Observation of bicarbonate in calcite by NMR spectroscopy

We show that the signal observed in ¹³C{¹H} cross-polarization magic-angle-spinning (CP/MAS) NMR spectra of synthetic calcite precipitated at room temperature arises from bicarbonate ion. Although this peak occurs at the same chemical shift as for the bulk carbonate signal, its observation by CP/MAS indicates that it corresponds to carbonate associated with H. The variation in the CP/MAS intensity with contact time shows oscillations characteristic of C-H pairs separated by 1.9(1) Å and remote from other H, consistent with bicarbonate. ¹³C{¹H} heteronuclear correlation spectra indicate that the hydrogen in the bicarbonate groups gives a relatively narrow ¹H NMR signal at +7.4 ppm. A peak at this chemical shift is also observed in direct-observe ¹H MAS NMR spectra of the synthetic sample, and also in natural biogenic and abiogenic calcite. This ¹H chemical shift indicates a moderate hydrogen bonding interaction d(OH...O) ≈ 1.85 Å, which suggests significant structural relaxation occurs near the bicarbonate.