19F high magnetic field NMR study of β-ZrF4 and CeF4: From spectra reconstruction to correlation between fluorine sites and 19F isotropic chemical shifts

Article abstract of DOI:10.1021/ic061339a

High magnetic field and high spinning frequency one- and two-dimensional one-pulse MAS ¹⁹F NMR spectra of β-ZrF₄ and CeF ₄ were recorded and reconstructed allowing the accurate determination of the ¹⁹F chemical shift tensor parameters for the seven different crystallographic fluorine sites of each compound. The attributions of the NMR resonances are performed using the superposition model for ¹⁹F isotropic chemical shift calculation initially proposed by Bureau et al. (Bureau, B.; Silly, G.; Emery, J.; Buzare, J.-Y. Chem. Phys. 1999, 249, 85-104). A satisfactory reliability is reached with a root-mean-square (rms) deviation between calculated and measured isotropic chemical shift values equal to 1.5 and 3.5 ppm for β-ZrF₄ and CeF₄, respectively.

Full text of DOI:10.1021/ic061339a

Inorg. Chem. 2006, 45, 10636−10641
19F High Magnetic Field NMR Study of
Reconstruction to Correlation between Fluorine Sites and F Isotropic
â-ZrF and CeF : From Spectra
4
4
19
Chemical Shifts
C. Legein,*,† F. Fayon,‡ C. Martineau,† M. Body,§ J.-Y. Buzar e´ ,§ D. Massiot,‡ E. Durand, A. Tressaud,
|
|
A. Demourgues, O. P e´ ron, and B. Boulard†
|
†
Laboratoire des Oxydes et Fluorures, CNRS UMR 6010, Laboratoire de Physique de l’Etat
Condens e´ , CNRS UMR 6087, Institut de Recherche en Ing e´ nierie Mol e´ culaire et Mat e´ riaux
Fonctionnels, CNRS FR 2575, UniVersit e´ du Maine, AVenue OliVier Messiaen, 72085 Le Mans
Cedex 9, France, Centre de Recherche sur les Mat e´ riaux a` Haute Temp e´ rature, CNRS UPR 4212,
1
D AVenue, Recherche Scientifique, 45071 Orl e´ ans Cedex 2, France, and Institut de Chimie de la
Mati e` re Condens e´ e de Bordeaux, CNRS UPR 9048, 87 AVenue du Dr. A. Schweitzer, 33608
Pessac Cedex, France
Received July 19, 2006
High magnetic field and high spinning frequency one- and two-dimensional one-pulse MAS 19F NMR spectra of
-ZrF and CeF
were recorded and reconstructed allowing the accurate determination of the 19F chemical shift
â
4
4
tensor parameters for the seven different crystallographic fluorine sites of each compound. The attributions of the
NMR resonances are performed using the superposition model for 19F isotropic chemical shift calculation initially
proposed by Bureau et al. (Bureau, B.; Silly, G.; Emery, J.; Buzar e´ , J.-Y. Chem. Phys. 1999, 249, 85
satisfactory reliability is reached with a root-mean-square (rms) deviation between calculated and measured isotropic
chemical shift values equal to 1.5 and 3.5 ppm for -ZrF and CeF , respectively.
−104). A
â
4
4
Introduction
attribution of the resonances was based on rough empirical
correlations related to experimental observations that similar
19F magic angle spinning (MAS) solid-state NMR can
provide important structural information on the environment
of fluorine atoms in ordered and disordered crystals, as well
as amorphous materials through F isotropic chemical shift
measurements. Because of the increasing MAS speed and
magnetic field, it is now possible to discriminate close
isotropic peaks on the experimental spectra. The challenge
1
9
F isotropic chemical shift values indicate similar local
environments.
1
1
Bureau et al. proposed an improvement hereafter called
the superposition model, which assigns isotropic chemical
shifts to fluorine crystallographic sites through the use of
phenomenological parameters. This model was successfully
19
applied to compounds from the BaF
binary systems: a refinement of the phenomenological
parameters led to the attribution of F NMR resonances to
fluorine sites with a root-mean-square (rms) value equal to
2
-AlF
3
and CaF
2
-AlF
3
1
9
is now to perform a reliable attribution of the F NMR
resonances to fluorine atoms. In numerous studies,
1
2
1
-10
the
1
9
*
To whom correspondence should be addressed. E-mail:
1
3
6
ppm, allowing a predictive use of this model. To assign
christophe.legein@univ-lemans.fr.
