Two-dimensional 43Ca-1H correlation solid-state NMR spectroscopy

Calcium-43 (nuclear spin, S=7/2) is an NMR insensitive low-γ quadrupolar nucleus and up until recently only one-dimensional solid-state 43Ca NMR spectra have been reported. Through-space correlation experiments are challenging between spin-12 and low-γ quadrupolar nuclei because of the intrinsically weak dipolar interaction and the often-low natural abundance of the quadrupolar nucleus. Rotary-resonance recoupling (R3) has recently been used to re-introduce hetero-nuclear dipolar interactions for sensitive high-γ quadrupolar nuclei, but has not yet been applied in the case of low-γ half-integer quadrupolar nuclei. Here an effective and robust 2D 1H-43Ca NMR correlation experiment combining the R3 dipole-recoupling scheme with 2D HMQC is presented. It is demonstrated that the weak 43Ca-1H dipolar coupling in hydroxyapatite and oxy-hydroxyapatite can be readily re-introduced and that this recoupling scheme is more efficient than conventional cross-polarization transfer. Moreover, three 43Ca-1H dipolar coupled calcium environments are clearly resolved in the structurally unknown oxy-hydroxyapatite. This local information is not readily available from other techniques such as powder XRD and high resolution electron microscopy. R3-HMQC is also a desirable experiment because the set-up is simple and it can be applied using conventional multi-resonance probes.