Jump to main content
Jump to site search
Access to RSC content Close the message box

Continue to access RSC content when you are not at your institution. Follow our step-by-step guide.


Issue 13, 2011
Previous Article Next Article

Towards homonuclear Jsolid-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

Author affiliations

Abstract

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 the quadrupolar frequency (ωPASQ/2π = 3CQ/[4I(2I − 1)]) is observed: the B3 (larger CQ) 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% 11B isotopic abundance, dephasing becomes faster for both sites as the 11B isotopic abundance increases. The experimental behaviour is rationalised using density matrix simulations for two and three dipolar-coupled 11B nuclei. The experimentally observed slower dephasing for the larger CQ (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 2J11B−11B couplings in lithium diborate, metaborate and triborate are presented: a clear trend is revealed whereby the 2J11B−11B couplings increase with increasing B–O–B bond angle and B–B distance. However, the calculated 2J11B−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% 11B where significant spin-echo intensity remains out to durations of ∼200 ms.

Graphical abstract: Towards homonuclear Jsolid-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

Back to tab navigation

Supplementary files

Article information


Submitted
01 Nov 2010
Accepted
25 Jan 2011
First published
14 Feb 2011

Phys. Chem. Chem. Phys., 2011,13, 5778-5789
Article type
Paper

Towards homonuclear Jsolid-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

N. S. Barrow, J. R. Yates, S. A. Feller, D. Holland, S. E. Ashbrook, P. Hodgkinson and S. P. Brown, Phys. Chem. Chem. Phys., 2011, 13, 5778
DOI: 10.1039/C0CP02343D

Social activity

Search articles by author

Spotlight

Advertisements