A solid-state 53Cr NMR study of chromate and dichromate salts

Solid-state Cr-53 NMR spectra of a series of chromate (CrO42-) and dichromate (Cr2O72-) salts have been examined by employing the stepped-frequency quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) experiment and high applied magnetic field strengths, 11.75 and 18.8 T. Cr-53 nuclear quadrupolar coupling constants, C-Q(Cr-53), ranging from 1.23 to 5.01 MHz for the CrO42- salts and 7.25 to 8.14 MHz for the Cr(2)O(7)2- salts have been measured. For the dichromate salts, this corresponds to central transition Cr-53 NMR lineshapes of 200-250 kHz at 18.8 T. The use of hyperbolic secant (HS) pulses in combination with the Hahn-echo (HE) or QCPMG experiment results in significant sensitivity enhancements when acquiring Cr-53 NMR spectra of magic-angle spinning (MAS) samples, provided the MAS rate is fast with respect to the second-order quadrupolar interaction. For the CrO42- and Cr2O72- salts, the anisotropic chromium magnetic shielding interaction is generally negligible compared to the second-order Cr-53 nuclear quadrupolar interaction. No simple correlation between the structure of the CrO42- and Cr2O72- anions and the observed C-Q(Cr-53) values has been found. Copyright (c) 2007 John Wiley & Sons, Ltd.