Article
pubs.acs.org/JPCC
High-Field 17O MAS NMR Reveals 1J(17O-127I) with its Sign and the
NMR Crystallography of the Scheelite Structures for NaIO4 and KIO4
Hans J. Jakobsen,*,† Henrik Bildsøe,† Michael Brorson,‡ Gang Wu,*,§ Peter L. Gor’kov,∥ Zhehong Gan,∥
and Ivan Hung∥
†
Danish Instrument Centre for Solid-State NMR Spectroscopy, Interdisciplinary Nanoscience Center (iNANO), Department of
Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark
‡
Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark
§
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
∥
National High Magnetic Field Laboratory, 1860 East Paul Dirac Drive, Tallahassee, Florida 32310, United States
ABSTRACT: High-field, ambient-temperature (AT) 17O MAS
NMR spectra of the tetraoxoanion IO4− in NaIO4 and KIO4
exhibit unusual line-shape features for the central transition
(CT), which so far have not been observed in any second-order
broadened line shape for the CT of a quadrupolar nucleus. These
features are caused by an unusually large isotropic 1J(17O-127I)
spin coupling (∼500 Hz) and appear like “teeth-on-a-saw”. This
study reports interesting results obtained from optimized fitting
of the spectra using our recently described XSTARS software.
The results include determination of a positive sign for
1 17
J( O-127I) (= +500 Hz), an unusual observation in solid-state
NMR. NMR crystallography shows a very precise correlation between extraordinary small changes for the 17O asymmetry
parameter (ηQ) and changes for a tetrahedral O−I−O angle upon distortion from an ideal tetrahedron. Similarly, the spectral
analysis shows that the NMR crystallography requires the principal axes Vzz(17O) and δzz(17O) of the PAS for these tensors are
both almost along the I−O bond. All the experimental data are in excellent agreement with our ADF and CASTEP calculations.
■
relaxation and/or self-decoupling. High-magnetic fields and
high-resolution techniques, such as MQMAS, DAS, and DOR,
have been used to reduce or remove the quadrupolar
broadening, and variable temperatures are often needed to
change the molecular dynamics and its line broadening effect in
order to reveal the relatively small scalar J couplings.2−4 A list of
references to recent determinations of such 1J(X-Y) indirect
coupling constants, using the specialized solid-state NMR
pulse-sequences of MQMAS, DAS, and DOR, can be found in
ref 4.
During these investigations, we have been asked by several
inorganic chemists to perform related 17O MAS NMR studies
for other tetraoxoanion salts. A series of tetraoxoanions for the
perhalides [i.e., perchlorates (ClO4−), perbromates (BrO4−),
and periodates (IO4−)], immediately came to mind. However,
it is known that the ClO4− ion does not exchange its oxygen
atoms to a measurable extent with water over long periods of
time at elevated temperatures and high acidities.5,6 This fact
was actually fully confirmed in the early stages of our 17O MAS
NMR studies by unsuccessful attempts to 17O-enrich the ClO4−
ion in KClO4, according to our standard procedure using 10%
INTRODUCTION
NMR crystallography, as related to high-resolution solid-state
NMR of powder samples, has evolved into an extremely
important box of tools for structural and dynamic studies within
chemistry, materials, and biochemistry during the past few
decades.1 In this respect, 17O appears to be a quite useful
quadrupolar nucleus because of the straightforward measurements of its isotropic/anisotropic chemical shifts, quadrupolar
coupling parameters, and in some cases even J(17O-Y) spin
couplings, but primarily due to the widespread occurrence of
oxygen in nature. 17O is a spin I = 5/2 nucleus, however, of very
low natural abundance, but many compounds can fairly easily
be 17O-enriched to an appropriate level for solid-state NMR
measurements. In a recent series of solid-state variabletemperature (VT) 17O MAS NMR investigations of some
tetraoxometal anions2−4 (WO42−,2,3 ReO4−,4 and MnO4−,3)
salts with different monovalent cations (e.g., K+,2−4 Cs+,2,3 and
NH4+,4), we determined the first detailed dynamics, thermodynamics, and 17O spectral parameters in such systems.2−4 Highresolution 17O MAS NMR obtained at low-temperatures has
allowed direct observation of one-bond indirect 1J(17O-187Re)
spin couplings in two perrhenates.4 This observation is among
the only few measurements of indirect spin couplings between
two quadrupolar nuclei that are often obscured by large
quadrupolar broadening and dynamic broadening effects like
© 2015 American Chemical Society
Received: April 18, 2015
Revised: June 1, 2015
Published: June 2, 2015
14434
DOI: 10.1021/acs.jpcc.5b03721
J. Phys. Chem. C 2015, 119, 14434−14442
Article
The Journal of Physical Chemistry C
(23 °C) (i.e., at an actual sample temperature of ∼50 °C),
based on the temperature calibration performed at the NHMFL
for the 3.2 mm Revolution NMR MAS systems using
Pb(NO3)2.14 Most importantly, a high-temperature 17O MAS
NMR experiment performed at 80 °C for the NaIO4 and KIO4
samples, with the aim to observe any effects on the line shape
and resolution, showed no change at all compared to the AT
experiment. All 17O MAS NMR spectra acquired on this
spectrometer used standard single-pulse excitation at 112.8
MHz, which focused on the central part of the spectrum [i.e.,
the 17O Central Transition (CT)]. 17O chemical shifts were
referenced to the 17O resonance of an external sample of
ordinary H2O. A 90° flip-angle pw(90)liquid = 5.0 μs was
obtained for the 17O resonance of ordinary H2O while a value
of pw = 1.2 μs, which corresponds to a liquid 22° flip-angle (or
a solids 65° flip angle) for the rf field strength of 50 kHz, was
used for the MAS experiments. This one-pulse excitation was
used for the solid-state MAS experiments along with a
relaxation delay of 10 s for both samples. The employed
spinning frequencies (νr) for the final spectra are shown in the
figure captions with an estimated stability Δνr < ± 2 Hz for
both samples. The magic angle θ = 54.736° was adjusted to the
highest possible precision (Δθ < ± 0.003°) by 23Na MAS NMR
using a sample of NaNO3 at 219.7 MHz, as described
elsewhere.15
Bruker 900 MHz Avance II 21.1 T Spectrometer. In order to
further separate the spinning sidebands, 17O MAS NMR spectra
were also recorded at the slightly higher 21.1 T field of the
Bruker 900 MHz Avance II spectrometer at the National
Ultrahigh-Field NMR Facility for Solids with a high spinning
frequency νr ∼ 31 kHz, using 2.5 mm o.d. Bruker MAS rotors.
