Effects of spin diffusion on spin-lattice relaxation in solid-state 2H MAS NMR spectroscopy

Article abstract of DOI:10.1016/j.cplett.2007.10.021

Recently, there has been increased interest in using ²H MAS NMR spectroscopy as an alternative to 'static' quadrupolar-echo NMR for studying dynamics in solids. Here we show that the phenomenon of enhanced spin diffusion as a result of n = 0 rotational resonance can lead to observation of a common spin-lattice (T₁) relaxation time for all deuterons. Thus, if rapidly relaxing groups are present, such as -CD₃ or - ND₃⁺ groups, then even highly immobile deuterons can exhibit T₁ relaxation times of ～1 s under MAS conditions, greatly facilitating data acquisition.

Full text of DOI:10.1016/j.cplett.2007.10.021

Available online at www.sciencedirect.com
Chemical Physics Letters 449 (2007) 86–91
www.elsevier.com/locate/cplett
Effects of spin diffusion on spin–lattice relaxation in solid-state
2H MAS NMR spectroscopy
*
Marica Cutajar, Moira H. Lewis, Stephen Wimperis
Department of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom
Received 13 August 2007; in final form 5 October 2007
Available online 11 October 2007
Abstract
Recently, there has been increased interest in using 2H MAS NMR spectroscopy as an alternative to ‘static’ quadrupolar-echo NMR
for studying dynamics in solids. Here we show that the phenomenon of enhanced spin diffusion as a result of n = 0 rotational resonance
can lead to observation of a common spin–lattice (T1) relaxation time for all deuterons. Thus, if rapidly relaxing groups are present, such
as –CD3 or ꢀNDþ3 groups, then even highly immobile deuterons can exhibit T1 relaxation times of ꢁ1 s under MAS conditions, greatly
facilitating data acquisition.
Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction
It has been observed that the spin–lattice relaxation (T1)
times of rigid or immobile deuterons are sometimes shorter
in spinning solids than in static solids, thereby increasing
2H (spin I = 1) NMR spectroscopy is widely used for
studying microsecond-timescale dynamics in solids [1–3].
2
the usefulness of the H MAS NMR experiment relative
2
The conventional approach is to acquire H quadrupolar-
to the static experiment [10]. As discussed by Alla et al.
in 1980, this effect occurs in solids where there are rapidly
reorienting deuterons, such as those in –CD3 or ꢀND3þ
groups, that relax very efficiently owing to modulation of
the 2H quadrupolar interaction on the timescale of the
inverse of the Larmor frequency and arises as a result of
spin order exchange (or ‘spin diffusion’) between the two
types of deuteron (immobile and rapidly reorienting) [10].
The observation of a greatly increased spin diffusion rate
under spinning conditions appears counter-intuitive as
echo spectra of the static powdered solid as a function of
temperature and echo duration and then to fit these with
a simulated lineshape based on a model for the dynamic
process. In this manner, the proposed motional model
can be confirmed and the rate constant determined over a
range of temperatures. In recent years, there has been an
increased interest in using 2H magic angle spinning
(MAS) NMR as an alternative to the quadrupolar-echo
technique [3–9]. The MAS spectrum consists of a series
of spinning sidebands with intensities that, at slower spin-
ning rates, mirror the ‘static’ quadrupolar lineshapes of
the individual H sites. Compared with the quadrupolar-
echo experiment, the sensitivity is improved because the
signal amplitude is concentrated into the spinning side-
bands, while individual H sites are resolved on the basis
of their distinct isotropic chemical shifts.
2
MAS is expected to suppress the effects of the H dipole–
dipole interaction. At slow spinning rates (ꢁ1 kHz), Alla
et al. have explained the phenomenon as follows: (i) in a
static solid, the energy levels of the immobile and rapidly
reorienting deuterons are, in general, non-degenerate
owing to the anisotropy of the large quadrupolar interac-
tion and spin diffusion cannot occur; and (ii) spinning pro-
duces a modulation of the quadrupolar interaction and,
hence, induces energy-level crossings during which
energy-conserving spin exchange or ‘flip–flops’ can occur
[10]. At faster MAS rates (ꢁ10 kHz), Gan and Robyr
2
2
*
Corresponding author. Fax: +44 01392 263 434.