†
Laboratoire des Oxydes et Fluorures, Universit e´ du Maine.
Centre de Recherche sur les Mat e´ riaux a` Haute Temp e´ rature.
Laboratoire de Physique de l’Etat Condens e´ , Universit e´ du Maine.
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux.
‡
(7) Kiczenski, T. J.; Stebbins, J. F. J. Non-Cryst. Solids 2002, 306, 160-
§
168.
|
(8) Youngman, R. E.; Dejneka, M. J. J. Am. Ceram. Soc. 2002, 85, 1077-
(
(
(
1) Miller, J. M. Prog. Nucl. Magn. Reson. Spectrosc. 1996, 28, 255-
1082.
281.
(9) Kiczenski, T. J.; Du, L.-S.; Stebbins, J. F. J. Non-Cryst. Solids 2004,
2) Bureau, B.; Silly, G.; Buzar e´ , J.-Y.; Emery, J.; Legein, C.; Jacoboni,
C. J. Phys.: Condens. Matter 1997, 9, 6719-6736.
3) Bureau, B.; Silly, G.; Buzar e´ , J.-Y.; Jacoboni, C. J. Non-Cryst. Solids
337, 142-149.
(10) Youngman, R. E.; Sen, S. Solid State Nucl. Magn. Reson. 2005, 27,
77-89.
1999, 258, 110-118.
(11) Bureau, B.; Silly, G.; Emery, J.; Buzar e´ , J.-Y. Chem. Phys. 1999, 249,
(4) Stebbins, J. F.; Zeng, Q. J. Non-Cryst. Solids 2000, 262, 1-5.
(5) Zeng, Q.; Stebbins, J. F. Am. Mineral. 2000, 85, 863-867.
(6) Chan, J. C. C.; Eckert, H. J. Non-Cryst. Solids 2001, 284, 16-21.
85-104.
(12) Body, M.; Silly, G.; Legein, C.; Buzar e´ , J.-Y. Inorg. Chem. 2004, 43,
2474-2485.
10636 Inorganic Chemistry, Vol. 45, No. 26, 2006
10.1021/ic061339a CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/22/2006

19F High Magnetic Field NMR Study of â-ZrF
the NMR peaks to fluorine sites in â-ZrF
and CeF
and CeF
4
4
, we
very high spinning frequency. Since these 19F MAS NMR
spectra recorded at a high magnetic field show numerous
intense spinning sidebands, we have used the two-dimen-
4
4
have applied, for the first time, the superposition model on
basic fluorides containing several inequivalent fluorine sites.
14
15
28
19
CeF
4
and â-ZrF
4
are isostructural and are both involved
sional one-pulse (TOP) processing to obtain F “infinite
spinning frequency” isotropic spectra, which allows a direct
quantification of the unequivalent fluorine sites in the
structure and is an alternative to the simulation of the
spinning sideband manifolds. Structural data are presented
for both studied compounds, which allow us to perform a
partial attribution of the NMR resonances on the basis of
their relative intensities. Finally, we show that the superposi-
tion model, initially proposed by Bureau et al.,11 can be
successfully applied to these compounds, leading to the
in numerous material science applications. Fluorozirconate
glasses16 have retained considerable attention over the past
few years due to their physical and optical properties.
Very recently, in the context of preparing glass-ceramic-
1
7-20
based integrated optical amplifiers for the C telecom band,
Boulard et al. have studied â-ZrF
obtained by vapor-phase deposition. Indeed, rare-earth
4
-LaF
3
3
-ErF vitreous films
21
doped transparent fluorozirconate glass ceramics with a high
2
2
degree of crystallinity have been recently reported. These
materials are very promising in the field of integrated optics
because they allow high erbium concentration (6-8 mol %)
and higher absorption and emission cross-sections of erbium
relative to that of the glassy host. Redox properties of Ce-
based materials are used in catalytic devices developed for
1
9
assignment of the F resonances to the seven unequivalent
fluorine sites present in the structure for each studied
compound.