This was done to confirm the spectral parameters determined
at 19.6 T from spectral simulations (in particular the 17O
chemical shift anisotropy (CSA) parameters due to the
decreased intensity of the first order ssbs caused by the higher
νr). The 17O MAS NMR experiments acquired on this
spectrometer, equipped with a standard narrow-bore (52 mm
i.d.) magnet, were obtained at 122.0 MHz using a Bruker
broadband X-{1H} double-resonance MAS probe. The AT 31
kHz MAS for the 2.5 mm rotor induced a sample heating of
∼40 °C, according to the temperature calibration for the 2.5
mm MAS probe, so the actual sample temperature is ∼63 °C.
Again 17O chemical shifts were referenced to the 17O resonance
of an external sample of ordinary H2O. A 90° flip-angle
pw(90)liquid = 2.0 μs was obtained for the 17O resonance of
ordinary H2O while a value of pw = 0.67 μs, corresponding to a
liquid 30° flip-angle (or a solids 90° flip angle) for the rf field
strength of 125 kHz, was used for the first pulse in a standard
solid-state spin−echo 90°−180° MAS experiments used on this
spectrometer. The few experiments acquired on this
spectrometer were performed mainly to confirm the magnitude
of the 17O quadrupole coupling and CSA parameters
determined at 19.6 T. The determination of the 17O CSA
parameters at 19.6 T are based on the relative intensities
between the centerband for the central transition (CT) and its
associated first- and second-order spinning sidebands (ssbs) as
observed from optimized fits of the experimental spectra. For
this reason, the 17O MAS NMR spectra acquired at 21.1 T used
a much higher spinning frequency νr ∼ 31 kHz in order to
confirm the expected increase in the relative intensity ratios
between the CT centerband and its first few ssbs.
Spectral Analysis. All 17O AT MAS NMR spectra have
been analyzed in terms of the 17O and 127I spectral parameters
O-enriched H2O at 100 °C for other tetraoxoanions,2−4 since
no 17O resonance was observed from 17O MAS NMR
experiments of the isolated KClO4 samples. On the other
hand, it is known, at least qualitatively, that the reactivity of
oxygen exchange with water increases in the series ClO3− <
BrO3− < IO3−.5 Thus, this study reports some exciting 17O
MAS NMR results for two periodates following successful 17Oenrichments of NaIO4 and KIO4 by our standard procedure
used recently.2−4 These results include: (i) the first direct
observation at ambient-temperature (AT) of a 1J(17O-Me)
indirect spin coupling between two quadrupolar nuclei in solid
tetraoxoanions (where Me is either a metal or metalloid/
nonmetal), (ii) the determination of an unusually large
magnitude for 1J(17O-127I), but most importantly its opposite
sign compared to our most recent results for 1J(17O-Me) spin
couplings between 17O and a true metal (Me),4 and (iii) first of
all the NMR crystallography revealed for the IO4− anion
resulting from in-depth simulations of the acquired experimental 17O AT MAS NMR spectra. All experimental 17O
spectral parameters are determined from optimized fits to these
spectra using our recently developed XSTARS software, which
includes all solid-state NMR interactions (including the direct
dipolar D(X-Y) and indirect J(X-Y) spin coupling), and finally
the relative tensor orientations for the two (X and Y)
quadrupolar nuclei.4 XSTARS is an extended version of the
most recently updated standard version of STARS.3 The
determined J-coupling constant including its sign, chemical
shift, and quadrupolar coupling parameters are all in excellent
agreement with ADF (Amsterdam density functional)7−10 and
CASTEP11 calculations that we performed for the corresponding parameters based on refined single-crystal XRD structures
for NaIO412 and KIO4.13
17
■
EXPERIMENTAL SECTION
Materials and Synthesis. Standard samples of NaIO4 and
KIO4 are commercially available. They were purchased from
Aldrich and used without further purification. 17O-enrichments
of the samples were achieved by 17O-exchange in sealed glass
ampules, each containing a 10% 17O-enriched H2O solution of
the sample and kept at a temperature of 90−100 °C for 7 days
according to the procedure used for 17O-enrichment of our
other tetraoxoanion salts.3,4 The 10% 17O-enriched H2O used
for the 17O-exchange was purchased from CortecNet, France.
Solid-State MAS NMR Spectroscopy. 17O MAS NMR
experiments were performed at the National High Magnetic
Field Lab (NHMFL), Florida State University (FSU),
Tallahassee, and at the National Ultrahigh-Field NMR Facility
for Solids, Ottawa, Ontario, Canada, on two different high-field
spectrometers.
Bruker DRX-830 Narrow-Bore 19.6 T Spectrometer. 17O
MAS NMR spectra intended to retrieve the 17O spectral
parameters from precise spectral simulations were acquired
using the Bruker DRX-830 narrow-bore 19.6 T spectrometer at
the NHMFL. This spectrometer is equipped with a special
Magnex narrow-bore (31 mm i.d.) magnet and a NHMFL
designed and home-built extremely narrow-bore (31 mm o.d.)