E-mail address: s.wimperis@chem.gla.ac.uk (S. Wimperis).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.10.021

M. Cutajar et al. / Chemical Physics Letters 449 (2007) 86–91
87
ꢂ
ꢃ
1
[11] have explained the increased spin diffusion rate as
being due to a rotational resonance effect [12–16] where
the MAS rate, mMAS, and the difference in centreband
frequencies of the two 2H sites, Dmiso, satisfies the condition
nmMAS = Dmiso, with n = 0 being the relevant case for
2H MAS NMR at currently available magnetic field
strengths.
DMAðtÞþDMBðtÞ¼ ½DMAð0ÞþDMBð0Þꢆexp ꢀ ðRf þRsÞt
2
ð4aÞ
DMAðtÞꢀDMBðtÞ¼ ½DMAð0ÞꢀDMBð0Þꢆexpfꢀ2Rsdtg
ð4bÞ
Thus any difference between the two magnetizations will
decay very rapidly indeed with a rate constant 2Rsd while
the sum magnetization will decay more slowly but still rap-
idly overall with a time constant T1,A+B = 2/(Rf + Rs) ꢄ 2/
Rf. Therefore, for example, if in the absence of spin diffu-
In a recent 2H MAS NMR study of selectively deuteꢀrated
þ
L-alanyl-glycine, ND3 ꢂCHðCH3ÞꢂCO ꢂ NDꢂCH2 ꢂCO2 , we
found that the spin–lattice relaxation time constant of
the ND deuterons was 150 s in the static solid but ꢁ1 s
2
under MAS, making H MAS very much the method of
sion (Rsd = 0) T1,A
= 1/Rf = 0.5 s and T1,B = 1/Rs =
choice on sensitivity grounds [17]. Further study showed
that the T1 constants of the immobile (ND) and rapidly
reorienting ðNDþ3 Þ deuterons were identical under MAS,
implying strong spin diffusion between the two. In this Let-
ter, we examine this very useful phenomenon in more detail
and investigate whether the Alla–Eckman–Pines or Gan–
100 s then, in the course of a non-selective inversion- or sat-
uration-recovery experiment where there was strong spin
diffusion (as induced by MAS), DMA(t) and DMB(t) would
both be observed decaying with a common spin–lattice
relaxation time constant T1,A+B = 1 s ꢄ 2T1,A. The most
noticeable effect under MAS would be the shortening of
the T1 of site B by two orders of magnitude (100 to 1 s)
but the lengthening of the T1 of site A (from 0.5 to 1 s)
should also be observable.
In the model of Alla et al. [10], spin diffusion between
sites A and B only occurs during the fraction x ꢃ 1 of
the rotor period when energy-level crossings occur. If one
assumes that spin diffusion is negligible when the energy
levels of the immobile and rapidly reorienting deuterons
are non-degenerate but that it is strong (with rate constant
Rsd) during the crossing periods, with 2xRsd ꢃ Rf ꢀ Rs
(because x ꢁ 10ꢀ3), then:
2
Robyr mechanism for increased H spin diffusion under
2
MAS applies. In particular, we will repeat the H spin–
lattice relaxation NMR study of perdeuterated and
selectively deuterated dimethyl terephthalate performed
by Alla et al., but using a higher static magnetic field
strength, B0, and higher MAS rates than were available in
1980.
2. Theoretical background
A simple model for the effects of spin diffusion on spin–
lattice relaxation can be constructed by considering the
longitudinal magnetizations of two sites, A and B, coupled
by a relaxation matrix R:
DMAðtÞ ¼ DMAð0Þ expfꢀðRf þ xRsdÞtg
DMBðtÞ ¼ DMBð0Þ expfꢀðRs þ xRsdÞtg
ð5aÞ
ð5bÞ
ꢀ
ꢁ
ꢀ
ꢁ
DMAðtÞ
DMBðtÞ
DMAð0Þ
DMBð0Þ
Alla et al. further assumed that Rs ꢃ xRsd and so DMB(t)
decays with a time constant T1,B ꢄ (xRsd)ꢀ1. In this model,
the shortening of the T1 of the immobile deuterons is inde-
pendent of spinning rate (indeed, Eckman has demon-
strated the effect at a spinning rate of 12 Hz [18]).
