Isomerization of the prenucleation building unit during crystallization ofALPO(4)-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 polarizatio n-MQMAS), and HETCOR (heteronuclear correlation) NMR spectroscopy. The CP-M QMAS method showed that the sample, when not allowed adequate time for crys tallization, included a substantial concentration of amorphous species. The crystalline component was cleanly singled out by this technique. We discus s the relative populations and the distributions of F and OH groups within the structural building units (SBUs) and the distribution of various SBUs w ithin the crystalline structure of AlPO4-CJ2. For the latter case, a mixtur e of SBUs is demonstrated from the F-19 --> P-31 HETCOR spectrum. Prenuclea tion building units (PNBUs) are the principal objects used for efficient cr ystal formation. They may undergo an isomerization to reach the proper conf iguration to integrate into the solid network. The isomerization implies fo rmation 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 isomerizati on, the network forms as domains containing a mixture of the different type s of SBUs.