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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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Barrow, Nathan S., Yates, Jonathan R., Feller, Steven A., Holland, Diane, Ashbrook, Sharon E., Hodgkinson, Paul and Brown, Steven P. (2011) 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. Physical Chemistry Chemical Physics, Vol.13 (No.13). pp. 5778-5789. doi:10.1039/c0cp02343d ISSN 1463-9076.
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WRAP_Barrow_0773279-px-031111-pccp135778-57892011.pdf - Accepted Version - Requires a PDF viewer. Download (596Kb) |
Official URL: http://dx.doi.org/10.1039/C0CP02343D
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 (ωQPAS/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 dipolarcoupled
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.
Item Type: | Journal Article | ||||
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Subjects: | Q Science > QC Physics | ||||
Divisions: | Faculty of Science, Engineering and Medicine > Science > Physics | ||||
Library of Congress Subject Headings (LCSH): | Nuclear magnetic resonance | ||||
Journal or Publication Title: | Physical Chemistry Chemical Physics | ||||
Publisher: | Royal Society of Chemistry | ||||
ISSN: | 1463-9076 | ||||
Official Date: | 2011 | ||||
Dates: |
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Volume: | Vol.13 | ||||
Number: | No.13 | ||||
Page Range: | pp. 5778-5789 | ||||
DOI: | 10.1039/c0cp02343d | ||||
Status: | Peer Reviewed | ||||
Publication Status: | Published | ||||
Access rights to Published version: | Restricted or Subscription Access | ||||
Date of first compliant deposit: | 18 December 2015 | ||||
Date of first compliant Open Access: | 18 December 2015 | ||||
Funder: | Engineering and Physical Sciences Research Council (EPSRC), University of Warwick |
Data sourced from Thomson Reuters' Web of Knowledge
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