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Observation of bicarbonate in calcite by NMR spectroscopy

¹ Center for Environmental Molecular Science, Department of Chemistry, State University of New York, Stony Brook, New York 11794-3400, U.S.A.
² Center for Environmental Molecular Science, Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, U.S.A.

Correspondence: ^* E-mail: brian.phillips{at}sunysb.edu

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.

Key Words: Calcite • NMR spectroscopy • bicarbonate • trona • nahcolite • hydrogen bond

This article has been cited by other articles:

				M. E. Fleet and X. Liu Hydrogen-carbonate ion in synthetic high-pressure apatite American Mineralogist, October 1, 2007; 92(10): 1764 - 1767. [Abstract] [Full Text] [PDF]