double-resonance X-{1H} broadband MAS probe for 3.2 mm
o.d. rotors (i.e., the magnet shim coil was removed). The
samples were spun at νr ∼ 18 kHz using a 3.2 mm o.d. MAS
rotor/stator module from Revolution NMR. All experiments
were performed using ambient-temperature (AT) nitrogen gas
for the air-bearing and drive gas. The effect of heating the
sample by MAS at 18 kHz is estimated to be ∼25 °C above AT
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DOI: 10.1021/acs.jpcc.5b03721
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The Journal of Physical Chemistry C
■
RESULTS AND DISCUSSION
NaIO4, 10% 17O-Enriched Sample. The experimental
single-pulse 19.6 T (112.8 MHz) 17O MAS NMR spectrum for
the ∼10% 17O-enriched sample of NaIO4 is displayed in Figure
1a for the central spectral region (for a spectral width of 150
and for parameters which describe the combined interactions
between the 17O and 127I quadrupolar nuclei [e.g., the direct
dipolar 1D(17O-127I) and indirect scalar 1J(17O-127I) couplings]
using our recently developed XSTARS software presented in
detail elsewhere.4 This new XSTARS software is an extension
of our standard16 and updated3 STARS simulation/iterative
fitting software. The XSTARS software has, similar to the
updated STARS version,3 been incorporated into the Varian
VnmrJ software running on a Linux RedHat PC. All spectra
were processed using a 28656 crystallite file generated with the
Zaremba-Conroy-Wolfsberg (ZCW) algorithm.17−19
For the quadrupole coupling and chemical shift parameters,
we employ the original conventions16 which still apply for
STARS and XSTARS, in other words
CQ = eQVzz /h
ηQ = (Vyy − Vxx )/Vzz
(1)
δσ = δiso − δzz
ησ = (δxx − δyy )/δσ
(2)
δiso = (1/3)(δxx + δyy + δzz) = (1/3)Tr(δ)
(3)
δJ = Jzz − Jiso
(4)
ηJ = (Jyy − Jxx )/δJ
Jiso = (1/3)(Jxx + Jyy + Jzz ) = (1/3)Tr(J )
Article
(5)
with the convention
|λzz − (1/3)Tr(λ)| ≥ |λ xx − (1/3)Tr(λ)|
≥ |λ yy − (1/3)Tr(λ)|
Figure 1. 112.8 MHz (19.6 T) experimental and simulated 17O MAS
NMR spectra of NaIO4 obtained for a spinning frequency νr = 18.55
kHz at AT and presented on a ppm scale corresponding to a total
width of the spectra on a frequency scale of 150 kHz. The
experimental spectrum was acquired using 18063 scans for about 2
days and 2 h for a relaxation delay of 10 s employing the flip angle
conditions described in the Experimental Section for this spectrometer
as noted in the section on “Spectral Analysis”. We point out that all
experimental and simulated spectra in Figures 1−5 were processed
using a 28656 crystallite file generated with the Zaremba-ConroyWolfsberg (ZCW) algorithm.17−19 (a) Expansion for the centerband
region of the experimental spectrum according to the two scales
mentioned above. (b) Optimized simulation, including the 17O CT
and all STs, of the experimental spectrum in (a) and shown on the
same frequency scales as in (a). (c) Corresponding simulation for the
CT only. (d) Corresponding simulation of the two outer STs for the
17
O spin I = 5/2 nucleus. Addition of the simulations in (c) and (d)
corresponds to the simulated spectrum in (b). All simulated spectra
used the optimized parameters listed in row 1 of Table 1.
(6)
for the principal elements (λαα = Vαα, δαα) of the quadrupole
and chemical shift tensors. The orientation of a tensor relative
to a molecular coordinate system is described by the three
Euler angles (ψ, χ, ξ) which correspond to positive rotations of
the tensor principal axis system around z(ψ), the new y(χ), and
the final z(ξ) axis.
ADF and CASTEP Calculations. Computations of NMR
tensors were performed using the Amsterdam density functional (ADF) software package.7−10 The Vosko-Wilk-Nausir
(VWN) exchange-correlation functional20 was used for the
local density approximation (LDA), and the Perdew−Burke−
Ernzerhof (PBE) exchange-correlation functional21 was used
for the generalized gradient approximation (GGA). Standard
Slater type-orbital (STO) basis sets with triple-ζ quality plus
polarization functions (TZ2P) were used for all of the atoms.
The spin orbital relativistic effect was incorporated in all
calculations via the zero order regular approximation
(ZORA).22−25 Plane-wave pseudopotential DFT calculations
of the NMR tensors were performed using Materials Studio
CASTEP software version 4.4 (Accelrys).11 The Perdew−
Burke−Ernzerhof (PBE) functionals21 were employed in all
calculations in the generalized gradient approximation (GGA)
for the exchange correlation energy. On-the-fly pseudo
potentials were used with a plane wave basis set cutoff energy
of 550 eV. The Monkhorst−Pack26 k-space grid sizes were 5 ×
5 × 2 (7 k-points used) and 4 × 4 × 2 (4 k-points used) for
NaIO4 and KIO4, respectively. The reported crystal structures
of NaIO4 (ICSD 14287)12 and KIO4 (ICSD 83376)13 were
used as already mentioned in the Experimental Section. In the
above calculations, computed 17O magnetic shielding values (σ
in ppm) were converted to chemical shifts (δ in ppm) by using
δ = 287.5 ppm − σ.27
kHz) and using a spinning frequency νr = 18.55 kHz. In
addition to the intense centerband for the CT, quite intense
first- and second-order ssbs are observed for the CT, due to a
large 17O CSA, along with several ssbs arising from the 17O
satellite transitions (STs). Optimized XSTARS fitting is
performed for both the CT and STs and the resulting final
fitted/simulated spectrum is shown in Figure 1b, while the
corresponding optimized 17O/127I spectral parameters are
summarized in Table 1. We note that for the simulation of
this 19.6 T (112.8 MHz) 17O MAS NMR spectrum, we used an
127
I NMR resonance frequency ν(127I) = 166.4 MHz and the
127
I quadrupole coupling parameters CQ = 42.2 MHz and ηQ =
0.0 reported earlier for NaIO4.28 In order to identify the origin
of the individual ssbs in the experimental/simulated 17O MAS
NMR spectra in Figure 1 (panels a and b), simulated 17O MAS
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The Journal of Physical Chemistry C
Table 1. 17O Quadrupole Coupling (CQ, ηQ), Chemical Shift Parameters (δσ, ησ, δiso), Effective Dipolar Coupling D′(17O-127I) =
D − δJ/2, Isotropic Coupling J = 1J(17O-127I), and Its Anisotropy δJ, Determined from 17O MAS NMR Spectra of NaIO4 and
KIO4 at ATa
sample experiment
NaIO4/19.6 T
NaIO4/21.1 T
KIO4/19.6 T
KIO4/21.1 T
temp (°C)
CQ (MHz)
ηQb
δσ (ppm)
ησ
δiso (ppm)
Jc (Hz)
D′ (Hz)
δJ (Hz)
∼
∼
∼
∼
11.19
11.17
10.87
10.89
0.066
0.062
0.032
0.030
155
140
148
147
0.20
0d
0.20
0d
250
251
243
243
500
502
506
504
−532
−528
−417
−442
−110
−118
−366
−316
AT,
AT,
AT,
AT,
50
63
50
63
a
