Isomerization of the prenucleation building unit during crystallization of ALPO4-CJ2:: An MQMAS, CP-MQMAS, and HETCOR NMR study

The structure of AlPO4-CJ2 aluminophosphate has been reinvestigated by MAS, MQMAS (multiple quantum magic angle spinning), CP-MQMAS (cross polarization-MQMAS), and HETCOR (heteronuclear correlation) NMR spectroscopy. The CP-MQMAS method showed that the sample, when not allowed adequate time for crystallization, included a substantial concentration of amorphous species. The crystalline component was cleanly singled out by this technique. We discuss the relative populations and the distributions of F and OH groups within the structural building units (SBUs) and the distribution of various SBUs within the crystalline structure of AlPO4-CJ2. For the latter case, a mixture of SBUs is demonstrated from the F-19 --> P-31 HETCOR spectrum. Prenucleation building units (PNBUs) are the principal objects used for efficient crystal formation. They may undergo an isomerization to reach the proper configuration to integrate into the solid network. The isomerization implies formation of a bridge within the PNBU, probably at random with respect to OH or F. As integration in the network is taking place parallel to isomerization, the network forms as domains containing a mixture of the different types of SBUs.