NMR and EPR studies on the charge transfer and formation of complexes through incompletely coordinated states

Changes in the state of coordination and electron distribution in a fine paniculate solid mixture under mechanical stressing are studied by solid state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), coupled with more conventional analytical tools, e.g., X-ray photoelectron spectroscopy or infrared spectroscopy. Systems examined are mainly Ca(OH)₂ + SiO₂ and metallic Al + hydrogel, TiO₂H₂O. Si-O-Ca or Al-O-Ti bridging bonds are formed either by charge transfer, i.e. an acid-base reaction in a broad definition, or by radical recombination. The charge transfer is also elucidated on the basis of electronegativity equalization, by taking low-coordination states into account.