Homonuclear correlation in solid-state NMR : developing experiments for half-integer quadrupolar nuclei

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Abstract

The objective was to develop solid-state nuclear magnetic resonance
(NMR) homonuclear correlation experiments for half-integer quadrupolar nuclei
so as to study atomic proximities and connectivities in disordered materials.
Nearby nuclear spins are coupled through space via their magnetic dipole moments.
Dipolar broadening is removed by magic angle spinning (MAS) for
isolated spin pairs. However, the noncommutation of the electric quadrupolar
interaction with the dipolar interaction means that the latter will not be removed
by MAS. This interplay between the dipolar and quadrupolar interactions,
combined with the effects of multiple noncommutating homonuclear
dipolar couplings, was investigated by observing spin-echo dephasing curves as
well as magnetisation transfer in 2D spin diffusion experiments. Polycrystalline
lithium diborate samples were synthesised to act as model compounds. The
preparation of samples with differing 11B isotopic abundances enabled a comparison
of samples with either predominantly isolated spin-pairs or multiple
coupled nuclei.
Spin diffusion experiments probed 11B–11B correlation at three magnetic
field strengths, 80% and 25% 11B isotopic abundances, MAS rates from
4427 Hz to 7602 Hz and under DOR. Enhanced magnetisation transfer was observed
for the higher 11B isotopic abundance and at slower spinning speeds. The
latter dependence was reproduced by four-spin computer simulations. Secondorder
quadrupolar broadened spin diffusion cross-peaks under MAS had a mixed
positive and negative appearance for the 80% 11B sample. A similar effect was
previously observed for four dipolar-coupled I = 1/2 nuclei.
Spin-echo dephasing curves were recorded for 5%, 25% and 100% 11B isotopic
abundances and MAS rates of 5 kHz to 20 kHz. Depletion of 11B isotopic
abundance prolonged the coherence dephasing time because of a reduction of
noncommuting homonuclear dipolar couplings. Faster dephasing was observed
for the smaller CQ = 0.51MHz site; four-spin computer simulations showed this
is consistent with the reintroduction of the dipolar coupling being most efficient
when the MAS rate and first-order quadrupolar interaction are of the same
magnitude. Speeding-up the MAS rate prolonged the dephasing time for the
CQ = 2.56MHz site but not for the CQ = 0.51MHz site because of an interplay
between the quadrupolar and multiple dipolar interactions. Through-bond Jcouplings
between 11B nuclei were not detected, setting an upper bound of 2JBB
<3 Hz in polycrystalline lithium diborate.

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