The δiso values (relative to H217O) have an error limit of ±0.5 ppm. The error limits for CQ and ηQ are ±0.02 MHz and ±0.005b, respectively. The
error limits for δσ and ησ are ±10 ppm and ±0.05, respectively, for the 19.6 T data and about ±15 ppm for δσ determined at 21.1 T. The error limits
for J = 1J(17O-127I) determined at both 19.6 and 21.1 T are only ±5 Hz. However, the error limits for D′ and δJ are generally much larger (∼ ±100
Hz) and could be even larger for the analyzed 21.1 T spectra. The Euler angles describing the tensor orientation for the 17O quadrupole and CSA
principal axis systems (PAS) relative to our definition of the molecular frame system are discussed in the text. It is observed that for the two sensitive
angles to the fitting process, β(Q) and β(δσ), it is only the difference β(Q) − β(δσ) which influences the optimized fitting. Thus, β(Q) = 0 is fixed
and β(δσ) is the only Euler angle being allowed to vary, resulting in difference values β(Q) − β(δσ) = −12° ± 3° in all optimizations of the 19.6 T
spectra. bSee text and Figure 6, which prove the extraordinary low error limit (±0.005) for these ηQ values. cSee text for the reasons why a positive
sign is deduced for J = 1J(17O-127I). dThe two ησ = 0 values in Table 1 for the two spectra acquired at 21.1 T (122.0 MHz) are fixed during the
optimization processes because of the much higher spinning frequency (νr ∼ 31 kHz) employed. This makes the small value ησ = 0.20 determined
for νr ∼ 18 kHz at 19.6 T a very uncertain parameter to determine for νr ∼ 31 kHz at 21.1 T.
coupling (127I being a spin I = 5/2 nucleus). As illustrated by
the two spectra in Figure 2 (panels a and b), the expected “sixpeak” splitting patterns on the flanks for both of the two
“horns”, due to the indirect scalar 1J(17O-127I) coupling, appear
as “seven-peak” patterns including the shoulders observed in
the outer-region of the patterns for the two “horns” in the
experimental as well as in the simulated spectrum. Optimized
simulated spectra unambiguously show that the additional
splitting into the apparent “seven-peak” pattern is caused by an
extremely small value for the 17O asymmetry parameter (i.e., ηQ
= 0.066). The fitting of the “seven-peak” pattern allows for a
very precise determination of the asymmetry parameter. For
example, changing ηQ = 0.066 to ηQ = 0.00 in a simulation,
keeping all other NaIO4 parameters unchanged in Table 1,
turns the “seven-peak” pattern into the originally expected “sixpeak” splitting pattern (not shown). This observation is in
complete agreement with both ADF7−10 and CASTEP11
calculations of the spectral parameters based on refined
single-crystal XRD structures for both NaIO412 and KIO413
(vide infra).
To obtain additional confirmation on the 17O quadrupole
coupling parameters and partly support the magnitude of the
17
O CSA parameters determined at 19.6 T (112.8 MHz), a 21.1
T (122.0 MHz) 17O MAS NMR experiment was performed at
the much higher spinning frequency νr ∼ 31 kHz using a 2.5
mm rotor and a standard spin−echo 90°−180° MAS
experiment. The experimental spin−echo 21.1 T 17O MAS
NMR spectrum of NaIO4 for νr = 31.25 kHz is displayed in
Figure 3a with an identical horizontal expansion in Hz (i.e., 150
kHz) as used for Figure 1. It clearly illustrates that the intensity
decreases considerably of the first- and second-order ssbs for
the CT due to the extensive increase in spinning speed,
followed by a corresponding increase in mainly the left, highfrequency “horn” of the CT centerband. A simulated spectrum
resulting from an optimized fit to the experimental spin−echo
spectrum in Figure 3a using the XSTARS software is shown in
Figure 3b. The resulting 17O spectral parameters are
summarized in Table 1 along with the parameters determined
from the 17O experimental 19.6 T spectrum in Figure 1a and its
expansion in Figure 2a.
Comparison for the two sets of 17O experimental parameters
for NaIO4 shows that the quadrupole coupling parameters (CQ,
ηQ) and indirect 1J(17O-127I) spin coupling constants are within
NMR spectra for the CT and STs are shown above Figure 1
(panels b−d). These simulations employed exactly the same
optimized parameters (Table 1) as was obtained for the
simulation in Figure 1b. The simulations in Figure 1 (panels c
and d) clearly identify both origin and positions of the ssbs
from the CT and STs.
The distinct and unusual observations for the experimental
and simulated AT 17O MAS spectra in Figure 1 (panels a and b,
respectively) are the quite small “spikes/peaks” which appear
on the flanks of both the CT and ST resonances. From the
optimized fitting of the experimental spectra (e.g., Figure 1a), it
is obvious that these “spikes/peaks” are caused by indirect
scalar 1J(17O-127I) couplings with a magnitude of ∼500 Hz. To
further explore the details of these 1J(17O-127I) spin couplings
and resulting J-splittings, Figure 2 (panels a and b) show
Figure 2. Expansions of the centerbands for the CT regions presented
in the two 112.8 MHz (19.6 T) experimental and simulated 17O MAS
NMR spectra of NaIO4 in Figure 1 (panels a and b). (a) Expansion of
the experimental spectrum. (b) Expansion of the simulated spectrum.
horizontal expansions for the CT centerband of the
experimental and simulated spectra, respectively, of the
corresponding central region spectra shown in Figure 1 (panels
a and b). These horizontal expansions in Figure 2 illustrate that
the two “horns” of the second order quadrupolar-broadened
doublet-pattern (CQ = 11.19 MHz and ηQ = 0.066, see Table 1)
appear like “teeth-on-a-saw” because of the 1J(17O-127I) spin
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The Journal of Physical Chemistry C
Figure 4. 112.8 MHz (19.6 T) experimental and simulated 17O MAS
NMR spectra of KIO4 obtained for a spinning frequency νr = 17.84
kHz at AT and presented on a ppm scale corresponding to a total
width of the spectra on a frequency scale of 150 kHz. The
experimental spectrum was acquired using 32768 scans for 3 days
and 19 h with a relaxation delay of 10 s. (a) Expansion for the
centerband region of the experimental spectrum according to the two
scales mentioned above. We note that the very narrow resonance at
0.0 ppm is caused by a minute presence of H217O in the dried sample,
which confirms the 17O resonance frequency for our external reference
sample of ordinary H217O water within ±1.0 ppm. (b) Optimized
simulation, including the 17O CT and all STs, of the experimental
spectrum in (a) and shown on the same frequency scales as in (a). The
simulated spectrum used the optimized parameters listed in row 3 of
Table 1.
