcium carbonate by magic-angle spinning NMR

Abstract

Characterizing the atomic-level structure of amorphous solids are challenging, because they lack the ordered arrangement of atoms found in crystals. A prominent example, which is omnipresent in nature, is amorphous calcium carbonate (ACC, CaCO3⋅H2O). ACC plays a key role in biomineralization processes and understanding its structure is key to understanding these processes. Despite its significance, a structural model of ACC is not yet available. Chapters 2 and 3 of this thesis show the strength of MAS NMR in studying the structure and dynamics of ACC. A series of one- and two-dimensional NMR experiments on ACC nanoparticles shows the presence of two chemically distinct environments. The first environment consists of immobile calcium and carbonate ions with embedded structural water molecules, which undergo 180° flips on a millisecond time scale. The second environment consists of water molecules, which undergo slow, but isotropic motion, and dissolved hydroxide ions. The second environment forms a network through the first environment, revealing the history of ACC as a colloid. The magnetic properties of the 1H and 13C nuclei in the first, rigid environment of ACC are similar to those in monohydrocalcite, a crystalline form of calcium carbonate with the same stoichiometry. These observations constitute a significant step forward in developing a structural model for ACC. Furthermore, the NMR-based structure-refinement approach presented in Chapter 3 can be used to characterize inorganic amorphous phases in general. While MAS NMR is thus a powerful spectroscopic tool, it suffers from a lack of sensitivity. This makes detecting specific atomic information in complex chemical systems very time-consuming or even impossible. Dynamic nuclear polarization (DNP) is a solution to this problem. By transferring polarization from unpaired electrons to nuclei, a 658-fold enhancement of NMR sensitivity can be achieved. In current commercial MAS DNP instruments, polarization transfer is induced by continuous microwave irradiation. This is clearly not the most efficient approach at high magnetic fields, which are essential to achieve chemical shift resolution, or when 𝑑- or 𝑓-block elements are used as a source of electron spin polarization. Chapter 4 presents an alternate approach namely to transfer polarization by pulse sequence, analogous to pulse sequences used in NMR, EPR, or MRI. The two-pulse phase modulation (TPPM) DNP pulse sequence is introduced which has a high-field compatible matching condition. A general theoretical description is provided for electron-proton polarization transfer by periodic DNP pulse sequences. Experiments and spin dynamics simulations at 1.2 T show that at high microwave peak power, the TPPM DNP pulse sequence outperforms other DNP pulse sequences with a high-field compatible matching condition, while the X-inverse-X (XiX) DNP pulse sequence is most efficient at low peak power. The experimental and theoretical results presented in Chapter 4 provide important reference points for developing DNP pulses sequences and effective pulsed MAS DNP. Finally, Chapter 5 explores triplet DNP, where enhancement of the NMR sensitivity is achieved by transferring polarization from highly polarized photo-excited triplet state. In triplet DNP, enhancement factors of over ~104 are possible. Numerical simulations show that efficient generation of triplet states of pentacene requires relatively long laser pulses (~1 μs). Preliminary experiments show an enhancement, but further optimization and testing are needed.