Danish Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.
J Magn Reson. 2013 May;230:98-110. doi: 10.1016/j.jmr.2013.01.012. Epub 2013 Feb 9.
(17)O MAS NMR spectra recorded at 14.1T and room temperature (RT) for (17)O-enriched samples of the two perrhenates, KReO4 and NH4ReO4, exhibit very similar overall appearances of the manifold of spinning sidebands (ssbs) for the satellite transitions (STs) and the central transition (CT). These overall appearances of the spectra are easily simulated in terms of the usual quadrupole coupling and chemical shift interaction parameters. However, a detailed inspection of the line shapes for the individual ssbs of the STs and, in particular, for the CT in the spectrum of KReO4 reveals line-shape features, which to our knowledge have not before been observed experimentally in 1D MAS NMR spectra for any quadrupolar nucleus, nor emerged from simulations for any combination of second-order quadrupolar interaction and chemical shift anisotropy. In contrast, such line-shape features are not observed for the corresponding ssbs (STs and CT) in the 14.1T RT (17)O MAS NMR spectrum of NH4ReO4. Considering the additional interaction of a combination of residual heteronuclear (17)O-(185/)(187)Re dipolar and scalar J coupling between this spin pair of two quadrupolar nuclei, spectral simulations for KReO4 show that these interactions are able to account for the observed line shapes, although the expected (1)J((17)O-(185/)(187)Re) six-line spin-spin splittings are not resolved. Low-temperature, high-field (21.1T) (17)O VT MAS NMR spectra of both KReO4 and NH4ReO4 show that full resolution into six-line multiplets for the centerbands are achieved at -90°C and -138°C, respectively. This allows determination of (1)J((17)O-(187)Re)=-268Hz and -278Hz for KReO4 and NH4ReO4, respectively, i.e., an isotropic (1)J coupling and its sign between two quadrupolar nuclei, observed for the first time directly from solid-state one-pulse 1D MAS NMR spectra, without resort to additional 1D or 2D experiments. Determination of T1((187)Re) spin-lattice relaxation times, observed indirectly through a 2D (17)O EXSY experiment for NH4ReO4 at several low temperatures, show that the dynamics observed for the ReO4(-) anion in the (17)O VT MAS NMR spectra at low temperatures are caused by self-decoupling of (1)J((17)O-(187)Re). The (1)J((17)O-(187)Re) values determined here for ReO4(-) from solid-state (17)O MAS NMR, along with literature (1)J((17)O-M) values for oxoanions (M being a quadrupolar nucleus) obtained from liquid-state NMR, have allowed correlations to be established between the reduced coupling constant (1)K((17)O-M)=2π(1)J((17)O-M)/(γ17OγMℏ) and the atomic number of M.
(17)O MAS NMR 光谱在 14.1T 和室温（RT）下记录，用于两种高铼酸盐，KReO4 和 NH4ReO4 的 (17)O 富集样品，卫星跃迁（ST）和中央跃迁（CT）的旋转边带（ssbs）的多重结构表现出非常相似的整体外观。这些光谱的整体外观很容易根据通常的四极耦合和化学位移相互作用参数进行模拟。然而，对 ST 的各个 ssbs 的线形状，特别是 KReO4 光谱中的 CT 的线形状进行详细检查，揭示出线形特征，据我们所知，这些特征以前在任何一维 MAS NMR 光谱中都没有在任何四极核中实验观察到，也没有从任何二阶四极相互作用和化学位移各向异性的组合的模拟中出现。相比之下，在 NH4ReO4 的 14.1T RT (17)O MAS NMR 光谱中，对应于 ST（ST 和 CT）的 ssbs 没有观察到这种线形状特征。考虑到两个四极核之间的残余异核（17）O-（185/）（187）Re 偶极和标量 J 耦合的组合的附加相互作用，对 KReO4 的光谱模拟表明，这些相互作用能够解释观察到的线形状，尽管预期的（1）J（（17）O-（185/）（187）Re）六线自旋-自旋分裂未得到解决。两种高铼酸盐的低温、高场（21.1T）（17）O VT MAS NMR 光谱表明，在 -90°C 和 -138°C 下，中心带完全实现了六线多重峰的完全分辨率。这允许分别确定 KReO4 和 NH4ReO4 的（1）J（（17）O-（187）Re）=-268Hz 和 -278Hz，这是两个四极核之间首次直接从固态单脉冲 1D MAS NMR 光谱中观察到的各向同性（1）J 耦合及其符号，而无需诉诸额外的 1D 或 2D 实验。通过在几个低温下对 NH4ReO4 进行二维（17）O EXSY 实验间接观察到（187）Re 的自旋晶格弛豫时间 T1（187）Re），表明在低温下观察到的 ReO4（-）阴离子在（17）O VT MAS NMR 光谱中的动力学是由（1）J（（17）O-（187）Re）的自去耦引起的。这里从固态（17）O MAS NMR 确定的 ReO4（-）的（1）J（（17）O-（187）Re）值，以及文献中液体状态 NMR 获得的氧阴离子（M 为四极核）的（1）J（（17）O-M）值，允许建立还原耦合常数（1）K（（17）O-M）=2π（1）J（（17）O-M）/（γ17OγMℏ）和 M 的原子数之间的关系。