Figure 3. 122.0 MHz (21.1 T) experimental (spin−echo; total spin−
echo time of 64 μs) and optimized simulation 17O MAS NMR spectra
of NaIO4 obtained for a spinning frequency νr = 31.25 kHz at AT. For
comparison with the spectra in Figure 1, the spectra are presented on a
ppm scale corresponding to a total width for the spectra on a
frequency scale of 150 kHz, as used in Figure 1. The experimental
spectrum was acquired using 2560 scans in 21.3 h for a relaxation delay
of 30 s.
the error limits for each of the two experiments. Because of the
high spinning rate employed at 21.1 T, the parameters obtained
for the 17O CSA interaction are quite inaccurate.
KIO4, 10% 17O-Enriched Sample. The experimental
single-pulse (19.6 T; 112.8 MHz) and spin−echo (21.1 T;
122.0 MHz) 17O MAS NMR spectra for the ∼10% 17Oenriched sample of KIO4 exhibit almost identical spectral
features as observed for the corresponding spectra shown in
Figures 1−3 for NaIO4. For that reason only the most
informative experimental and simulated single-pulse 19.6 T 17O
MAS spectra are presented here in Figures 4 and 5 using a
spinning frequency νr = 17.84 kHz. The analysis and optimized
fitting to the experimental spectrum in Figure 4a follow the
exact same procedures as described above in the section on the
analysis and results for the spectra of NaIO4. Accordingly, we
refrain to repeat this procedure here and refer the reader to the
details in the above section. The result of the optimized fit to
the experimental spectrum of KIO4 in Figure 4a is displayed in
Figure 4b, and the resulting spectral parameters are summarized
in Table 1. A most important difference between the 17O MAS
spectra of NaIO4 and KIO4 is observed from the horizontal
expansion for the CT centerband of the experimental and
simulated spectra for KIO4 shown in Figure 5, when compared
to the corresponding expansions in Figure 2 for NaIO4. The
additional splitting for the expected “six-peak” splitting pattern
for both two “horns” is much smaller in Figure 5 spectra
(KIO4) compared to the larger splitting leading to a “sevenpeak” pattern in Figure 2 spectra (NaIO4). Thus, both
expanded spectra for KIO4 in Figure 5 clearly show the “sixpeak” splitting with a further small/shoulder splitting for each
of the total of 12 “six-peaks”, which corresponds to ηQ = 0.032
as opposed to the ηQ = 0.066 for NaIO4. These additional
different small splitting in the 17O MAS spectra of NaIO4 and
KIO4 partly explores the NMR crystallography for the two
slightly different crystal structures of NaIO412 and KIO413 as will
be discussed next in Discussion, NMR Crystallography, ADF,
and CASTEP Calculations.
Figure 5. Expansions of the centerbands for the CT regions presented
in the two 112.8 MHz (19.6 T) experimental and simulated 17O MAS
NMR spectra of KIO4 in Figure 4 (panels a and b). (a) Expansion of
the experimental spectrum. (b) Expansion of the simulated spectrum.
Discussion, NMR Crystallography, ADF, and CASTEP
Calculations. In addition to the above results and data shown
in Table 1, the optimized fitting of the experimental 17O MAS
spectra for NaIO4 and KIO4 all lead to the intriguing result, that
the effective dipolar coupling constant D′ and 1J(17O-127I) are
of opposite sign. Because most solid-state NMR experiments
are unable to either observe or even determine the sign of
indirect J-couplings in solids, as opposed to the numerous
methods available for this purpose in liquid-state NMR, we here
elaborate on why this becomes possible for the present crystal
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structures. This appears to be due to the NMR crystallography
determined for the present samples, however, with two
different options, of which one can be disregarded partly on
experimental grounds due to an insufficient optimized fit of the
experimental spectrum based on rms errors and partly because
of unusual orientations of the 17O quadrupole and CSA tensors.
This experimental finding is supported by CASTEP calculations
of the tensor orientations (vide infra) and is in accordance with
our ADF calculations of the principal elements of the J-tensors
and thereby with the sign for 1J(17O-127I) (vide infra).
As in our recent 17O VT MAS NMR study on the
1 17
J( O-187Re) coupling in two perrhenates (KReO4 and
NH4ReO4),4 with analogous Scheelite-type structure (space
group I41/a, Z = 4)13,29 as for the periodates, the molecular
frame system for the IO4− tetrahedron has been selected with
its z axis along the I−O bond and with the 127I quadupole
principal z axis [Vzz(127I)] in the zx plane of the molecular
frame. Thus, the orientation of the 127I quadupolar principal
axis system (PAS) is given by the three angles (0,57,0) for
NaIO412 and (0,56,0) for KIO413, while the D′ dipolar PAS is
given by the three angles (0,0,0). Two sets of Euler angles
describing the PAS orientation for the 17O quadrupole coupling
[α(Q), β(Q), γ(Q)] and CSA [α(δσ), β(δσ), γ(δσ)] tensors
relative to the definition of the molecular frame system have
been used in the fitting procedure. Four of these angles have
been fixed to zero, since only β(Q) and β(δσ) appear to be
sensitive to the optimized fitting. This is exactly similar to the
situation encountered in the analysis of the low-temperature
17
O MAS spectra of the perrhenates.4 Two widely different
combinations of values for the pair of β(Q) and β(δσ) angles
immediately became clear as possible candidates for a follow-up
on two final optimized fits of the experimental periodate
spectra, using these two different combinations of values for
β(Q) and β(δσ). The first and most obvious combination is
identical to the one, and only possible, found during the
analysis of the 17O MAS spectra of the perrhenates4 and
involves optimization of values in the region ∼0° for both β(Q)
and β(δσ) [i.e., the principal axes Vzz(17O) and δzz(17O) of the
PAS for these two tensors are almost along the I−O bond].
