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

Solid-state (53)Cr NMR spectra of a series of chromate (CrO4(2-)) and dichromate (Cr2O7(2-)) 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, CQ(53Cr), ranging from 1.23 to 5.01 MHz for the Cr(4(2-) salts and 7.25 to 8.14 MHz for the Cr2O7(2-) salts have been measured. For the dichromate salts, this corresponds to central transition 53Cr 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 53Cr NMR spectra of magic-angle spinning (MAS) samples, provided the MAS rate is fast with respect to the second-order quadrupolar interaction. For the CrO4(2-) and Cr2O7(2-) salts, the anisotropic chromium magnetic shielding interaction is generally negligible compared to the second-order 53Cr nuclear quadrupolar interaction. No simple correlation between the structure of the CrO4(2-) and Cr2O7(2-) anions and the observed CQ(53Cr) values has been found.