However, the dipolar coupling constant of two deuterons
separated by r = 0.15 nm is only l0hc2D=ð16p3r3Þ ¼
0:84 kHz and hence one generally predicts complete sup-
pression of the 2H dipole–dipole interaction, including
the flip-flop terms responsible for spin diffusion, at typical
MAS rates of ꢁ10 kHz. Therefore, this model applies only
to the case of slow MAS rates, such as the 1 kHz spinning
used in the original work.
¼
expfꢀRtg
ð1Þ
where DMiðtÞ ¼ MiðtÞ ꢀ Mei q is the deviation of the magne-
tization of site i from thermal equilibrium and
ꢀ
ꢁ
Rf þ Rsd
ꢀRsd
ꢀRsd
Rs þ Rsd
R ¼
ð2Þ
where Rf is the rate constant for the fast decay of DMA(t) in
the absence of spin diffusion, Rs is the much smaller rate
constant for the slow decay of DMB(t) in the absence of
spin diffusion, and Rsd is the rate constant for spin diffusion
between A and B. If spin diffusion is slow (2Rsd ꢃ Rf ꢀ Rs)
then:
Although not referring directly to its effects upon spin–
2
lattice relaxation, Gan and Robyr have discussed H spin
diffusion under typical MAS conditions in terms of a rota-
tional resonance effect where the spinning rate, mMAS, and
the difference in centreband frequencies, Dmiso, satisfies
the condition nmMAS = Dmiso [11]. Rather than following
the instantaneous positions of the energy levels, the rota-
tional resonance model concentrates on their time-aver-
aged energies under MAS and indicates that strong spin
diffusion will only occur when there are degeneracies
between these ‘levels’. These degeneracies will arise when
DMAðtÞ ¼ DMAð0Þ expfꢀðRf þ RsdÞtg
DMBðtÞ ¼ DMBð0Þ expfꢀðRs þ RsdÞtg
ð3aÞ
ð3bÞ
and DMA(t) decays rapidly with a spin–lattice relaxation
time constant T1,A = (Rf + Rsd)ꢀ1 ꢄ 1/Rf and DMB(t) de-
cays slowly with a time constant T1,B = (Rs + Rsd)ꢀ1. If
spin diffusion is strong (2Rsd ꢅ Rf ꢀ Rs) then the sum
and difference magnetizations relax separately:

88
M. Cutajar et al. / Chemical Physics Letters 449 (2007) 86–91
the centreband frequencies of the two 2H sites overlap
(n = 0) or, since in the Floquet picture of dynamical sys-
tems one can associate time-averaged energy levels with
the spinning sidebands, when the centreband of one site
overlaps with the sideband of another (n = 1, 2, etc.).
a
b
c
d
2
Owing to the small range of H chemical shifts in diamag-
netic solids (Dmiso < 1 kHz), however, the only experimen-
tally realizable resonance condition is likely to be that
with n = 0. The spin diffusion rate will depend on the
degree of overlap of the two MAS NMR resonances (or,
equivalently, on the amplitude of their zero-quantum spec-
trum at m = 0) and be strongest in the case of complete
overlap.
+
ND3
ND
3. Experimental details
Selectively deuterated L-alanyl-glycine (ala-gly-d4) was
prepared by dissolving 0.5 g of non-deuterated L-alanyl-
glycine (Sigma) in excess D2O (99.9 at.% 2H, Aldrich)
and allowing the solution to stand overnight before gently
evaporating the solvent. Dimethyl terephthalate was pre-
pared by reacting 0.5 g of terephthalic acid with 5 cm3 of
methanol in the presence of 0.9 cm3 of thionyl chloride.
Full details of this procedure including the necessary safety
precautions can be found in Ref. [19]. Perdeuterated
dimethyl terephthalate-d10 (DMT-d10), CD3ÆCO2ÆC6D4Æ
CO2ÆCD3, was prepared using terephthalic acid-d4
(98 at.% 2H, Aldrich) and methanol-d4 (99.8 at.% 2H,
Aldrich), while DMT-d4, CH3ÆCO2ÆC6D4ÆCO2ÆCH3, and
DMT-d6, CD3ÆCO2ÆC6H4ÆCO2ÆCD3, were prepared using
the appropriate deuterated and non-deuterated starting
materials.