Experimental Procedures
2
3
the post-combustion process for Ce-based oxides or the
oxidative coupling of methane in the case of CeO /CaF
4
â-ZrF was obtained from Astron. The product was characterized
with powder X-ray diffraction (XRD); the X-ray powder pattern
was compared to the one reported in the Powder Diffraction File
2
2
2
4
system since cerium is a regulator of the oxygen partial
pressure over the catalyst. Moreover, the unique optical
characteristics of ceria, its band gap around 3.1 eV, and the
low value of refractive index around 2.05 suggest also that
(
PDF) number 00-033-1480, confirming the presence of the â-phase
only.
CeF
CeO
4
samples were prepared by fluorination of highly divided
with F gas at T ) 400-500 °C. The samples were
can be used as an UV absorber.25 To characterize the
2
2
CeO
structure of zirconium- and cerium-based fluoride com-
pounds, we have studied in a first step crystalline â-ZrF
2
characterized with powder X-ray diffraction (XRD); the X-ray
powder patterns were compared to the one reported in the Powder
Diffraction File (PDF) number 04-007-3514, confirming the purity
4
1
9
19
4
and CeF by F solid-state NMR. F NMR studies of such
of the samples.
crystalline phases are most of the time required prior the
determination of the short-range structure and the assignment
19F one- and two-dimensional one-pulse MAS NMR spectra were
recorded on an Avance 750 Bruker spectrometer operating at 17.6
19
4
of fluorine environments in ZrF -based glasses and glass
T ( F Larmor frequency of 705.85 MHz), using a 2.5 mm CP-
MAS probehead. All spectra were acquired using a Hahn echo
sequence with an inter-pulse delay equal to one rotor period. For
ceramics as well as in cerium IV- and fluorine-based
materials.
We present here what are apparently the first 19F solid-
â-ZrF
corresponding to a F nutation frequency of 170 kHz. As the CeF
spectrum extended over more than 600 kHz, shorter pulse durations
0.5 and 0.75 µs) were used, with the same 19F nutation frequency,
4
, a 1.5 µs 90° pulse and a 3 µs 180° pulse were used,
19
4
state NMR data for CeF
4 4
. â-ZrF was recently studied by
this technique at 11.7 T, but the assignement of the NMR
(
1
0
resonances to the fluorine sites was not complete.
to ensure a homogeneous irradiation of the whole spectrum. The
complete irradiation of the spectra was carefully checked by
recording spectra with various offsets. The discrimination of
isotropic peaks from spinning sidebands was achieved by recording
spectra at various spinning frequencies from 24 to 34 kHz. The
19
In this work, we have obtained F MAS NMR spectra of
â-ZrF and CeF at a very high magnetic field (17.6 T) and
4
4
(
13) Body, M.; Silly, G.; Legein, C.; Buzar e´ , J.-Y.; Calvayrac, F.; Blaha,
P. J. Solid State Chem. 2005, 178, 3655-3661.
recycle delay was set to 20 and 10 s for â-ZrF
4 4
and CeF ,
(14) Schmidt, R.; Mueller, B. G. Z. Anorg. Allg. Chem. 1999, 625, 605-
respectively, to ensure no saturation. The 19F chemical shifts are
608.
(
15) Burbank, R. D.; Bensey, F. N., Jr. USAEC Rep. 1956, K-1280, 1-19.
3
referenced to CFCl at 0 ppm.
The processing of these 19F MAS spectra with extensive spinning
sidebands manifold was carried out carefully, ensuring that all
(16) Poulain, M.; Poulain, M.; Lucas, J. Mater. Res. Bull. 1975, 10, 243-
246.
(
(
(
17) Lucas, J. J. Mater. Sci. 1989, 24, 1-6.
Fourier transforms start at t ) 0. The two-dimensional one-
18) Adam, J. L. Chem. ReV. 2002, 102, 2461-2476.
pulse2
6-28
(TOP) MAS spectra were reconstructed by stacking
19) Leroy, D.; Lucas, J.; Poulain, M.; Ravaine, D. Mater. Res. Bull. 1978,
13, 1039-1046.
subspectra shifted by the spinning frequency from the conventional
one-dimensional MAS spectra. Interpolation inside the one-
dimensional spectrum when extracting subspectra was used to
(
(
(
(
20) Hasz, W. C.; Wang, J. H.; Moynihan, C. T. J. Non-Cryst. Solids 1993,
161, 127-132.