This full optimization for the two periodates involves all
relevant parameters, including β(Q) ∼ β(δσ) ∼ 0°, for the
experimental spectra and leads to the simulated spectra
presented above in Figures 1−5 for both NaIO4 and KIO4.
The corresponding spectral parameters are summarized in
Table 1. The optimizations for both compounds lead to the
conclusion that for β(Q) ∼ β(δσ) ∼ 0°, the indirect J-coupling
and D′ are of opposite sign (i.e., D′ × J < 0). All attempts to
find a minimum in the rms surface for J and D′ having the same
sign (D′ × J > 0) were unsuccessful. At this stage, we here
already note (vide infra) that the sign of the effective dipolar
coupling D′ has been safely estimated to be negative (i.e., D′ <
0) and therefore used as such throughout the following
discussion. In fact, for the optimum NaIO4 parameters obtained
at 19.6 T and listed in Table 1, a sign-change in J (i.e., J < 0)
increased the rms error by 12% for this parameter set. With this
rms error and J < 0 as a starting point, a following optimization
always converged to the parameter set with opposite sign for J
and D′ (D′ × J < 0) shown in Table 1. To illustrate the changes
in the 17O MAS spectrum of NaIO4 caused by the sign reversal
in J, Figure 6 shows a comparison between the experimental
spectrum (Figure 6a) and two simulated spectra using the
parameter set in row 1 of Table 1 (Figure 6b, blue spectrum)
and the same set with a negative sign for J (Figure 6b, red
Figure 6. Experimental and simulated 17O MAS NMR spectra
obtained at 19.6 T of the centerband for the CT in NaIO4. The
experimental spectrum is an expansion of the spectrum in Figure 1a.
The two simulated spectra both used the optimized spectral
parameters shown in Table 1 (row 1), except for the sign of the J
coupling (i.e., J > 0 and J < 0, respectively). (a) Experimental
spectrum. (b) Overlay of the two simulated spectra: J > 0 in blue and J
< 0 in red. (c) Difference spectrum between the experimental
spectrum (a) and the simulated spectrum for J > 0 in blue. (d)
Difference spectrum between the experimental spectrum (a) and the
simulated spectrum for J < 0 in red. (e) Difference spectrum between
the two simulated spectra in blue and red, respectively.
spectrum). The spectra in Figure 6 clearly show how the sign
reversal of the J coupling changes the intensity pattern near the
left “horn” of the CT line shape. This effect can be rationalized
in terms of the “effective” CSA for the six manifolds defined by
the 127I spin states, mz = 5/2, 3/2, 1/2, −1/2, −3/2, and −5/2.
For coincident PAS of the 17O CSA and D′ tensors, the
“effective” CSA is given by [δσ + 2mz(127I)D′/ν0], where ν0 is
the 17O resonance frequency.4 A sign reversal for 1J(17O-127I)
reverses the order of the manifolds by swapping the “effective”
CSA for the corresponding mz(127I) high- and low-frequency
manifolds. The second, alternate solution with β(Q) ∼ β(δσ) ∼
90°, resulting in a negative sign for the J coupling, is considered
below.
The effective dipolar coupling constant is defined as D′ = D
− δJ/2, where the direct dipolar coupling constant is defined as
D = (γIγSμ0ℏ)(8π2rIS3)−1 with I = 17O and S = 127I and δJ is the
anisotropy in the indirect J spin−spin coupling as defined and
used in our XSTARS,4 simulation/iterative fitting software. We
note30 that alternatively ΔJ is also used in the literature to
denote the anisotropy in the J coupling (see, for example, ref
30), and that the two conventions are related by the equation δJ
= 2ΔJ/3 (i.e., similar to the two conventions also used in the
literature for the chemical shift anisotropy, δσ and Δσ, where δσ
= 2Δσ/3). From the r(17O-127I) bond lengths obtained from
refinements of the crystal structures for NaIO412 and KIO413,
r(17O-127I) = 1.775 and 1.762 Å, respectively, we obtain the two
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direct dipolar couplings D = −587 Hz and D = −600 Hz for
NaIO4 and KIO4, respectively. Generally, the magnitude of the
anisotropy in the indirect spin−spin coupling (δJ) is small
compared to D and/or the J coupling itself and usually it is
assumed that δJ = 0. Thus, for a calculated direct dipolar
coupling D ∼ −600 Hz, and an observed magnitude for the
indirect J coupling 1J(17O-127I) ∼ 500 Hz for both NaIO4 and
KIO4, we feel we are on safe ground in assuming that the
effective dipolar coupling D′ = D − δJ/2 is negative,
independent of the sign for δJ. Thus, combined with the
above fascinating experimental result that D′ and 1J(17O-127I)
are of opposite sign for both NaIO4 and KIO4, we conclude
that 1J(17O-127I) is positive in sign for both compounds. These
conclusions are fully supported following our ADF calculations
with respect to the magnitudes and positive sign for 1J(17O-127I)
and fairly large magnitudes and negative sign for δJ in both
NaIO4 and KIO4, vide infra. We note that the positive sign
determined here for 1J(17O-127I) is opposite to the negative sign
determined in the same manner for 1J(17O-187Re).4 Furthermore, the positive sign and much larger magnitude of
1 17
J( O-127I) (∼ +500 Hz) results in an extreme large negative
reduced coupling constant 1K(17O-127I) = (2π 1J(17O-127I))/
(γ17Oγ 127Iℏ) = −152.4 (10 20 NA−2 m−3), which is way outside
the linear correlation observed recently between the reduced
coupling constants 1K( 17O-M) and the atomic number for M,
where M is a quadrupolar metal nucleus (e.g., M = 51V, 53Cr,
55
Mn, 95Mo, 99Tc, and 187Re). The 1J(17O-127I) spin coupling
constants reported here are not only the first determined onebond J-coupling between 17O and a halogen atom but to our
knowledge also the first determined one-bond coupling
between 17O and a quadrupolar nonmetal nucleus along with
its sign.