20
10
0
–10 ppm
2H NMR spectra were recorded on a Bruker Avance 400
spectrometer equipped with a widebore (89 mm) 9.4 T
magnet, yielding a 2H Larmor frequency of 61.4 MHz,
respectively. Powdered samples were packed into 4-mm
rotors for both static and MAS NMR experiments. Spin–
lattice relaxation (T1) times were measured using either
the inversion-recovery (for MAS spectra) or saturation-
recovery experiments (for static spectra). Static 2H line-
shapes were recorded using the Exorcycled quadrupolar-
echo pulse sequence [20] and this was also appended to
the saturation-recovery technique for static samples. The
radiofrequency field strength (m1 = cB1/2p) used was
approximately 100 kHz, corresponding to a 90° pulse
length of 2.5 ls.
100
0
–100
kHz
Fig. 1. 2H static and MAS NMR spectra of ala-gly-d4: (a) static spectrum
recorded with a relaxation interval of 900 s; (b) fitting of spectrum in (a)
showing broad (ND) and narrow ðNDþ3 Þ powder patterns making up total
lineshape; (c) static spectrum recorded with a relaxation interval of 1 s; (d)
MAS (10 kHz) spectrum recorded with a relaxation interval of 1 s; the
inset shows the centreband.
recorded with a 1 s relaxation interval (Fig. 1c) consists of
only the narrower of the two Pake patterns. The reason for
this is that, while the NDþ3 deuterons relax efficiently owing
to the rapid modulation of the 2H quadrupolar interaction,
the immobile ND deuterons have a very long spin–lattice
relaxation time. In the MAS spectrum recorded with a
1 s relaxation interval (Fig. 1d), however, both the broad
and narrow Pake patterns are clearly present within the
spinning sideband manifold.
4. Results and discussion
2
Fig. 1 shows H static and MAS NMR spectra of ala-
gly-d4. The static spectrum recorded with a 900 s relaxation
interval (Fig. 1a) consists of two overlapping spin I = 1
Pake powder patterns, as confirmed by lineshape fitting
(Fig. 1b). The broader powder pattern is assigned to the
immobile ND deuterons while the narrower (on account
of motional averaging) pattern is assigned to the rapidly
reorienting NDþ3 deuterons. In contrast, the static spectrum
2
Table 1 gives the H spin–lattice relaxation times of the
NDþ3 and ND deuterons in ala-gly-d4 measured under sta-
tic and MAS conditions. The short (0.8 s) and very long
(150 s) T1 time constants of the NDþ3 and ND deuterons,
respectively, are confirmed in the static solid.

M. Cutajar et al. / Chemical Physics Letters 449 (2007) 86–91
89
Table 1
We believe that this apparent dependence on the MAS rate
is largely due to frictional heating of the rotor to tempera-
tures above that of the bearing gas as the MAS rate
increases [21]. Variable-temperature spin–lattice relaxation
time measurements at a MAS rate of 3 kHz support this
proposal, with the T1 of NDþ3 and ND falling from 1.2 s
at 290 K to ꢁ0.1 s at 330 K, presumably as a result of
restricted rotational dynamics of the NDþ3 group about
its C3 axis. It can be noted that at a MAS rate of 3 kHz,
where there is negligible frictional heating, the common
T1 of 1.2 s is intermediate between the short (0.8 s) and
long (150 s) T1 values in the static solid in accordance with
the model of strong spin diffusion presented in Section 2.
Fig. 2 shows 2H static and MAS NMR spectra of DMT-
d10, which was one of the materials studied originally by
Alla et al. [10]. As with ala-gly-d4, the static spectrum
recorded with a long (500 s) relaxation interval (Fig. 2a)
consists of two overlapping spin I = 1 Pake patterns, with
the broader pattern assigned to the immobile aromatic
(ArD) deuterons and the narrower to the rapidly reorient-
ing CD3 deuterons. The static spectrum recorded with a
short (4 s) relaxation interval (Fig. 2b) again consists of
only the narrower of the two Pake patterns. In the MAS
spectrum shown in Fig. 2c both the broad and narrow
2H powder lineshapes are evident within the spinning side-
band manifold; however, it should be noted that a much
longer relaxation interval (120 s) has been used to achieve
this result than with ala-gly-d4.
