Solid State Nuclear Magnetic Resonance 20, 100–107 (2001)
doi:10.1006/snmr.2001.0033, available online at http://www.idealibrary.com on
High-Field
127
I NMR of Solid Sheelite Structures:
Periodates Revisited
Gang Wu1 and Shuan Dong
Department of Chemistry, Queen’s University,
Kingston, Ontario, Canada K7L 3N6
Received August 15, 2000; revised May 14, 2001; published online September 20, 2001
We report new solid-state 127 I NMR results for sheelite periodates, MIO4 (M = Na+ , K+ ,
Rb+ , and NH+4 ), and for pseudo-scheelite CsIO4 and HIO4 . The observed 127 I quadrupole
coupling constants were between 1.0 and 43.0 MHz in agreement with previous NQR data. In
contrast to an early 127 I NMR study (S. L. Segel and H. M. Vyas, 1980, J. Chem. Phys. 72, 1406),
we found that the 127 I chemical shift anisotropy is negligibly small in sheelite periodates. A
small but definite 127 I chemical shift tensor was observed for pseudo-scheelite CsIO4 . © 2001
Elsevier Science
Key Words: 127 I NMR; sheelite structure; periodates; quadrupole coupling constant; chemical
shift tensor.
INTRODUCTION
Iodine-127 (spin-5/2, 100% natural abundance) has been extensively studied over
the past 50 years by nuclear quadrupole resonance (NQR) spectroscopy [1, 2].
However, most iodine-containing compounds are inaccessible by nuclear magnetic
resonance (NMR) because of the prohibitively large 127 I quadrupole interactions.
For example, it is common to have compounds where the 127 I quadrupole coupling
constant (QCC), e2 qQ/h, is on the order of 1000–3000 MHz [1, 2]. A very limited
number of solution 127 I NMR studies are available in the literature, the majority of
which deal with symmetrical species such as I− and IO−
4 [3, 4].
The first solid-state 127 I NMR study was the early work of Watkins and Pound on
a single crystal of KI [5]. Subsequently, a small number of solid-state 127 I NMR studies have been reported [6–11]. Recently, Harbison and his co-workers demonstrated
the utility of two-dimensional zero-field 127 I NMR [12]. The quadrupole interaction
at the 127 I nucleus may also be detected indirectly from NMR spectra of a neighboring spin-1/2 nucleus [13]. Among all previous solid-state 127 I NMR studies, the most
peculiar results are those in the paper by Segel and Vyas [8], who reported astonishingly large values of 127 I chemical shift anisotropy (CSA) for seemingly symmetrical species. For example, the span of the 127 I chemical shift tensor, = δ11 − δ33 ,
for solid sheelite NaIO4 was reported to be 59,000 ppm [8]! However, the same
1
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127
I NMR OF SHEELITE STRUCTURES
101
study showed that the isostructural NH4 IO4 exhibited a vanishing 127 I chemical shift
anisotropy. This was truly puzzling because the span of the only other 127 I chemical
shift tensor available in the literature is 1488 ppm, which was derived from the spin
rotation coupling tensor of gaseous HI obtained from a molecular beam experiment
[14]. For this reason, we decided to reexamine the solid-state 127 I NMR of sheelite periodates, MIO4 . Using a modern high-field NMR approach, we hope that the
new results may shed some light on the magnitude of the 127 I CSA in sheelites.
EXPERIMENTAL
Sample preparation. Sodium periodate (NaIO4 ) and potassium periodate
(KIO4 ) and periodic acid (H5 IO6 ) were obtained form commercial sources. CsIO4 ,
RbIO4 , and NH4 IO4 were prepared simply by reacting excessive CsCl (aq), RbCl
(aq), and NH3 (aq) with H5 IO6 , respectively. The reactions were carried out at
room temperature. In each reaction, the precipitate product was filtrated, washed
with cold water, and dried. HIO4 was prepared by dehydration of H5 IO6 at 100◦ C
under vacuum for 5 h.
Solid-state NMR. Solid-state 127 I NMR spectra were recorded on a Bruker
Avance-500 spectrometer operating at 500.13 and 100.36 MHz for 1 H and
127
I nuclei, respectively. Polycrystalline samples were packed into zirconium
oxide rotors (4 mm o.d.). For static experiments, an echo sequence employing two 90◦ pulses was used: P1 (φ1 )–τ1 –P2 (φ2 )–τ2 –Acq(φ3 ), where φ1 =
x x x x y y y y −x −x −x −x −y −y −y −y; φ2 = 4 × (x y −x −y); φ3 =
−y y −y y −x x −x x y −y y −y x −x x −x. Typical delays between the two
pulses in the half-echo and whole-echo experiments were 30 and 400 µs, respectively. The RF strength at the 127 I frequency was approximately 80 kHz. All stationary 127 I NMR spectra were recorded with a fast ADC (12 bit) which gives a
maximum sweep width of 4 MHz. Recycle delays were ranged from 5 to10 s. A
solid sample of NaI was used for RF power calibration and for 127 I chemical shift
referencing, δ (solid NaI) = 0 ppm.