Further extremely important results with respect to the NMR
crystallography of NaIO4 and KIO4 arise from the optimized
17
O fitted data (Table 1) of the experimental 17O MAS spectra
in Figures 1a and 2a and Figures 4a and 5a for the two (19.6 T)
17
O asymmetry parameters ηQ = 0.066 and ηQ = 0.032 in NaIO4
and KIO4, respectively. It turns out that the error limit for these
ηQ values is < ± 0.005 and that the magnitude of ηQ is related
to the magnitude of the additional splitting observed within the
expected “six-peak” J-coupling patterns on the two “horns” for
the second-order broadened CT. We point out that the “sevenpeak” pattern observed on the horns in the spectrum of NaIO4
(Figure 2) is due to the fact that the additional splitting (caused
by ηQ = 0.066) is accidentally of similar magnitude as
1 17
J( O-127I) (i.e., ∼500 Hz). Assuming an ideal/perfect
tetrahedron for the IO4− ion (i.e., ηQ = 0.000), a simulation
using the 19.6 T parameters for NaIO4 listed in Table 1, but for
ηQ = 0.000, turns the observed “seven-peak” pattern in Figure 2
into the expected “six-peak” J-coupling pattern. Thus, the two
quite small, but very precise 17O asymmetry parameters ηQ =
0.066 and ηQ = 0.032 determined for NaIO4 and KIO4,
respectively, show that the IO4− tetrahedron for NaIO4 and
KIO4 are both slightly distorted. This is in complete agreement
with the Scheelite-type crystal structures determined for
NaIO4,12 KIO4,13 and other periodates (space group I41/
a)12,13 which show that the tetrahedra for the IO4− ions are
slightly compressed in the crystal axis c direction. Thereby, the
group of six tetrahedral angles, each equal to 109.47° for an
ideal tetrahedron, splits into two groups. A group of two equalsized angles with an increased angle opening and a group of
four equal-sized angles with a slight decrease of the angle
opening, both groups with respect to the ideal tetrahedral angle
of 109.47°. The refined crystal structure for NaIO412 reports a
value 114.05° for the group of two angles and a value 107.33°
for the group of four tetrahedral angles. Since these angles were
not reported from the refined crystal structure of KIO413, we
have used this structure to calculate a value of 112.06° for the
group of two angles and a value 108.19° for the group of four
tetrahedral angles. Recognizing the increase for the tetrahedral
angles (in the group of two angles) in going from KIO4 to
NaIO4, accompanied by a corresponding increase for the ηQ
values (or the corresponding additional splitting), and taking
the disappearance of the additional splitting for ηQ = 0.000 for
the ideal tetrahedron, the possibility of a linear correlation
between the increase in tetrahedral angle and the ηQ values for
these three points is looked into. Indeed, as shown in Figure 7,
Figure 7. Linear plot of the increase in the 17O quadrupole asymmetry
parameter ηQ versus the increase in tetrahedral angle for the two large
and equal-sized tetrahedral-angle openings upon compression of the
IO4− tetrahedron along the crystal-axis c in NaIO4 and KIO4. As
shown in the text an ideal tetrahedron with the coordinates (109.47°,
0.000) serves as the third (reference) point.
an excellent linear correlation (R = 0.991) is observed for the
compression of the ideal tetrahedron in the region represented
by the distortions determined for KIO4 and NaIO4. From the
linear plot in Figure 7, we find that an increase by 1° (for the
group of two equal-sized angles) in the ideal tetrahedral angle
(109.47°) corresponds to increase in ηQ = 0.014.
Finally, we believe that the experimental observation and
determination of the extremely small and very precise ηQ values
reported in this study would not have been possible without the
presence of the large 1J(17O-127I) spin−spin coupling. This J
coupling paved the way for the observation of the “teeth-on-asaw” pattern within the CT and thereby the resolution required
for the present study in relation to solid-state NMR
crystallography.
We now return to the possible alternate solution which is
observed for a ∼ 90° change in the orientation of both the 17O
quadrupole and CSA tensor [i.e., for β(Q) ∼ β(δσ) ∼ 90°]. An
optimized fit for these tensor orientations results in parameter
values similar to the optimized parameters discussed and
determined above for the NaIO4 data at 19.6 T in Table 1;
however, most importantly with the result that J and D′ have
the same sign (D′ × J > 0) (i.e., J < 0).
With these conditions for β(Q) ∼ β(δσ) ∼ 90°, optimization
of a fit for the experimental 17O MAS spectrum of NaIO4 at
19.6 T gives an rms error only slightly above (∼3%) the rms
error for the optimized fit discussed above, corresponding to
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Table 2. ADF and CASTEP Calculations of the 17O Quadrupole Coupling (CQ, ηQ), Chemical Shift Parameters (δσ, ησ, δiso),
Effective Dipolar Coupling D′(17O-127I) = D − δJ/2, Isotropic Coupling J = 1J(17O-127I), and Its Anisotropy δJ in NaIO4 and
KIO4 for Comparison with the Experimental 17O MAS NMR Data Reported in Table 1a
sample AFD/CASTEP
CQ (MHz)
ηQ
δσ (ppm)
ησ
δiso (ppm)
J (Hz)
D′b (Hz)
δJ (Hz)
NaIO4/ADF
KIO4/ADF
NaIO4/CASTEP
KIO4/CASTEP
−10.68
−10.51
−12.03
−11.69
0.07
0.04
0.08
0.04
178
167
180
173
0.17
0.10
0.24
0.11
278
261
374
357
449
461
−
−
−470
−482
−
−
−234
−236
−
−
a
See text for the definitions of the relevant parameters used here and in our previous publications. bThe calculated D′ values are determined using
the equation D′(17O-187Re) = D − δJ/2, employing the calculated values direct dipolar couplings D = −587 and −600 Hz for NaIO4 and KIO4,
respectively, and their corresponding calculated anisotropic values δJ = −234 and −236 Hz shown in the last column of this table.
