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

摘要: Magic-angle spinning (MAS) NMR spin-echo dephasing is systematically investigated for the spin I = 3/2 11B nucleus in lithium diborate, Li2O·2B2O3. A clear dependence on quadrupolar frequency (ωPASQ/2π 3CQ/[4I(2I − 1)]) observed: B3 (larger CQ) site dephases more slowly than B4 at all MAS frequencies (5 to 20 kHz) 14.1 T. Increasing leads markedly slower site, while there a much less evident effect site. Considering samples 5, 25, 80 (natural abundance) and 100% isotopic abundance, becomes faster both sites as abundance increases. The experimental behaviour rationalised using density matrix simulations two three dipolar-coupled nuclei. experimentally observed larger CQ (B3) reproduced explained by reintroduction of dipolar coupling so-called “spontaneous quadrupolar-driven recoupling mechanism” having different frequencies. Specifically, isolated spin-pair show that spontaneous mechanism most efficient when equal twice frequency. While simulations, increasing dephasing, agreement with experiment three-spin which additionally include homogeneous nature homonuclear network. First-principles calculations, GIPAW approach, 2J11B−11B couplings metaborate triborate are presented: trend revealed whereby increase B–O–B bond angle B–B distance. However, calculated small (0.95, 1.20 2.65 Hz diborate), thus explaining why no zero crossing due J modulation experimentally, even sample 25% where significant intensity remains out durations ∼200 ms.