21) Boulard, B.; P e´ ron, O.; Chiasera, A.; Ferrari, M.; Jestin, Y. Proc. SPIE-
Int. Soc. Opt. Eng. 2006, 6183, 369-376.
28
19
eliminate acquisition timing constraints. The F isotropic “infinite
spinning rate” MAS spectra were then obtained from the full
22) Mortier, M.; Monteville, A.; Patriarche, G.; Maz e´ , G.; Auzel, F. Opt.
Mater. 2001, 16, 255-267.
23) Laachir, A.; Perrichon, V.; Badri, A.; Lamotte, J.; Catherine, E.;
Lavalley, J. C.; Fallah, J. E.; Hilaire, L.; Le Normand, F.; Quemere,
E.; Sauvion, G. N.; Touret, O. J. Chem. Soc., Faraday Trans. 1991,
(26) Bl u¨ mich, B.; Blumler, P.; Jansen, J. Solid State Nucl. Magn. Reson.
1992, 1, 111-113.
8
7, 1601-1609.
(27) Blumler, P.; Bl u¨ mich, B.; Jansen, J. Solid State Nucl. Magn. Reson.
1994, 3, 237-240.
(
24) Zhou, X. P.; Wang, S. J.; Weng, W. Z.; Wan, H. L.; Tsai, K. R. J.
Nat. Gas Chem. 1993, 4, 280-289.
25) Sato, T.; Katakura, T.; Yin, S.; Fujimoto, T.; Yabe, S. Solid State
Ionics 2004, 172, 377-382.
(28) Massiot, D.; Hiet, J.; Pellerin, N.; Fayon, F.; Deschamps, M.;
Steuernagel, S.; Grandinetti, P. J. J. Magn. Reson. 2006, 181, 310-
315.
(
Inorganic Chemistry, Vol. 45, No. 26, 2006 10637

Legein et al.
Table 1. Fractional Atomic Coordinates,15 Fluorine Environments, and
F-Zr Bond Lengths (Å) for â-ZrF4
nearest
Zr atoms bond lenghts
F-Zr
atoms site symmetry
x
y
z
Zr1 4e
Zr2 8f
2
1
0
0.2148 1/4
0.2944 0.9289 0.1278
F1
F2
F3
F4
F5
F6
F7
4d
4e
8f
8f
8f
8f
8f
-1
1/4
0
1/4
3/4
Zr2
Zr2
Zr2
Zr2
Zr1
Zr2
Zr1
Zr2
Zr2
Zr2
Zr1
Zr2
Zr1
Zr2
2.072
2.072
2.132
2.132
2.139
2.148
2.048
2.180
2.031
2.127
2.052
2.118
2.088
2.131
2
1
1
1
1
1
0.598 1/4
0.111 0.289 0.040
0.115 0.059 0.161
0.212 0.526 0.104
0.379 0.125 0.162
0.378 0.346 0.028
Table 2. Fluorine Environments and F-Ce Bond Lengths (Å) for CeF4
nearest
Ce atoms
F-Ce
Figure 1. Perspective view of â-ZrF4 structure showing ZrF84- square
atoms
F1
site
4e
symmetry
2
bond lengths
Archimedean antiprism.