Table 3. ADF Calculated Values for the Three Principal Components of the Chemical Shift (δ) and Indirect Spin-Coupling (J)
Tensors. Corresponding CASTEP Calculated Values Only for the Three Principal Components for the Chemical Shift (δ)
Tensor. These Values for NaIO4 and KIO4 are used to calculate the Corresponding Anisotropic Parameters Shown in Table 2.a
sample AFD/CASTEP
δxx (ppm)
δyy (ppm)
δzz (ppm)
Jxx (Hz)
Jyy (Hz)
Jzz (Hz)
NaIO4/ADF
KIO4/ADF
NaIO4/CASTEP
KIO4/CASTEP
381.3
352.3
485.1
453.1
351.9
335.9
442.5
433.6
99.7
94.1
193.5
184.0
569.7
580.8
−
−
561.4
576.5
−
−
214.6
225.0
−
−
a
See text for the definitions of the relevant parameters used here and in our previous publications. It is noted that the ADF calculations used the
isolated geometries for the two IO4− anions extracted from the crystal structures of NaIO412 and KIO413, while the CASTEP calculations used their
full crystal structures.
β(Q) ∼ β(δσ) ∼ 0°. Thus, this new optimization for β(Q) ∼
β(δσ) ∼ 90° converges to a local, rather than to a global
minimum on the rms surface. From a spectral simulation point
of view, the small increase of ∼3% in rms error indicates, but
does not definitively prove, that the parameter set shown for
NaIO4 in Table 1 (row 1) with J > 0 is most likely the correct
set. Furthermore, comparison of the orientations for the 17O
quadrupole and CSA tensors [β(Q) ∼ β(δσ) ∼ 0°] are also in
agreement with the results obtained for the perrhenates4
(identical crystal structure as for the periodates; space group
I41/a) provided the parameter set shown in Table 1 for J > 0 is
selected. Finally, the results of the ADF and CASTEP
calculations to be presented below are in excellent agreement
with the experimental data shown in Table 1.
To confirm the results on the NMR crystallography of the
two Scheelite structures for the IO4− ion obtained here from
analysis of 17O solid-state MAS NMR spectra, some ADF7−10
and CASTEP11 calculations have been performed for both
NaIO4 and KIO4 using the refined crystal structures for these
compounds.12,13 The ADF calculations were performed for the
two isolated IO4− anions whose geometries were extracted from
the refined crystal structures of NaIO412 and KIO413. First of all,
the tensor orientations resulting from both the CASTEP and
ADF calculations confirm the orientations, β(Q) ∼ β(δσ) ∼ 0°
for Vzz(17O) and δzz(17O), and with Vzz(127I) in the zx plane
[β(127I) = −56°] along the crystallographic c axis, as discussed
above for the molecular frame system for the IO4− tetrahedron
and used in the optimized fitting of the present spectra and
those recently for the perrhenate anions.4 The ADF and
CASTEP calculated 17O spectral parameters for NaIO4 and
KIO4 listed in Table 2 are all generally in very good agreement
with the experimental values in Table 1. We note that the
present version of our CASTEP software does not allow
calculations of the J coupling data and therefore the J-coupling
parameters reported here are only the results from ADF
calculations of the principal components of the J tensor. To
calculate the spectral parameters for the definitions of the CSA
and J tensor in the above eqs 1−6, the ADF and CASTEP
calculated principal components in these tensors for both
NaIO4 and KIO4 are summarized in Table 3. It is encouraging
to observe that the positive sign and large magnitude
determined experimentally for 1J(17O-127I) (approximately
+500 Hz for both NaIO4 and KIO4) is fully confirmed by the
ADF calculations. Similarly, the negative sign determined
experimentally for 1J(17O-127I) (approximately −500 Hz), using
the possible alternate tensor orientation β(Q) ∼ β(δσ) ∼ 90° as
starting parameters for the optimized fitting leads to a slightly
larger rms error. By changing the sign to positive for
1 17
J( O-127I) in a further optimization of all other parameters,
this change to a fixed value 1J(17O-127I) = +500 Hz immediately
changes the tensor orientation β(Q) ∼ β(δσ) to ∼0° in order to
achieve an even lower rms error, as described above in detail.
Finally, in conclusion, the ADF and CASTEP calculations in
Table 3 fully support the unambiguous results reported here on
the 17O solid-state NMR crystallography reported here for
NaIO4 and KIO4.
Conclusions. Analysis of RT 17O MAS NMR spectra of the
periodate ion in NaIO4 and KIO4 has allowed determination of
the magnitude and a positive sign for the indirect 1J(17O-127I)
coupling [i.e., 1J(17O-127I) = +500 Hz]. In addition, the spectral
analysis shows that the effective dipolar coupling (D′) and the
indirect 1J(17O-127I) coupling (J) have the opposite sign (D′ × J
< 0), which is in agreement with J > 0 since D′ has been safely
estimated to be negative (D′ < 0). Furthermore, of two possible
orientations for the quadrupole and CSA tensor, the spectral
analysis (i.e., the NMR crystallography) favors the one where
the molecular frame system for the IO4− tetrahedron has its z
axis along the I−O bond and the principal axes Vzz(17O) and
δzz(17O) of the PAS for both tensors are almost along the I−O
bond. Finally, a further contribution to the NMR crystallography is observed from a precise linear correlation between very
small changes for the 17O quadrupole asymmetry parameter
(ηQ) and changes for the tetrahedral-angle (O−I−O) opening
resulting from compression of the IO4− tetrahedron along the
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■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The analysis and simulations of the acquired 17O MAS NMR
spectra were performed at the Danish Instrument Centre for
Solid-State NMR Spectroscopy, Aarhus University, DK-8000
Aarhus C, which is supported by two Danish National Research
Councils. Thanks to Dr. Jacob Overgaard, Department of
Chemistry, Aarhus University, for fruitful discussions and
calculations of the tetrahedral angles in IO4− for KIO4. The 17O
MAS NMR experiments were performed at the National High
Magnetic Field Laboratory, which is supported by National
Science Foundation Cooperative Agreement DMR-1157490
and the State of Florida. The high spinning-frequency spin−
echo spectra were obtained at the National Ultrahigh-Field
NMR Facility for Solids (Ottawa, Ontario, Canada). G.W.
thanks NSERC of Canada for funding and Dr. Victor Terskikh
and Abouzar Toubaei for assistance in performing both
experiments and calculations.
■
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