Nuclear Overhauser Effect (NOE) Enhancement of 11B NMR Spectra
The nuclear Overhauser effect (NOE) is one of the classic NMR
phenomena and one of the most widely exploited.1,2 The observation
of an enhancement or a reduction in the intensity of the resonance
of a spin, I, upon saturation or inversion of the resonance of a second
spin, S, that lies close in space (<0.5 nm) is a key step in the NMR
structure determination of both small and large molecules in
solution.3 In addition, the heteronuclear NOE is routinely used to
enhance the spectra of insensitive nuclei, such as 13C, in solution.4
The NOE is generally considered to be an NMR phenomenon of
spin I ) 1/2 nuclei in solution, but here we show that a strong 11B
{1H} NOE can be observed in the solid-state 11B NMR spectra of
borane adducts, yielding fractional enhancements, fI{S} ) (I - I0)/
I0, of the magic angle spinning (MAS) NMR signal of up to 155%.
This is an interesting and unusual observation as 11B (spin I )
3/2) is a quadrupolar nucleus and the corresponding NOE is absent
in solutions of the same materials.
The NOE arises as a consequence of spin-lattice relaxation driven
by random modulation of the IS dipole-dipole interaction by thermal
motions on the picosecond or nanosecond (or “fast”) time scale
(10-12-10-9 s). Such rapid motions are a general property of liquids
but are less common in solids, where dynamics on the millisecond
to second (or “slow”) time scale (10-3-1 s) are considered to
dominate. Despite this, a heteronuclear 13C {1H} NOE has been
observed in a number of solids, especially those containing methyl
(CH3) groups as these reorient about their C3 axes on the fast time
scale.5 Even when fast time scale motions are present, the NOE is
usually not observed in either liquids or solids if I or S is a spin I
g 1 nucleus as the quadrupolar interaction is typically much
stronger than the dipole-dipole one and, in this case, efficient
quadrupolar spin-lattice relaxation will restore the quadrupolar
nuclei to equilibrium before the IS dipole-dipole interaction has
sufficient time in which to act.
The widely used steady-state I {S ) 1H} NOE experiment is
not the optimal choice for solid-state NMR as it requires long
periods of high-power irradiation of the 1H nuclei, and this is not
practicable on MAS NMR probeheads. Instead, here we utilize the
transient NOE,2 observing it by applying a 180° pulse to the 1H
nuclei at an interval ô > 0 before the start of I spin data acquisition,
as shown in Figure 1. For comparison, a spectrum without an NOE
can be recorded with the same sequence by setting ô ) 0. Care
should be taken to avoid the possibility of an inadvertent NOE by
leaving a recycle interval during time averaging that is very long
compared with the 1H spin-lattice relaxation time. All the experiments
described below were performed at a magnetic field strength
of B0 ) 9.4 T (unless otherwise specified) and at T ) 300 K.
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