Spin–lattice relaxation (T1) time constants in ala-gly-d4 measured at
B0 = 9.4 T
MAS rate
(kHz)
Bearing gas
temperature (K)
NDþ3
T1 (s)
ND
T1 (s)
0 (static)
3
5
10
14
290
290
290
290
290
0.8 0.1
1.2 0.1
0.40 0.04
0.19 0.03
0.13 0.03
150 20
1.2 0.1
0.38 0.04
0.16 0.03
0.14 0.03
3
330
0.13 0.04
0.11 0.04
2
In the centreband of the H MAS spectrum, the ND3þ
and ND resonances are fully degenerate (see inset in
Fig. 1d) but the individual spin–lattice relaxation times
can be determined from a spinning sideband at the edge
of the powder pattern (ND) and from one near the centre
of the powder pattern under the envelope of the narrower
Pake doublet ðNDþ3 Þ. Table 1 shows that the T1 time con-
stants of the NDþ3 and ND deuterons are short and identi-
cal under MAS conditions, indicating the presence of very
strong MAS-induced spin diffusion. However, this com-
mon T1 appears to depend on the MAS rate, being 1.2 s
at a spinning rate of 3 kHz but only ꢁ0.1 s at 14 kHz.
a
b
c
CD3
a
H2
H1
H1
H2
D3C
H3C
D3C
O
O
O
O
CD3
CH3
CD3
b
c
D2
D1
D1
D2
O
O
O
O
ArD1 ArD2
D2
D1
D1
D2
O
O
O
O
100
0
–100
kHz
Fig. 2. 2H static and MAS NMR spectra of dimethyl terephthalate-d10
(DMT-d10); (a) static spectrum recorded with a relaxation interval of
500 s; (b) static spectrum recorded with a relaxation interval of 4 s; (c)
MAS (10 kHz) spectrum recorded with a relaxation interval of 120 s.
15
10
5
ppm
0
Fig. 3. 2H MAS centreband spectra of (a) DMT-d6, (b) DMT-d4 and (c)
DMT-d10
.

90
M. Cutajar et al. / Chemical Physics Letters 449 (2007) 86–91
2
Table 2
Table 2 gives the H spin–lattice relaxation times of the
CD3 and ArD deuterons in DMT-d10 measured under sta-
tic and MAS conditions. The short (1.3 s) and very long
(230 s) T1 time constants of the rapidly reorienting CD3
and immobile ArD deuterons, respectively, are confirmed
in the static solid. Under MAS, the T1 constants of the
ArD deuterons are shorter than in the static solid but
remain longer than that of the CD3 deuterons; there is no
common T1 in this case.
Spin–lattice relaxation (T1) time constants in DMT-d10 measured at
B0 = 9.4 T
MAS rate
(kHz)
Bearing gas
temperature (K)
CD3
T1 (s)
ArD1
T1 (s)
ArD2
T1 (s)
0 (static)
2
5
8
290
290
290
290
290
1.3 0.2
1.3 0.1
1.4 0.1
1.5 0.1
1.6 0.1
230 20
150 10
68
27
15
7
5
55
36
32
5
3
3
5
4
1
10
It can be seen both from the examples of inversion-
recovery centreband and sideband spectra in Fig. 4a and
b and the T1 values in Table 2 that the ArD2 deuterons
in DMT-d10, whose centreband overlaps significantly with
that of the CD3 deuterons, relax faster under MAS than
the ArD1 deuterons. In contrast, in the inversion-recovery
MAS spectra of DMT-d4 shown in Fig. 4c, the ArD1 and
ArD2 peaks relax at a similar rate, which is much slower
than that of the corresponding peaks in DMT-d10. These
results show that it is spin diffusion enhanced by n = 0
rotational resonance (the Gan–Robyr mechanism [11]) that
is dominating spin–lattice relaxation under typical MAS
conditions: in DMT-d10, the ArD2 peak relaxes fairly rap-
idly because of its degree of centreband overlap with the
CD3 peak; the ArD1 peak relaxes slower than the ArD2
peak because of its negligible centreband overlap with the
2
330
1.8 0.2
52
5
36
5
2
Fig. 3 shows the centreband H MAS spectra of DMT-
d6, DMT-d4 and DMT-d10. The DMT-d6 centreband con-
sists of a single peak (Fig. 3a), arising from the two CD3
groups, but the DMT-d4 centreband consists of two peaks
(Fig. 3b), showing that there are two distinct chemical envi-
ronments for the four aromatic deuterons, which can be
grouped into crystallographically distinct pairs (labelled
ArD1 and ArD2) as a result of the way the molecules pack
in the solid. As expected, therefore, the DMT-d10 centre-
band (Fig. 3c) consists of three peaks (CD3, ArD1 and
ArD2), with significant overlap between the CD3 and
ArD2 peaks.
