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\udI = 3/2 11B nucleus in lithium diborate, Li2O.2B2O3. A clear dependence on the quadrupolar\udfrequency (ωQPAS/2π = 3CQ/[4I (2I – 1)]) is observed: the B3 (larger CQ) site dephases more slowly\udthan the B4 site at all investigated MAS frequencies (5 to 20 kHz) at 14.1 T. Increasing the MAS\udfrequency leads to markedly slower dephasing for the B3 site, while there is a much less evident\udeffect for the B4 site. Considering samples at 5, 25, 80 (natural abundance) and 100 % 11B isotopic\udabundance, dephasing becomes faster for both sites as the 11B isotopic abundance increases. The\udexperimental behaviour is rationalised using density matrix simulations for two and three dipolarcoupled\ud11B nuclei. The experimentally observed slower dephasing for the larger CQ (B3) site is\udreproduced in all simulations and is explained by the reintroduction of the dipolar coupling by the\udso-called “spontaneous quadrupolar-driven recoupling mechanism” having a different dependence\udon the MAS frequency for different quadrupolar frequencies. Specifically, isolated spin-pair\udsimulations show that the spontaneous quadrupolar-driven recoupling mechanism is most efficient\udwhen the quadrupolar frequency is equal to twice the MAS frequency. While for isolated spin-pair\udsimulations, increasing the MAS frequency leads to faster dephasing, agreement with experiment is\udobserved for three-spin simulations which additionally include the homogeneous nature of the\udhomonuclear dipolar coupling network. First-principles calculations, using the GIPAW approach,\udof the 2J11B-11B couplings in lithium diborate, metaborate and triborate are presented: a clear trend\udis revealed whereby the 2J11B-11B couplings increase with increasing B-O-B bond angle and B-B\uddistance. However, the calculated 2J11B-11B couplings are small (0.95, 1.20 and 2.65 Hz in lithium\uddiborate), thus explaining why no zero crossing due to J modulation is observed experimentally,\udeven for the sample at 25 % 11B where significant spin-echo intensity remains out to durations of\ud~200 ms.