Chemistry Reference
In-Depth Information
hydroxyfluorides derived from b-AlF
3
[8]. The high magnetic field, by reducing the
influence of the contribution of the quadrupolar interaction to the spectrum, allows the
resolution of
27
Al resonances with close
iso
values. As shown in Section 5.3 of this
chapter,
27
Al high-field NMR is a powerful tool to probe and quantify the various
aluminium species in aluminium based fluorides.
27
Al NMR
5.2.2
5.2.2.1
SATRAS Experiments
Satellite Transition Spectroscopy (SATRAS) [9,10] has been proven to be a useful
technique to determine the NMR parameters of quadrupolar spin systems, in particular
when the quadrupolar frequency
Q
is so small that the structure of the central transition
(CT) 1/2
!
1/2 due to second-order effects is not resolved. In that case, reliable informa-
tion can be obtained through the reconstruction of the spinning sideband manifold of the
satellite transitions. The extent of the NMR spectrum gives
Q
while the shape of the
envelope of the spinning sidebands provides the asymmetry parameter
Q
. When the
effects of pulse duration can be neglected (i.e. using short pulse and small flip angle),
the SATRAS spectrum is quantitative. As an example, the
27
Al NMR spectrum of
-BaCaAlF
7
, which contains one aluminium site, is presented in Figure 5.4. The shapeless
central transition and the overall SATRAS spectrum expansion indicate a low
Q
value.
The reconstruction of the SATRAS spectrum (Figure 5.4) was achieved using a home-
made code based on the theoretical treatment developed by Skibsted et al. [9,11], and
including a correction for the second-order frequency shift [12,13]. In this way, we
precisely determined the
27
Al NMR parameters (
iso
,
Q
and
Q
) in numerous alkali and
alkaline-earth fluoraluminates [2,14-17].
5.2.2.2 MQ-MAS Experiments
In 1995, Frydman and Harwood [18] introduced the multiple-quantum MAS
(MQ-MAS) experiment, which makes use of the multi-quanta transitions of a quad-
rupolar nucleus. This two-dimensional (2D) NMR experiment correlates a high resolu-
tion isotropic spectrum (F1 vertical dimension) to the anisotropic central transition of
the MAS spectrum (F2 horizontal dimension). The development of this method has
considerably extended the applicability of solid-state NMR to quadrupolar nuclei in
multisite crystalline phases and glasses. In the presence of strong heteronuclear dipolar
couplings (such as
19
F-
27
Al), resolution of the
27
Al MQ-MAS spectrum can also be
improved by applying composite decoupling schemes during the MQ evolution and
the acquisition periods, as shown for Na
5
Al
3
F
14
[19]. In crystalline compounds, the
separation of the aluminium sites in two dimensions allows extraction of their quad-
rupolar interaction parameters individually [2,14-17,19] (Figure 5.5). In disordered
crystalline compounds, an improved resolution of the resonances is obtained, as shown
on the
27
Al MQ-MAS spectrum of a high surface area (HS) pyrochlore aluminium
hydroxyfluoride [20].