Ce1
Ce1
Ce2
Ce1
Ce2
Ce1
Ce1
Ce1
Ce2
Ce1
Ce1
Ce2
Ce1
Ce1
2.271
2.271
2.222
2.237
2.228
2.254
2.213
2.213
2.238
2.269
2.240
2.304
2.200
2.241
projection of the TOP spectra onto the frequency dimension. The
1
9
F2
F3
F4
F5
F6
F7
8f
8f
4d
8f
8f
8f
1
1
F chemical shift anisotropies were determined from the spinning
sidebands intensities according to Herzfeld and Berger.29 The
reconstructions of the one-dimensional MAS spectra (including
spinning sidebands) and the TOP processing were both performed
with the DMFIT30 software. The chemical shift tensor is described
by three parameters: the isotropic chemical shift, δiso, the chemical
shift anisotropy, δaniso, and the asymmetry parameter, η, according
to the following formulas:
-1
1
1
1
ν - νref
1
3
6
δiso (ppm) )
× 10 ) (δxx + δyy + δzz),
νref
CeF
4
(ICSD No. 89621, space group C2/c (No. 15), with
unit cell dimensions: a ) 12.5883 Å; b ) 10.6263 Å; c )
δyy - δxx
δaniso(ppm) ) δ - δ , and η )
zz
iso
δaniso
14
8.2241 Å; and â ) 126.24°)
4
is isostructural with â-ZrF .
The fluorine atom environments are described in Table 2.
with the principal components defined in the sequence
δ - δ | g |δ - δ | g |δ - δ |
19
Figures 2a and 3a show the F two-dimensional one-pulse
TOP) MAS NMR spectra of â-ZrF and CeF , respectively.
(
4
4
|
zz
iso
xx
iso
yy
iso
These spectra contain a large number of intense spinning
Results
The crystal structure of â-ZrF
dimensional network of corner-sharing ZrF
Archimedean antiprism (Figure 1). Each fluorine atom is
sidebands that indicate fairly large chemical shift anisotropy
values for the fluorine sites of â-ZrF and CeF . It should
4 4
be mentioned that spinning frequencies higher than 30 kHz
are required to prevent overlap between isotropic lines and
4
consists of a three-
4
-
8
square
4
spinning sidebands on the CeF one-dimensional MAS
1
5
coordinated by two zirconium atoms. The crystal structure
of â-ZrF reported in ref 15 is surprisingly lacking in the
spectrum that would result in a skewed sideband pattern in
4
2
8
the corresponding TOP spectrum. Figures 2b and 3b show
Inorganic Crystal Structure Database (ICSD), where there
are only reported crystal data from ref 31. Unfortunately,
fluorine atomic positions were undetermined in this study.
1
9
the F experimental and reconstructed “infinite spinning
rate” isotropic spectra of â-ZrF and CeF , respectively,
4
4
obtained from the projection of the TOP spectra onto the
4
â-ZrF crystallographic data required for discussion are
frequency dimension.
gathered in Table 1 (space group I2/c (No. 15), with unit
cell dimensions: a ) 9.57 Å; b ) 9.93 Å; c ) 7.73 Å; and
â ) 94°28′). There are seven inequivalent crystallographic
fluorine sites, two with a multiplicity of 4 (4d and 4e) and
five with 8f multiplicity.
The 19F isotropic spectrum of â-ZrF
is reconstructed
Figure 2b) using seven contributions with relative intensities
4
(
that are in fine agreement with structural data since five peaks
with relative intensity equal to 16.7% and two peaks with
relative intensity equal to 8.3% are expected (Tables 1 and
3
). It should be noted that this spectrum recorded at 17.6 T
(
(
29) Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021-6030.
30) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calv e´ , S.; Alonso,
B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson.
Chem. 2002, 40, 70-76.
clearly shows an improved resolution relative to that previ-
ously obtained at a lower magnetic field. The better
resolution at high field is due to the greater chemical shift
1
0
(31) Zachariasen, W. H. Acta Cryst. 1949, 2, 388-390.
10638 Inorganic Chemistry, Vol. 45, No. 26, 2006

19F High Magnetic Field NMR Study of â-ZrF
and CeF
4 4
Figure 2. (a) 19F experimental two-dimensional one-pulse (TOP) MAS
spectrum of â-ZrF4 obtained at a magnetic field of 17.6 T and a spinning
frequency of 30 kHz. (b) Experimental (top) and reconstructed (bottom)
Figure 3. (a) 19F experimental two-dimensional one-pulse (TOP) MAS
spectrum of CeF4 obtained at a magnetic field of 17.6 T and a spinning
frequency of 34 kHz. (b) Experimental (top) and reconstructed (bottom)
“infinite spinning rate” isotropic spectra of â-ZrF4. The individual contribu-
tions to the reconstructed spectrum are shown.
