Forgeron Michelle A M, Wasylishen Roderick E
Department of Chemistry, Gunning/Lemieux Chemistry Centre, University of Alberta, Edmonton, Alberta, T6G 2G2, Canada.
Magn Reson Chem. 2008 Mar;46(3):206-14. doi: 10.1002/mrc.2164.
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.
通过采用步进频率四极 Carr - Purcell Meiboom - Gill（QCPMG）实验以及 11.75 和 18.8 T 的高外加磁场强度，对一系列铬酸盐（CrO4(2 -)）和重铬酸盐（Cr2O7(2 -)）盐的固态 (53)Cr NMR 光谱进行了研究。已测量出 Cr(4(2 -)盐的 Cr - 53 核四极耦合常数 CQ(53Cr)在 1.23 至 5.01 MHz 范围内，Cr2O7(2 -)盐的该常数在 7.25 至 8.14 MHz 范围内。对于重铬酸盐，这对应于 18.8 T 时中心跃迁 53Cr NMR 线宽为 200 - 250 kHz。当采集魔角旋转（MAS）样品的 53Cr NMR 光谱时，若 MAS 速率相对于二阶四极相互作用较快，则使用双曲正割（HS）脉冲与哈恩回波（HE）或 QCPMG 实验相结合会显著提高灵敏度。对于 CrO4(2 -)和 Cr2O7(2 -)盐，与二阶 53Cr 核四极相互作用相比，各向异性铬磁屏蔽相互作用通常可忽略不计。尚未发现 CrO4(2 -)和 Cr2O7(2 -)阴离子的结构与观测到的 CQ(53Cr)值之间存在简单的相关性。
