NMR spectroscopy of citrate in solids: Cross-polarization kinetics in weakly coupled systems

Solid-state NMR spectroscopy is a potentially powerful method for obtaining molecular level structural information crucial for understanding the specific relationship between calcite crystals and occluded organic molecules that are important in biomineralization and biomimetic materials. In this work, a method is developed based on cross-polarization/magic angle spinning (CP/MAS) NMR to measure the heteronuclear distances and obtain structural information for large intracrystalline citrate defects in a synthetic calcite/citrate composite. Using compounds with well-characterized crystal structures, Mg(II) citrate and Sr(II) citrate, a correlation is established between T₁₅, the CP time, and M₂¹⁵, the van Vleck heteronuclear dipolar second moment, which contains distance and structural information. This correlation is supported by peak assignments obtained from calculations of the ¹³C chemical shifts for crystalline Mg(II) citrate. On the basis of T _IS^-1 versus M₂¹⁵ correlation, measurement of T_I5 for carbonate ions associated with citrate defects in a cakite(¹³C-enriched)/citrate coprecipitate yields an estimate for the distance between citrate and the nearest carbonate carbon that indicates close spatial proximity and provides useful constraints for future computational study. The applicability of T_IS versus M ₂¹⁵ correlations to other weakly coupled spin-1/2 systems is discussed in terms of the effects of ¹H homonuclear dipolar coupling, using the CP kinetics of Zn(II) dihydroxybenzoate and kaolinite for comparison. The results suggest a limited range of correlation constants and indicate that quantitative information can be obtained from CP/MAS kinetics obtained under similar experimental conditions.