“
infinite spinning rate” isotropic spectra of CeF4. The individual contribu-
Table 3. δiso,exp (ppm), δaniso (ppm), and η; Relative Intensities (%)
1
9
tions to the reconstructed spectrum are shown. The arrows point out
resonances attributed to unidentified impurity.
Deduced from F Isotropic Spectrum Simulation and from
a
One-Dimensional MAS Spectrum Simulation; and Line Attributions
b
and δiso,cal (ppm) for â-ZrF4
δiso,exp
δaniso
η
intensity intensity
line ((0.5) ((5) ((0.05)
((2)
((2)
attribution δiso,cal
1
2
3
4
5
6
7
30.6 -170
28.7 -180
26.7 -170
19.6 -170
15.8 -165
10.4 -160
9.5 -175
0.45
0.55
0.55
0.4
0.4
0.3
16.2
8.1
17.3
16.0
17.0
8.3
16.0
8.5
17.8
15.7
16.0
7.7
F5
F1
F6
F7
F4
F2
F3
28.4
30.4
26.4
18.6
17.7
11.6
8.2
0.3
17.1
18.3
a
In italics. b Unambiguous attributions are in bold.
dispersion (in frequency units). The chemical shift anisotro-
pies (CSA) of the seven individual contributions were
2
9
determined from the spinning sideband intensities. The
obtained chemical shift anisotropy values range between
-
160 and -180 ppm, and the η values range between 0.3
and 0.55 (Table 3), in agreement with the rhombic fluorine
site symmetries (Table 1). The simulation of the one-
dimensional MAS spectrum of â-ZrF
4
using these CSA
parameters is shown in Figure 4. The relative intensities of
Figure 4. Experimental (bottom) and reconstructed (top) 19F one-
dimensional MAS NMR spectra of â-ZrF4 obtained at a magnetic field of
19
the seven F NMR resonances obtained from the simulation
of the conventional one-dimensional MAS spectrum are in
good agreement with those determined from the “infinite
spinning frequency” isotropic spectrum. The distinction
between fluorine sites with different multiplicities is unam-
biguous, allowing us to definitely assign lines 2 and 6 to the
F1 and/or F2 fluorine sites as these sites are expected to be
half as abundant as the five other fluorine sites in the structure
1
7.6 T and a spinning frequency of 30 kHz. Spinning sidebands are marked
by asterisks.
4
spectrum of CeF can be reconstructed with seven peaks
(Figure 3b) with relative intensities in agreement with
19
structural data (Tables 2 and 4). The associated F chemical
shift anisotropies determined from the spinning sideband
2
9
intensities are gathered in Table 4. It should be noted that
1
9
(Table 1). At this stage, there is no way a priori of choosing
4
the F experimental isotropic spectrum of CeF exhibits
between F1 and F2 for these two peaks. The assignment of
lines 2 and 6 to less intense resonances differ with those
obtained in ref 10, where reconstruction was done by
analyzing only the isotropic lines of the one-dimensional
MAS lower resolution spectrum.
some extra lines of weak intensities (Figure 3b). These extra
lines were attributed to an impurity since their relative
intensities (as compared to the intensities of the resonances
attributed to CeF
Moreover, their relative intensities are lower than 8%, and
they cannot be attributed to fluorine sites of CeF according
4
) differ from one sample to another.
As for â-ZrF
isotropic spectrum obtained from the projection of the TOP
4
, the 19F “infinite spinning frequency”
4
to the expected site multiplicities. This impurity, which is
Inorganic Chemistry, Vol. 45, No. 26, 2006 10639

Legein et al.
1
a
Table 4. δiso,exp (ppm), δaniso (ppm), and η; Relative Intensities (%)
Table 5. Rl (Å- ), d0 (Å), and σl (ppm) Parameters Before and After
0
1
9
Deduced from F Isotropic Spectrum Simulation and from
Refinement for â-ZrF4 and CeF4
a
One-Dimensional MAS Spectrum Simulation; and Line Attributions
b
compound
Rl
d0
σl0
and δiso,cal (ppm) for CeF4
â-ZrF4
3.26
3.15
0.99
1.18
2.105
2.243
2.109
2.245
-155.8
-253.9
-155.0
-252.8
δiso,exp
δaniso
η
intensity intensity
CeF4
line ((0.5) ((7) ((0.05)
((2)
((2)
attribution δiso,cal
a
In italics.
