Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate
Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate
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DOI：
10.1039/c0cp02343d
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发表时间：
2011-01-01
影响因子：
3.3
通讯作者：
Brown, Steven P.
中科院分区：
文献类型：
--
作者：
Barrow, Nathan S.;Yates, Jonathan R.;Brown, Steven P.
系统地研究了二硼酸锂 Li2O.2B(2)O(3) 中自旋 I = 3/2 B-11 核的魔角旋转 (MAS) NMR 自旋回波相移。观察到对四极频率 (omega(PAS)(Q)/2 pi = 3C(Q)/[4I(2I - 1)]）的明显依赖性：在 14.1 T 下所有研究的 MAS 频率（5 至 20 kHz）下，B3（较大的 C-Q）位点相移速度比 B4 位点慢。增加 MAS 频率会导致 B3 位点的相移速度明显减慢，而效果则不太明显对于 B4 站点。考虑到 5、25、80（自然丰度）和 100% B-11 同位素丰度的样本，随着 B-11 同位素丰度的增加，两个位点的相移变得更快。使用两个和三个偶极耦合 B-11 核的密度矩阵模拟使实验行为合理化。在所有模拟中都再现了实验观察到的较大 C-Q (B3) 位点的较慢相移，并通过所谓的“自发四极驱动重耦合机制”重新引入偶极耦合来解释，该机制对不同四极频率的 MAS 频率具有不同的依赖性。具体而言，孤立自旋对模拟表明，当四极频率等于 MAS 两倍时，自发四极驱动重耦合机制最有效。而对于孤立自旋对模拟，增加 MAS 频率会导致更快的相移，与三自旋模拟的实验结果一致，其中还包括使用 GIPAW 方法对二硼酸锂、偏硼酸锂和三硼酸锂中的 (2)J(11B-11B) 耦合进行的第一原理计算：揭示了明显的趋势。 (2)J(11B-11B) 耦合随着 B-O-B 键角和 B-B 距离的增加而增加，然而，计算出的 (2)J(11B-11B) 耦合很小（二硼酸锂中为 0.95、1.20 和 2.65 Hz），从而解释了为什么在实验中没有观察到 J 调制引起的零交叉，即使对于 25% B-11 的样品，其中显着的自旋回波强度仍然存在于 25% 的持续时间内。类似于 200 毫秒。
Magic-angle spinning (MAS) NMR spin-echo dephasing is systematically investigated for the spin I = 3/2 B-11 nucleus in lithium diborate, Li2O.2B(2)O(3). A clear dependence on the quadrupolar frequency (omega(PAS)(Q)/2 pi = 3C(Q)/[4I(2I - 1)]) is observed: the B3 (larger C-Q) site dephases more slowly than the B4 site at all investigated MAS frequencies (5 to 20 kHz) at 14.1 T. Increasing the MAS frequency leads to markedly slower dephasing for the B3 site, while there is a much less evident effect for the B4 site. Considering samples at 5, 25, 80 (natural abundance) and 100% B-11 isotopic abundance, dephasing becomes faster for both sites as the B-11 isotopic abundance increases. The experimental behaviour is rationalised using density matrix simulations for two and three dipolar-coupled B-11 nuclei. The experimentally observed slower dephasing for the larger C-Q (B3) site is reproduced in all simulations and is explained by the reintroduction of the dipolar coupling by the so-called "spontaneous quadrupolar-driven recoupling mechanism'' having a different dependence on the MAS frequency for different quadrupolar frequencies. Specifically, isolated spin-pair simulations show that the spontaneous quadrupolar-driven recoupling mechanism is most efficient when the quadrupolar frequency is equal to twice the MAS frequency. While for isolated spin-pair simulations, increasing the MAS frequency leads to faster dephasing, agreement with experiment is observed for three-spin simulations which additionally include the homogeneous nature of the homonuclear dipolar coupling network. First-principles calculations, using the GIPAW approach, of the (2)J(11B-11B) couplings in lithium diborate, metaborate and triborate are presented: a clear trend is revealed whereby the (2)J(11B-11B) couplings increase with increasing B-O-B bond angle and B-B distance. However, the calculated (2)J(11B-11B) couplings are small (0.95, 1.20 and 2.65 Hz in lithium diborate), thus explaining why no zero crossing due to J modulation is observed experimentally, even for the sample at 25% B-11 where significant spin-echo intensity remains out to durations of similar to 200 ms.