RESULTS AND DISCUSSION
127
I quadrupole coupling constant. The stationary 127 I NMR spectra of solid peri+
+
+
odates (MIO4 , M = NH+
4 , Rb , K , and Na ) are shown in Fig. 1. Each of the
spectra exhibits a typical lineshape arising from second-order quadrupole interactions. The total linewidth varies from approximately 30 kHz for NH4 IO4 to over
0.6 MHz for NaIO4 ! As seen from Fig. 1, all four spectra exhibit features characteristic of an axially symmetric electric field gradient (EFG) at the 127 I nucleus. All
four periodates belong to the sheelite structure where the iodine atom is at a site of
4̄ symmetry [15]. The observation of axially symmetric EFGs at the 127 I nucleus in
these compounds is in agreement with the crystallographic symmetry present in the
sheelite structure. In such circumstances, analysis of the observed 127 I NMR spectra
is straightforward. Figure 2 shows the results for NH4 IO4 and KIO4 to illustrate the
excellent agreement between the observed and calculated spectra. The obtained 127 I
102
WU AND DONG
FIG. 1. Experimental 127 I stationary NMR spectra of periodates with a sheelite structure.
(A) NH4 IO4 , (B) RbIO4 , (C) KIO4 , and (D) NaIO4 . All spectra were obtained at 11.75 T.
solid-state NMR parameters are given in Table 1. Our results confirm the 127 I QCC
values obtained from NQR studies [16–19]. Most importantly, our analysis did not
include any 127 I CSA. This suggests that the earlier results of Segel and Vyas [8] for
the 127 I chemical shift tensor in sheelite periodates were incorrect. Their erroneous
results may be related to the experimental difficulties in measuring 127 I NMR signals at a very low magnetic field and to the complex nature of their data analysis. In
light of our new results, the correctness of the 127 I CSA results for iodates (MIO3 ,
M = Li+ , Cs+ , and Rb+ ) reported by Segel and Vyas in the same study [8] may also
be questionable.
The whole-echo experiment. It is generally quite difficult to obtain undistorted
wide-line NMR spectra. Very often an echo sequence must be used to avoid spectral
distortion [20]. If the intrinsic spin–spin relaxation time (T2 ) of the sample is long,
it is more advantageous to record the whole echo rather than only the half echo.
As demonstrated by Massiot et al. [21], if the whole echo is detected, the signal-tonoise ratio in the resultant spectrum can be improved by a factor of 21/2 . Figure 3
shows the two different types of 127 I free induction decay (FID) signals for NaIO4 .
Figure 3A was obtained with a relatively short delay, τ = 30 µs, between the two
pulses. The resultant FID exhibits an echo whose maximum intensity appears at
t = τ = 30 µs. As shown in Fig. 3B, when the delay is increased to τ = 380 µs,
127
103
I NMR OF SHEELITE STRUCTURES
FIG. 2. Comparison between experimental (upper trace) and calculated (lower trace)
NMR spectra for (A) NH4 IO4 and (B) KIO4 .
127
I stationary
the echo still appears at t = τ. However, since τ is much longer than the apparent T2 , the whole echo can be observed. Figure 3C shows the direct FT of the
whole-echo FID. The obvious spectral distortion is due to the fact that the echo
maximum is not at t = 0. To obtain an undistorted spectrum, one simply applies
first-order phase correction, which is equivalent to a shift in the time domain. The
Solid-State
TABLE 1
I NMR Parameters
127
Compound
δiso (ppm)a
NH4 IO4
3960
RbIO4
3960
KIO4
3960
NaIO4
3950
CsIO4
3972 ± 2
δ11 = δ22 = 3978 ppm
δ33 = 3960 ppm
3300
HIO4
e2 qQ/h (MHz)b
η
Ref.
1000
1002
1565
160
2066
2073
4224
4239
100
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
This
[16]
This
[17]
This
[18]
This
[18]
This
4300
0.75
This work
work
work
work
work
work
a
The uncertainty in the chemical shifts was estimated to be ±10 ppm except for
CsIO4 .
b
The uncertainty in the e2 qQ/h values was estimated to be ±001 MHz.