1
2
3
4
5
6
7
235.0 -390
228.6 -410
221.8 -390
220.3 -390
210.4 -380
199.5 -370
196.2 -370
0.3
0.3
16.7
11.2
16.9
15.5
15.2
16.3
8.2
15.2
10.6
16.4
16.8
17.5
15.4
8.1
F7
F4
F2
F3
F5
F6
F1
229.5
233.9
223.8
217.0
209.5
199.0
199.2
0.25
0.25
0.2
0.15
0.1
the simulation of the conventional one-dimensional MAS
spectrum are in good agreement with those determined from
the “infinite spinning frequency” isotropic spectrum.
Discussion
a
In italics. b Unambiguous attributions are in bold.
The superposition model for 19F isotropic chemical shift
calculation was proposed by Bureau et al. for ionic
fluorides. In this model, the F isotropic chemical shift is
considered to be a sum of one constant diamagnetic term
and several paramagnetic contributions from the l neighbor-
ing cations and is calculated according to the main following
1
1
1
9
1
1
formula:
δ
) -127.1 - σ and σ ) σ exp[-R (d - d )],
iso/C6F6
∑l
l
l
l
l
0
0
which corresponds to δiso/CFCl3 ) -291.3 - σl
∑l
where d
0
is the characteristic F-M distance that is taken
equal to the bond length in the related basic fluoride, and
l0 is the parameter that determines the order of magnitude
σ
of the cationic paramagnetic contribution to the shielding
and was deduced from measurements in the related basic
fluoride where δiso/CFCl3 ) -291.3 - nσl0 (n is the coordina-
tion number of the fluorine atom).
The first step, using this model, is to determine R
l
. For
atoms whose atomic radial wave functions are known, R
l
was deduced from the behavior of the cation isotropic
p
paramagnetic contribution σj (see formula in Appendix A
Figure 5. Experimental (bottom) and reconstructed (top) 19F one-
dimensional MAS NMR spectra of CeF4 obtained at a magnetic field of
of ref 11) calculated for several d distances on the basis of
3
2
Ramsey’s theory with molecular orbitals obtained by
1
7.6 T and a spinning frequency of 34 kHz. The dashed lines delimit the
3
3
isotropic resonance range.
L o¨ wdin’s orthogonalization method. A linear relation
between R and the ionic radius r of the ligand l has been
established: R ) -0.806r + 4.048. This formula was
applied to cations for which radial wave functions are
l
l
not detected on the X-ray powder pattern, likely would be a
cerium IV oxy-(hydroxy)-fluoride since the observed δiso
1
1
4
+
4+
values are close to the CeF
4
ones and are then characteristic
unknown. As is the case for Zr and Ce cations, we use
it to calculate the corresponding initial R parameters (Table
5). The Zr and Ce crystal radii are taken equal to 0.98
of fluorine ions with Ce4 ions as nearest neighbors. The
+
l
4
+
4+
agreement between expected and experimental relative
3
4
intensities is not as fine as for â-ZrF
4
, certainly due to the
and 1.11 Å, respectively.
The second step consists in determining d
Since the structure of â-ZrF and CeF contains several
presence of the impurity. Nevertheless, the distinction
between fluorine sites with different multiplicities is unam-
biguous. Lines 2 and 7 can be easily assigned to F1 and/or
F4 fluorine sites, as these sites are expected to be half as
abundant as the five other fluorine sites in the structure (Table
0
and σl0 values.
4
4
distinct crystallographic fluorine sites with several different
F-Zr distances and δiso values, this determination is not as
straighforward as for basic fluorides containing a single
1
1
2
(
). The CSA values ranging between -370 and -410 ppm
Table 4) are larger than those measured for â-ZrF fluorine
, the η values ranging between 0.1 and
fluorine crystallographic site.
determined for both atoms, considering weighted average
values of d and δiso (Table 5).