DMT-d10
DMT-d4
DMT-d10
sideband
a
c
b
centreband
centreband
τ
τ
τ
τ = 90 s
= 120 s
= 15 s
= 0 s
τ
τ
= 5 s
= 0 s
15
10
5
0
ppm
15
10
5
0
ppm
15
10
5
0
ppm
Fig. 4. Inversion-recovery 2H MAS (10 kHz) spectra recorded using the s intervals (between the 180° and 90° pulses) shown: (a) DMT-d10 centreband
spectra; (b) DMT-d10 sideband spectra (where the CD3 peak is absent); (c) DMT-d4 centreband spectra.

M. Cutajar et al. / Chemical Physics Letters 449 (2007) 86–91
91
CD3 peak but its relaxation is still enhanced relative to that
in the static solid due to its partial overlap with the ArD2
peak.
appears to play a much less significant role. It seems likely
that the MAS-induced effects described here for the spin
I = 1 H nucleus will also be important in NMR spectro-
2
The T1 values in Table 2 again depend on the MAS rate;
in this case, however, the dependence does not appear to be
due primarily to temperature-induced dynamical changes
in the solid (the changes in T1 values from 290 to 330 K
at a MAS rate of 2 kHz are relatively small) but seems to
be a genuine effect. Consider first the results at a 2 kHz
MAS rate: the Gan–Robyr mechanism appears to be
already active (the T1 of ArD2 is shorter than that of
ArD1) but the Alla–Eckman–Pines mechanism is possibly
still important in enhancing the overall spin diffusion rate
constant. As the MAS rate is increased, we would expect
the latter mechanism to decline rapidly in importance;
however, we observe the T1 values of the ArD1 and, in par-
ticular, ArD2 deuterons becoming progressively shorter,
indicating enhanced spin diffusion through a rotational res-
onance effect. This observation is perhaps unexpected in
view of theoretical predictions for n = 0 rotational reso-
nance among spin I = 1/2 nuclei [22] but is not inconsistent
with the spin diffusion rate being very small both in the sta-
tic solid and under extremely rapid MAS (ꢅ10 kHz).
scopy of other quadrupolar nuclei, including those with
half-integer spin quantum numbers (I = 3/2, 5/2, etc.),
and Kwak et al. have recently shown some interesting
results that are relevant to this area [23].
Acknowledgements
We are grateful to EPSRC for support (Grant No. GR/
S96579), to Dr. Z. Gan (NHFML, Tallahassee) for direct-
ing us to some relevant literature in this area, and to Dr.
R.C. Hartley (Glasgow) for help with the synthesis of the
deuterated dimethyl terephthalate.
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5. Conclusions
In materials where the phenomenon occurs, the shorten-
ing of the 2H T1 time constants of immobile deuterons
under MAS is extraordinarily useful. For example, in ala-
gly-d4, the reduction of T1 from ꢁ150 s in the static solid
to ꢁ1 s under MAS represents a greater than ten-fold
increase in the integrated-signal-to-noise ratio per unit
experiment time. This increase is multiplicative, of course,
with the well known gain in sensitivity that occurs as a
result of the concentration of the signal amplitude into
the spinning sidebands. In this Letter we have discussed
the origin of the effect and have shown that it occurs as a
result of increased spin diffusion arising largely, as dis-
cussed by Gan and Robyr [11], from an n = 0 rotational
resonance effect. In materials such as ala-gly-d4 where there
2
is complete spectral overlap of the H MAS NMR centre-
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bands of the immobile and rapidly reorienting deuterons,
spin diffusion will be very strong and a common T1 will
be observed. In contrast, in materials such as DMT-d10
where there is only incomplete overlap of the MAS centre-
bands, spin diffusion will still be enhanced and reduced T1
values observed for immobile deuterons but these will
remain longer than those of the rapidly reorienting deute-
rons. At the fast MAS rates used routinely nowadays, the
mechanism for reduced 2H T1 time constants proposed
originally by Alla et al. [10] for the slow spinning case
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