104
WU AND DONG
FIG. 3. (A) Half-echo 127 I FID. (B) Whole-echo 127 I FID. (C) The direct FT of the whole-echo FID
shown in (B). (D) FT followed by first-order phase correction of the whole-echo FID shown in (B). The
sample was NaIO4 .
corrected spectrum is shown in Fig. 3D. The main advantage of the whole-echo
approach is to increase sensitivity and reduce spectral distortion.
Using the whole-echo approach, we also obtained the 127 I NMR spectrum for
solid HIO4 . As shown in Fig. 4, the 127 I NMR spectrum covers a frequency range
of approximately 1 MHz. Spectral simulations yield that QCC = 43.0 MHz, η =
075, and δiso = 3300 ppm. The spectral distortion at the low-frequency end of
the spectrum is presumably due to a lower excitation efficiency which is a common
problem for observing very wide NMR spectra. The chemical shift for HIO4 is
clearly more shielded than those for periodates. However, the crystal structure of
HIO4 appears to be unavailable in the literature.
Chemical shift anisotropy. As mentioned earlier, in contrast to the report of
Segel and Vyas [8], we have found that the span of the 127 I chemical shift tensor
is negligibly small in sheelite periodates (span < 50 ppm). In order to accurately
determine any CSA, it is desirable to find a sample whose 127 I QCC is reasonably
small. Interestingly, the 127 I QCC in pseudo-scheelite CsIO4 was reported to be
quite small at room temperature [22]. The 127 I magic-angle-spinning (MAS) NMR
127
FIG. 4.
HIO4 .
I NMR OF SHEELITE STRUCTURES
105
Experimental (upper trace) and calculated (lower trace) 127 I stationary NMR spectra of solid
spectrum of CsIO4 is shown in Fig. 5. From the observed satellite manifold, we
estimated that QCC = 1.0 MHz, which is in excellent agreement with a previous
determination [22]. This compound may represent an ideal system for detecting
small 127 I CSA.
The static 127 I central-transition NMR spectrum of CsIO4 is shown in Fig. 6.
Figures 6A and 6B show the simulated 127 I NMR spectra with QCC = 1.00 MHz
but different line-broadening treatment. It can be seen that the resultant lineshape
(Fig. 6B) is symmetric and much narrower than the observed one shown in Fig. 6D.
To reproduce the observed asymmetric feature in the stationary 127 I spectrum, it is
necessary to include a small 127 I chemical shift tensor: δ11 = δ22 = 3978 ± 2 and
δ33 = 3960 ± 2 ppm. The span of this chemical shift tensor is only 18 ppm and
represents an upper limit for the 127 I CSA in solid CsIO4 . This further confirms that
the 127 I CSA for periodate ions must be negligibly small.
FIG. 5. Magic-angle-spinning
νQ = (3/20)e2 qQ/h.
127
I NMR spectrum of pseudo-scheelite CsIO4 at room temperature.
106
WU AND DONG
FIG. 6. (A) Simulated stationary 127 I NMR spectrum for pseudo-scheelite CsIO4 , QCC = 1.0 MHz
and η = 00. (B) Same as in (A) but convoluted with a Gaussian function of full width at half-height
(FWHH) of 600 Hz. (C) Addition of a 127 I chemical shift tensor: δ11 = δ22 = 3978 and δ33 = 3960 ppm.
(D) Observed stationary 127 I central-transition NMR spectrum of CsIO4 .
CONCLUSIONS
We have reported new solid-state 127 I NMR results for several sheelite periodates,
127
I QCCs determined from our
MIO4 (M = Na+ , K+ , Rb+ , NH+
4 ). The values of
solid-state NMR experiments are in good agreement with NQR results. However,
in contrast to a previous 127 I NMR study [8], our results have proved that 127 I CSAs
are negligibly small in sheelite periodates. We have also obtained 127 I NMR spectra
for CsIO4 and HIO4 . We have demonstrated the utility of various NMR techniques
such as half-echo, whole-echo, and MAS in obtaining high-quality 127 I NMR spectra for solid samples. The present study has also demonstrated that the utility of
modern high-field NMR techniques has made detection of very wide NMR spectra
possible.
ACKNOWLEDGMENTS
We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for
research and equipment grants. G.W. thanks Queen’s University for a Chancellor’s Research Award
(2000) and the Ontario Government for a Premier’s Research Excellence Award (2000). We also
thank Mr. Alan Wong for helpful discussions and Professor R. J. C. Brown for his interest in this
research.
127
I NMR OF SHEELITE STRUCTURES
107
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