0
d and σl0 values were
4
sites. As for â-ZrF
4
0
0
.3 (Table 4) are all different from zero, in agreement with
the rhombic fluorine site symmetries (Table 2). The simula-
tion of the one-dimensional MAS spectrum of CeF using
The last step is to define the number of neighboring cations
M whose contributions have to be taken into account. As
4
(
(
32) Ramsey, N. F. Phys. ReV. 1950, 78, 699-703.
33) L o¨ wdin, P. O¨ . AdV. Phys. 1956, 5, 1-172.
these CSA parameters is shown in Figure 5. The relative
intensities of the seven 19F NMR resonances obtained from
(34) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751-767.
10640 Inorganic Chemistry, Vol. 45, No. 26, 2006

19F High Magnetic Field NMR Study of â-ZrF
and CeF
4
4
was previously done for basic fluorides, only the first fluorine
This is certainly due to the fact that the F-Zr bond lengths
used in ref 10 were clearly inaccurate.
11
coordination sphere was considered in the calculation. The
next nearest neighboring cations in â-ZrF and CeF are at
4
4
A comparison between initial and refined parameters
distances larger than 3.75 and 4.0 Å, respectively, and it was
previously shown that it is needless to perform calculations
with sphere radii larger than 3.5 Å.12
(
Table 5) of the superposition model shows that the only
sensitive parameter is R , which describes the behavior of
the paramagnetic contribution with the F-M distance. The
decrease for the R parameter may be correlated with
l
The values obtained with this model are compared with
l
the experimental results for â-ZrF
4
4
and CeF . The calculated
12
deviation from the purely ionic model for F-Zr and F-Ce
bonds. In other words, the orbital overlap between F and Zr
or Ce atoms is larger than in a purely ionic model.
and experimental isotropic chemical shifts are then matched,
having regard for the first attribution based on the relative
intensities, to minimize the difference: ∆δiso ) δiso,exp
iso,cal. The obtained rms deviations of 19 and 22 ppm for
â-ZrF and CeF , respectively, are too large to perform any
attribution. Therefore, a minimization of the sum S )
-
δ
Conclusion
4
4
High magnetic field and high spinning frequency one- and
1
9
2
two-dimensional one-pulse F MAS NMR spectra were
recorded for the two isostructural compounds â-ZrF and
CeF . From the reconstruction of these experimental spectra,
the F chemical shift parameters of the seven resonances
corresponding to the seven crystallographic inequivalent
fluorine sites in the â-ZrF and CeF structures were
4 4
determined. The attributions of the F resonances to the
various fluorine sites were performed using the superposition
model initially proposed by Bureau et al., which was
applied for the first time on basic fluorides containing several
fluorine sites. A good agreement is obtained between
experimental and calculated isotropic chemical shift values,
the rms deviation values being equal to 1.5 and 3.5 ppm for
∑
l 0
(δiso,cal - δiso,exp) was carried out with R , d , and σl0 as
adjustable parameters. A new parameter set is obtained, and,
if necessary, the line attribution is modified, with respect to
the relative intensities, to keep the difference between
calculated and experimental δiso at a minimum. Usually, this
process is repeated until the line attribution remains un-
changed, but in this study, only one new attribution of the
NMR lines was performed: permutation between F4 and F7
4
4
1
9
1
9
1
1
attributions for â-ZrF
ppm were achieved for â-ZrF
4
. The rms deviations of 1.5 and 3.5
and CeF , respectively. The
4
4
calculated isotropic chemical shift values and the corre-
sponding line attributions are collected in Tables 3 and 4
for â-ZrF
the assignment of the F4 and F7 resonances in â-ZrF
remains ambiguous since the difference between their
4 4
and CeF , respectively. It should be noted that
4 4
â-ZrF and CeF , respectively.
4
Acknowledgment. We thank J.-P. Laval, from the
SPCTS, CNRS UMR 6638, Universit e´ de Limoges, who
kindly supplied us with ref 15.
1
9
calculated F isotropic chemical shifts is small, within the
rms deviation of 1.5 ppm (Table 3). Nevertheless, our results
19
lead to a more detailed attribution of the F resonances to
0
the fluorine sites in â-ZrF
4
than that previously proposed.1
IC061339A
Inorganic Chemistry, Vol. 45, No. 26, 2006 10641